diff --git a/CMakeLists.txt b/CMakeLists.txt
index ffb0ea1ab..1d4b40b47 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -99,6 +99,11 @@ set(_SwiftFoundation_FutureAvailability "macOS 10000, iOS 10000, tvOS 10000, wat
 
 # All versions to define for each availability name
 list(APPEND _SwiftFoundation_versions
+    "5.5"
+    "5.7"
+    "5.8"
+    "5.9"
+    "6.0"
     "6.0.2"
     "6.1"
     "6.2"
@@ -107,10 +112,12 @@ list(APPEND _SwiftFoundation_versions
 # Each availability name to define
 list(APPEND _SwiftFoundation_availability_names
     "FoundationPreview"
+    "FoundationAttributedString"
     "FoundationSpan")
 
 # The aligned availability for each name (in the same order)
 list(APPEND _SwiftFoundation_availability_releases
+    ${_SwiftFoundation_BaseAvailability}
     ${_SwiftFoundation_BaseAvailability}
     ${_SwiftFoundation_macOS26Availability})
 
diff --git a/Package.swift b/Package.swift
index cedb5599b..297f35c7d 100644
--- a/Package.swift
+++ b/Package.swift
@@ -8,9 +8,10 @@ import CompilerPluginSupport
 
 let availabilityTags: [_Availability] = [
     _Availability("FoundationPreview"), // Default FoundationPreview availability
+    _Availability("FoundationAttributedString"),
     _Availability("FoundationSpan", availability: .macOS26), // Availability of Span types
 ]
-let versionNumbers = ["6.0.2", "6.1", "6.2"]
+let versionNumbers = ["5.5", "5.7", "5.8", "5.9", "6.0", "6.0.2", "6.1", "6.2"]
 
 // Availability Macro Utilities
 
diff --git a/Sources/FoundationEssentials/AttributedString/AttributeContainer.swift b/Sources/FoundationEssentials/AttributedString/AttributeContainer.swift
index 10c2dc35c..78c0350fe 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributeContainer.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributeContainer.swift
@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 @dynamicMemberLookup
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public struct AttributeContainer : Sendable {
     internal var storage : AttributedString._AttributeStorage
     
@@ -24,7 +24,7 @@ public struct AttributeContainer : Sendable {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeContainer {
     @preconcurrency
     public subscript<T: AttributedStringKey>(_: T.Type) -> T.Value? where T.Value : Sendable {
@@ -87,26 +87,27 @@ extension AttributeContainer {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeContainer: Equatable {}
 
-@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+@available(FoundationAttributedString 5.7, *)
 extension AttributeContainer: Hashable {}
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeContainer: CustomStringConvertible {
     public var description: String {
         storage.description
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeContainer {
     internal var _hasConstrainedAttributes: Bool {
         storage.hasConstrainedAttributes
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributeContainer {
     /// Returns a copy of the attribute container with only attributes that specify the provided inheritance behavior.
     /// - Parameter inheritedByAddedText: An `inheritedByAddedText` value to filter. Attributes matching this value are included in the returned container.
diff --git a/Sources/FoundationEssentials/AttributedString/AttributeScope.swift b/Sources/FoundationEssentials/AttributedString/AttributeScope.swift
index 2fac17426..a0901fa05 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributeScope.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributeScope.swift
@@ -16,14 +16,14 @@
 //     var foregroundColor : ForegroundColor
 // }
 // An AttributeScope can contain other scopes as well.
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol AttributeScope : DecodingConfigurationProviding, EncodingConfigurationProviding {
     static var decodingConfiguration: AttributeScopeCodableConfiguration { get }
     static var encodingConfiguration: AttributeScopeCodableConfiguration { get }
 }
 
 @frozen
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public enum AttributeScopes { }
 
 @available(macOS, unavailable, introduced: 12.0)
@@ -171,7 +171,7 @@ internal func _loadDefaultAttributes() -> [String : any AttributedStringKey.Type
 }
 
 // TODO: Support AttributeScope key finding in FoundationPreview
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScope {
     private static var scopeDescription: ScopeDescription {
         if let cached = _loadedScopeCache.withLock({ $0[Self.self] }) {
@@ -207,7 +207,7 @@ extension AttributeScope {
     }
     
     /// A list of all attribute keys contained within this scope and any sub-scopes.
-    @available(FoundationPreview 6.2, *)
+    @available(FoundationAttributedString 6.2, *)
     public static var attributeKeys: some Sequence<any AttributedStringKey.Type> {
         Self.scopeDescription.attributes.values
     }
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+AttributeTransformation.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+AttributeTransformation.swift
index acf2a878d..4bcbf1757 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+AttributeTransformation.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+AttributeTransformation.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     @preconcurrency
     public struct SingleAttributeTransformer<T: AttributedStringKey> : Sendable where T.Value : Sendable {
@@ -45,7 +45,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal func applyRemovals<K>(
         withOriginal orig: AttributedString.SingleAttributeTransformer<K>,
@@ -78,7 +78,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     @preconcurrency
     public func transformingAttributes<K>(
@@ -251,7 +251,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     @preconcurrency
     public func transformingAttributes<K>(
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+CharacterView.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+CharacterView.swift
index 12af67060..d6fce4b29 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+CharacterView.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+CharacterView.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public struct CharacterView : Sendable {
         /// The guts of the base attributed string.
@@ -80,13 +80,14 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.CharacterView {
     internal var _characters: BigSubstring {
         BigSubstring(_unchecked: _guts.string, in: _range)
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension Slice<AttributedString.CharacterView> {
     internal var _rebased: AttributedString.CharacterView {
         let bounds = Range(uncheckedBounds: (self.startIndex._value, self.endIndex._value))
@@ -100,7 +101,7 @@ extension Slice<AttributedString.CharacterView> {
 
 // FIXME: AttributedString.CharacterView needs to publicly conform to Equatable & Hashable.
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.CharacterView: BidirectionalCollection {
     public typealias Element = Character
     public typealias Index = AttributedString.Index
@@ -123,7 +124,7 @@ extension AttributedString.CharacterView: BidirectionalCollection {
         return _defaultCount
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal var _count: Int {
         _characters.count
@@ -153,7 +154,7 @@ extension AttributedString.CharacterView: BidirectionalCollection {
         return _defaultIndex(i, offsetBy: distance)
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal func _index(_ i: AttributedString.Index, offsetBy distance: Int) -> AttributedString.Index {
         precondition(i >= startIndex && i <= endIndex, "AttributedString index out of bounds")
@@ -176,7 +177,7 @@ extension AttributedString.CharacterView: BidirectionalCollection {
         return _defaultIndex(i, offsetBy: distance, limitedBy: limit)
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal func _index(
         _ i: AttributedString.Index,
@@ -207,7 +208,7 @@ extension AttributedString.CharacterView: BidirectionalCollection {
         return _defaultDistance(from: start, to: end)
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal func _distance(from start: AttributedString.Index, to end: AttributedString.Index) -> Int {
         precondition(start >= startIndex && start <= endIndex, "AttributedString index out of bounds")
@@ -245,7 +246,7 @@ extension AttributedString.CharacterView: BidirectionalCollection {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.CharacterView: RangeReplaceableCollection {
     internal mutating func _ensureUniqueReference() {
         if !isKnownUniquelyReferenced(&_guts) {
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+Guts.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+Guts.swift
index 163215139..2a2e80dfa 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+Guts.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+Guts.swift
@@ -18,6 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal final class Guts : @unchecked Sendable {
         typealias Index = AttributedString.Index
@@ -54,6 +55,7 @@ extension AttributedString {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Guts {
     __consuming func copy() -> AttributedString.Guts {
         AttributedString.Guts(string: self.string, runs: self.runs)
@@ -73,6 +75,7 @@ extension AttributedString.Guts {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Guts {
     internal static func characterwiseIsEqual(
         _ left: AttributedString.Guts,
@@ -182,6 +185,7 @@ extension AttributedString.Guts {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Guts {
     internal func description(in range: Range<BigString.Index>) -> String {
         self.description(in: RangeSet(range))
@@ -199,6 +203,7 @@ extension AttributedString.Guts {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Guts {
     var stringBounds: Range<BigString.Index> {
         Range(uncheckedBounds: (string.startIndex, string.endIndex))
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+IndexTracking.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+IndexTracking.swift
index c4646fc6c..1c0977670 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+IndexTracking.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+IndexTracking.swift
@@ -20,6 +20,7 @@ internal import _FoundationCollections
 
 // MARK: - Internal Index Updating
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Guts {
     func _prepareTrackedIndicesUpdate(mutationRange: Range<BigString.Index>) {
         // Move any range endpoints inside of the mutation range to outside of the mutation range since a range should never end up splitting a mutation
@@ -75,7 +76,7 @@ extension AttributedString.Guts {
 
 // MARK: - Public API
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString {
     // MARK: inout API
     
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift
index 8b617aaab..98df21dde 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift
@@ -14,6 +14,7 @@
 internal import Synchronization
 #endif
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Guts {
     typealias Version = UInt
     
@@ -43,7 +44,7 @@ extension AttributedString.Guts {
 
 // MARK: - Public API
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString.Index {
     /// Indicates whether the index is valid for use with the provided attributed string.
     /// - Parameter text: An attributed string used to validate the index.
@@ -63,7 +64,7 @@ extension AttributedString.Index {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension Range<AttributedString.Index> {
     /// Indicates whether the range is valid for use with the provided attributed string.
     /// - Parameter text: An attributed string used to validate the range.
@@ -88,7 +89,7 @@ extension Range<AttributedString.Index> {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension RangeSet<AttributedString.Index> {
     /// Indicates whether the range set is valid for use with the provided attributed string.
     /// - Parameter text: An attributed string used to validate the range set.
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+AttributeSlices.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+AttributeSlices.swift
index 943bbcb56..d84919989 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+AttributeSlices.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+AttributeSlices.swift
@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     @preconcurrency
     public struct AttributesSlice1<T : AttributedStringKey> : BidirectionalCollection, Sendable
@@ -142,7 +142,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     @preconcurrency
     public struct AttributesSlice2<
@@ -298,7 +298,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     @preconcurrency
     public struct AttributesSlice3<
@@ -466,7 +466,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     @preconcurrency
     public struct AttributesSlice4<
@@ -648,7 +648,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     @preconcurrency
     public struct AttributesSlice5<
@@ -846,7 +846,7 @@ extension AttributedString.Runs {
 
 #if FOUNDATION_FRAMEWORK
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     @_spi(AttributedString)
     public struct NSAttributesSlice : BidirectionalCollection, Sendable {
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+Run.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+Run.swift
index fce658af4..631cf2170 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+Run.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+Runs+Run.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     @dynamicMemberLookup
     public struct Run: Sendable {
@@ -42,7 +42,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs.Run: Equatable {
     public static func == (lhs: Self, rhs: Self) -> Bool {
         lhs._range._utf8OffsetRange.count == rhs._range._utf8OffsetRange.count
@@ -50,14 +50,14 @@ extension AttributedString.Runs.Run: Equatable {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs.Run: CustomStringConvertible {
     public var description: String {
         AttributedSubstring(_guts, in: _range).description
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs.Run {
     public var range: Range<AttributedString.Index> {
         let lower = AttributedString.Index(_range.lowerBound, version: _guts.version)
@@ -78,7 +78,7 @@ extension AttributedString.Runs.Run {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs.Run {
     @preconcurrency
     @inlinable // Trivial implementation, allows callers to optimize away the keypath allocation
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+Runs.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+Runs.swift
index f1c9c8abc..08afba4b1 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+Runs.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+Runs.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public struct Runs: Sendable {
         internal typealias _InternalRun = AttributedString._InternalRun
@@ -95,7 +95,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs: Equatable {
     public static func == (lhs: Self, rhs: Self) -> Bool {
         // Note: Unlike AttributedString itself, this is comparing run lengths without normalizing
@@ -119,14 +119,14 @@ extension AttributedString.Runs: Equatable {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs: CustomStringConvertible {
     public var description: String {
         _guts.description(in: _strBounds)
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     public struct Index: Sendable {
         /// The offset of this run from the start of the attributed string.
@@ -185,7 +185,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs.Index: Comparable {
     public static func == (lhs: Self, rhs: Self) -> Bool {
         lhs._runOffset == rhs._runOffset && lhs._stringIndex == rhs._stringIndex
@@ -210,11 +210,11 @@ extension AttributedString.Runs.Index: Comparable {
 }
 
 #if !FOUNDATION_FRAMEWORK
-@available(macOS, deprecated: 10000, introduced: 12, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
-@available(iOS, deprecated: 10000, introduced: 15, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
-@available(tvOS, deprecated: 10000, introduced: 15, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
-@available(watchOS, deprecated: 10000, introduced: 8, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
-@available(visionOS, deprecated: 10000, introduced: 1, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
+@available(macOS, deprecated: 10000, introduced: 26, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
+@available(iOS, deprecated: 10000, introduced: 26, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
+@available(tvOS, deprecated: 10000, introduced: 26, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
+@available(watchOS, deprecated: 10000, introduced: 26, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
+@available(visionOS, deprecated: 10000, introduced: 26, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
 @available(*, deprecated, message: "AttributedString.Runs.Index should not be used as a Strideable and should instead be offset using the API provided by AttributedString.Runs")
 extension AttributedString.Runs.Index: Strideable {
     public func distance(to other: Self) -> Int {
@@ -230,14 +230,14 @@ extension AttributedString.Runs.Index: Strideable {
 }
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension Range<AttributedString.Runs.Index> {
     var _runOffsetRange: Range<Int> {
         Range<Int>(uncheckedBounds: (lowerBound._runOffset, upperBound._runOffset))
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs: BidirectionalCollection {
     public typealias Element = Run
 
@@ -339,7 +339,7 @@ extension AttributedString.Runs: BidirectionalCollection {
     }
     #endif
     
-    @available(FoundationPreview 6.2, *)
+    @available(FoundationAttributedString 6.2, *)
     @usableFromInline
     internal func _distance(from start: Index, to end: Index) -> Int {
         guard _isDiscontiguous else {
@@ -366,7 +366,7 @@ extension AttributedString.Runs: BidirectionalCollection {
     #endif
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal func _index(_ index: Index, offsetBy distance: Int) -> Index {
         guard _isDiscontiguous else {
@@ -433,7 +433,7 @@ extension AttributedString.Runs: BidirectionalCollection {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     // FIXME: Make public, with a better name. (Probably no need to state "run" -- `index(containing:)`?)
     internal func indexOfRun(at position: AttributedString.Index) -> Index {
@@ -488,7 +488,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Runs {
     internal func _slicedRunBoundary(
         after i: AttributedString.Index,
@@ -620,7 +620,7 @@ extension AttributedString.Runs {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension BigString {
     internal func _firstConstraintBreak(
         in range: Range<Index>,
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+UTF16View.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+UTF16View.swift
index a9566bebd..355a266ba 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+UTF16View.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+UTF16View.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString {
     /// A view of an attributed string’s contents as a collection of UTF-16 code units.
     public struct UTF16View: Sendable {
@@ -42,7 +42,7 @@ extension AttributedString {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedSubstring {
     /// A view of the attributed substring's contents as a collection of UTF-16 code units.
     public var utf16: AttributedString.UTF16View {
@@ -50,14 +50,14 @@ extension AttributedSubstring {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString.UTF16View {
     var _utf16: BigSubstring.UTF16View {
         BigSubstring.UTF16View(_unchecked: _guts.string, in: _range)
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString.UTF16View: BidirectionalCollection {
     public typealias Element = UTF16.CodeUnit
     public typealias Index = AttributedString.Index
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+UTF8View.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+UTF8View.swift
index 7f68cf8b2..281adc001 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+UTF8View.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+UTF8View.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString {
     /// A view of an attributed string’s contents as a collection of UTF-8 code units.
     public struct UTF8View: Sendable {
@@ -42,7 +42,7 @@ extension AttributedString {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedSubstring {
     /// A view of the attributed substring's contents as a collection of UTF-8 code units.
     public var utf8: AttributedString.UTF8View {
@@ -50,14 +50,14 @@ extension AttributedSubstring {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString.UTF8View {
     var _utf8: BigSubstring.UTF8View {
         BigSubstring.UTF8View(_unchecked: _guts.string, in: _range)
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString.UTF8View: BidirectionalCollection {
     public typealias Element = UTF8.CodeUnit
     public typealias Index = AttributedString.Index
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+UnicodeScalarView.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+UnicodeScalarView.swift
index a0f8ed991..42c1d04c3 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+UnicodeScalarView.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+UnicodeScalarView.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public struct UnicodeScalarView: Sendable {
         internal var _guts: Guts
@@ -73,12 +73,14 @@ extension AttributedString {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.UnicodeScalarView {
     var _unicodeScalars: BigSubstring.UnicodeScalarView {
         BigSubstring.UnicodeScalarView(_unchecked: _guts.string, in: _range)
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension Slice<AttributedString.UnicodeScalarView> {
     internal var _rebased: AttributedString.UnicodeScalarView {
         let bounds = Range(uncheckedBounds: (self.startIndex._value, self.endIndex._value))
@@ -90,6 +92,7 @@ extension Slice<AttributedString.UnicodeScalarView> {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.UnicodeScalarView {
     // FIXME: AttributedString.UnicodeScalarView needs to publicly conform to Hashable.
     internal func _isEqual(to other: Self) -> Bool {
@@ -101,7 +104,7 @@ extension AttributedString.UnicodeScalarView {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.UnicodeScalarView: BidirectionalCollection {
     public typealias Element = UnicodeScalar
     public typealias Index = AttributedString.Index
@@ -124,7 +127,7 @@ extension AttributedString.UnicodeScalarView: BidirectionalCollection {
         return _defaultCount
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal var _count: Int {
         _unicodeScalars.count
@@ -165,7 +168,7 @@ extension AttributedString.UnicodeScalarView: BidirectionalCollection {
         return _defaultIndex(i, offsetBy: distance, limitedBy: limit)
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal func _index(
         _ i: AttributedString.Index,
@@ -189,15 +192,19 @@ extension AttributedString.UnicodeScalarView: BidirectionalCollection {
         from start: AttributedString.Index,
         to end: AttributedString.Index
     ) -> Int {
+        #if FOUNDATION_FRAMEWORK
         if #available(macOS 14, iOS 17, tvOS 17, watchOS 10, *) {
             return _distance(from: start, to: end)
         }
         precondition(start >= startIndex && start <= endIndex, "AttributedString index out of bounds")
         precondition(end >= startIndex && end <= endIndex, "AttributedString index out of bounds")
         return _defaultDistance(from: start, to: end)
+        #else
+        return _distance(from: start, to: end)
+        #endif
     }
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal func _distance(
         from start: AttributedString.Index,
@@ -226,7 +233,7 @@ extension AttributedString.UnicodeScalarView: BidirectionalCollection {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.UnicodeScalarView: RangeReplaceableCollection {
     internal mutating func _ensureUniqueReference() {
         if !isKnownUniquelyReferenced(&_guts) {
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRun.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRun.swift
index ffd6fd179..f1d36098a 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRun.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRun.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal struct _InternalRun: Sendable {
         // UTF-8 Code Unit Length
@@ -32,12 +32,14 @@ extension AttributedString {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRun: Equatable {
     internal static func == (left: Self, right: Self) -> Bool {
         left.length == right.length && left.attributes == right.attributes
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRun: Hashable {
     internal func hash(into hasher: inout Hasher) {
         hasher.combine(length)
@@ -45,6 +47,7 @@ extension AttributedString._InternalRun: Hashable {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRun: RopeElement {
     typealias Index = Int
 
@@ -72,12 +75,14 @@ extension AttributedString._InternalRun: RopeElement {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRun {
     internal func get<T: AttributedStringKey>(_ k: T.Type) -> T.Value? where T.Value : Sendable {
         attributes[k]
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRun {
     struct Summary: Sendable {
         var count: Int
@@ -95,6 +100,7 @@ extension AttributedString._InternalRun {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRun.Summary: RopeSummary {
     @inline(__always)
     static var maxNodeSize: Int { 15 }
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift
index 0c0e3c6fe..cb2cdedcb 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift
@@ -18,6 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     /// An internal convenience wrapper around `Rope<_InternalRun>`, giving it functionality
     /// that's specific to attributed strings.
@@ -41,6 +42,7 @@ extension AttributedString {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns {
     /// A metric that assigns each run a size of 1; i.e., the metric corresponding to run offsets.
     ///
@@ -89,6 +91,7 @@ extension AttributedString._InternalRuns {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns {
     struct Index {
         /// The underlying index in the rope.
@@ -108,17 +111,20 @@ extension AttributedString._InternalRuns {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns.Index: Equatable {
     static func ==(left: Self, right: Self) -> Bool {
         left.utf8Offset == right.utf8Offset
     }
 }
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns.Index: Comparable {
     static func <(left: Self, right: Self) -> Bool {
         left.utf8Offset < right.utf8Offset
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns: BidirectionalCollection {
     typealias Element = _InternalRun
     typealias SubSequence = Slice<Self>
@@ -187,12 +193,14 @@ extension AttributedString._InternalRuns: BidirectionalCollection {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension Slice<AttributedString._InternalRuns> {
     var utf8Count: Int {
         self.base.distance(from: self.startIndex, to: self.endIndex)
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns {
     func index(atRunOffset runOffset: Int) -> Index {
         let r = _rope.find(at: runOffset, in: RunMetric(), preferEnd: false)
@@ -221,6 +229,7 @@ extension AttributedString._InternalRuns {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns {
     var utf8Count: Int {
         _rope.count(in: UTF8Metric())
@@ -249,6 +258,7 @@ extension AttributedString._InternalRuns {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRuns {
     /// Update the run at `index` with the provided `run` and coalesce it with its
     /// neighbors if necessary. `index` is updated to a valid index addressing the
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRunsSlice.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRunsSlice.swift
index 5386c8217..de5396832 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRunsSlice.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRunsSlice.swift
@@ -18,6 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     /// A mutable rope of `_InternalRun` values, sliced on an arbitrary UTF-8 offset range.
     /// Boundary runs may be sliced into two parts; this collection transparently constructs
@@ -49,6 +50,7 @@ extension AttributedString {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRunsSlice: BidirectionalCollection {
     // Note: This is a true bidirectional collection, only supporting step-by-step access.
     // (It would be possible to implement `index(_:offsetBy:)` in logarithmic time, but we have
@@ -102,6 +104,7 @@ extension AttributedString._InternalRunsSlice: BidirectionalCollection {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._InternalRunsSlice {
     func update(
         at index: inout _InternalRuns.Index,
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString.swift b/Sources/FoundationEssentials/AttributedString/AttributedString.swift
index 1f871c647..27e08fa54 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString.swift
@@ -19,7 +19,7 @@ internal import _FoundationCollections
 #endif
 
 @dynamicMemberLookup
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public struct AttributedString : Sendable {
     internal var _guts: Guts
 
@@ -28,7 +28,7 @@ public struct AttributedString : Sendable {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal static let currentIdentity = LockedState(initialState: 0)
     internal static var _nextModifyIdentity : Int {
@@ -40,7 +40,7 @@ extension AttributedString {
 }
 
 // MARK: Initialization
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public init() {
         self._guts = Guts()
@@ -126,7 +126,7 @@ extension AttributedString {
 #endif // FOUNDATION_FRAMEWORK
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal static func _bstring<S: Sequence<Character>>(from elements: S) -> BigString {
         if let elements = _specializingCast(elements, to: String.self) {
@@ -147,7 +147,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString { // Equatable
     public static func == (lhs: Self, rhs: Self) -> Bool {
         AttributedString.Guts.characterwiseIsEqual(lhs._guts, to: rhs._guts)
@@ -156,14 +156,14 @@ extension AttributedString { // Equatable
 
 // Note: The Hashable implementation is inherited from AttributedStringProtocol.
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString: ExpressibleByStringLiteral {
     public init(stringLiteral value: String) {
         self.init(value)
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString { // AttributedStringAttributeMutation
     public mutating func setAttributes(_ attributes: AttributeContainer) {
         ensureUniqueReference()
@@ -199,7 +199,7 @@ extension AttributedString { // AttributedStringAttributeMutation
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString: AttributedStringProtocol {
     public struct Index : Comparable, Sendable {
         internal var _value: BigString.Index
@@ -273,7 +273,7 @@ extension AttributedString: AttributedStringProtocol {
 }
 
 // MARK: Mutating operations
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal mutating func ensureUniqueReference() {
         if !isKnownUniquelyReferenced(&_guts) {
@@ -304,7 +304,7 @@ extension AttributedString {
 }
 
 // MARK: Concatenation operators
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public static func +(lhs: AttributedString, rhs: some AttributedStringProtocol) -> AttributedString {
         var result = lhs
@@ -328,7 +328,7 @@ extension AttributedString {
 }
 
 // MARK: Substring access
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public subscript(bounds: some RangeExpression<Index>) -> AttributedSubstring {
         get {
@@ -365,7 +365,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension Range where Bound == AttributedString.Index {
     internal var _bstringRange: Range<BigString.Index> {
         Range<BigString.Index>(uncheckedBounds: (lowerBound._value, upperBound._value))
@@ -376,12 +376,14 @@ extension Range where Bound == AttributedString.Index {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension RangeSet where Bound == AttributedString.Index {
     internal var _bstringIndices: RangeSet<BigString.Index> {
         RangeSet<BigString.Index>(self.ranges.map(\._bstringRange))
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension RangeSet where Bound == BigString.Index {
     internal func _attributedStringIndices(version: AttributedString.Guts.Version) -> RangeSet<AttributedString.Index> {
         RangeSet<AttributedString.Index>(self.ranges.lazy.map {
@@ -390,7 +392,7 @@ extension RangeSet where Bound == BigString.Index {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension Range where Bound == BigString.Index {
     internal var _utf8OffsetRange: Range<Int> {
         Range<Int>(uncheckedBounds: (lowerBound.utf8Offset, upperBound.utf8Offset))
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedStringAttribute.swift b/Sources/FoundationEssentials/AttributedString/AttributedStringAttribute.swift
index ec6cc8487..95b74ede0 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedStringAttribute.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedStringAttribute.swift
@@ -12,7 +12,7 @@
 
 // MARK: AttributedStringKey API
 
-@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+@available(FoundationAttributedString 5.7, *)
 extension AttributedString {
     public enum AttributeRunBoundaries : Hashable, Sendable {
         case paragraph
@@ -34,7 +34,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+@available(FoundationAttributedString 5.7, *)
 extension AttributedString.AttributeRunBoundaries {
     var _isScalarConstrained: Bool {
         if case .character = self { return true }
@@ -49,7 +49,7 @@ extension AttributedString.AttributeRunBoundaries {
     }
 }
 
-@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+@available(FoundationAttributedString 5.7, *)
 extension AttributedString {
     public struct AttributeInvalidationCondition : Hashable, Sendable {
         private enum _Storage : Hashable {
@@ -94,39 +94,39 @@ extension AttributedString {
 }
 
 // Developers define new attributes by implementing AttributeKey.
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol AttributedStringKey {
     associatedtype Value : Hashable
     static var name : String { get }
     
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     static var runBoundaries : AttributedString.AttributeRunBoundaries? { get }
     
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     static var inheritedByAddedText : Bool { get }
     
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     static var invalidationConditions : Set<AttributedString.AttributeInvalidationCondition>? { get }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringKey {
     public var description: String { Self.name }
 
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public static var runBoundaries : AttributedString.AttributeRunBoundaries? { nil }
 
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public static var inheritedByAddedText : Bool { true }
 
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public static var invalidationConditions : Set<AttributedString.AttributeInvalidationCondition>? { nil }
 }
 
 // MARK: Attribute Scopes
 
 @dynamicMemberLookup @frozen
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public enum AttributeDynamicLookup {
     public subscript<T: AttributedStringKey>(_: T.Type) -> T {
         get { fatalError("Called outside of a dynamicMemberLookup subscript overload") }
@@ -141,7 +141,7 @@ public enum AttributeDynamicLookup {
 extension AttributeDynamicLookup : Sendable {}
 
 @dynamicMemberLookup
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public struct ScopedAttributeContainer<S: AttributeScope> : Sendable {
     internal var storage : AttributedString._AttributeStorage
     
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeConstrainingBehavior.swift b/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeConstrainingBehavior.swift
index edf767c00..17ea247ef 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeConstrainingBehavior.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeConstrainingBehavior.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._AttributeStorage {
     var hasConstrainedAttributes: Bool {
         self.contents.values.contains { value in
@@ -56,7 +56,7 @@ extension AttributedString._AttributeStorage {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._AttributeValue {
     var hasConstrainedAttributes: Bool {
         runBoundaries != nil
@@ -68,7 +68,7 @@ extension AttributedString._AttributeValue {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringKey {
     static var constraintsInvolved: [AttributedString.AttributeRunBoundaries] {
         guard let constraint = Self.runBoundaries else { return [] }
@@ -76,21 +76,21 @@ extension AttributedStringKey {
     }
 }
 
-@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+@available(FoundationAttributedString 5.7, *)
 extension Collection where Element == AttributedString.AttributeRunBoundaries {
     var _containsScalarConstraint: Bool {
         self.contains { $0._isScalarConstrained }
     }
 }
 
-@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+@available(FoundationAttributedString 5.7, *)
 extension Collection where Element == AttributedString.AttributeRunBoundaries? {
     var _containsScalarConstraint: Bool {
         self.contains { $0?._isScalarConstrained ?? false }
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Guts {
     
     // MARK: Index/Range Utilities
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeStorage.swift b/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeStorage.swift
index d10c84ff8..34f6e2457 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeStorage.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedStringAttributeStorage.swift
@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal struct _AttributeValue : Hashable, CustomStringConvertible, Sendable {
         private typealias RawValue = any Sendable & Hashable
@@ -108,7 +108,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 internal extension Dictionary where Key == String, Value == AttributedString._AttributeValue {
     var _attrStrDescription : String {
         let keyvals = self.reduce(into: "") { (res, entry) in
@@ -118,7 +118,7 @@ internal extension Dictionary where Key == String, Value == AttributedString._At
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal struct _AttributeStorage: Hashable, Sendable {
         internal typealias AttributeMergePolicy = AttributedString.AttributeMergePolicy
@@ -139,14 +139,14 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._AttributeStorage: CustomStringConvertible {
     var description: String {
         contents._attrStrDescription
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._AttributeStorage {
     var isEmpty: Bool {
         contents.isEmpty
@@ -166,7 +166,7 @@ extension AttributedString._AttributeStorage {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._AttributeStorage {
     internal func isEqual(to other: Self, comparing attributes: [String]) -> Bool {
         assert(!attributes.isEmpty)
@@ -179,7 +179,7 @@ extension AttributedString._AttributeStorage {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString._AttributeStorage {
     func attributesForAddedText() -> Self {
         var storage = Self()
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedStringCodable.swift b/Sources/FoundationEssentials/AttributedString/AttributedStringCodable.swift
index 33f28ab47..f8655da7f 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedStringCodable.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedStringCodable.swift
@@ -30,27 +30,27 @@ extension Decoder {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol EncodableAttributedStringKey : AttributedStringKey {
     static func encode(_ value: Value, to encoder: Encoder) throws
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol DecodableAttributedStringKey : AttributedStringKey {
     static func decode(from decoder: Decoder) throws -> Value
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias CodableAttributedStringKey = EncodableAttributedStringKey & DecodableAttributedStringKey
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension EncodableAttributedStringKey where Value : Encodable {
     public static func encode(_ value: Value, to encoder: Encoder) throws {
         try value.encode(to: encoder)
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension DecodableAttributedStringKey where Value : Decodable {
     public static func decode(from decoder: Decoder) throws -> Value {
         return try Value.init(from: decoder)
@@ -58,18 +58,18 @@ extension DecodableAttributedStringKey where Value : Decodable {
 }
 
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol MarkdownDecodableAttributedStringKey : AttributedStringKey {
     static func decodeMarkdown(from decoder: Decoder) throws -> Value
     static var markdownName: String { get }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension MarkdownDecodableAttributedStringKey {
     public static var markdownName: String { name }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension MarkdownDecodableAttributedStringKey where Self : DecodableAttributedStringKey {
     public static func decodeMarkdown(from decoder: Decoder) throws -> Value {
         try Self.decode(from: decoder)
@@ -77,7 +77,7 @@ extension MarkdownDecodableAttributedStringKey where Self : DecodableAttributedS
 }
 
 #if FOUNDATION_FRAMEWORK
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension EncodableAttributedStringKey where Value : NSSecureCoding & NSObject {
     public static func encode(_ value: Value, to encoder: Encoder) throws {
         let data = try NSKeyedArchiver.archivedData(withRootObject: value, requiringSecureCoding: true)
@@ -86,7 +86,7 @@ extension EncodableAttributedStringKey where Value : NSSecureCoding & NSObject {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension DecodableAttributedStringKey where Value : NSSecureCoding & NSObject {
     public static func decode(from decoder: Decoder) throws -> Value {
         let container = try decoder.singleValueContainer()
@@ -103,7 +103,7 @@ extension DecodableAttributedStringKey where Value : NSSecureCoding & NSObject {
 
 // MARK: AttributedString CodableWithConfiguration Conformance
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public struct AttributeScopeCodableConfiguration : Sendable {
     internal let attributesTable : [String : any AttributedStringKey.Type]
     
@@ -124,7 +124,7 @@ public struct AttributeScopeCodableConfiguration : Sendable {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScope {
     public static var encodingConfiguration: AttributeScopeCodableConfiguration { AttributeScopeCodableConfiguration(Self.self) }
     public static var decodingConfiguration: AttributeScopeCodableConfiguration { AttributeScopeCodableConfiguration(Self.self) }
@@ -132,7 +132,7 @@ extension AttributeScope {
 
 #if FOUNDATION_FRAMEWORK
 // TODO: Support AttributedString codable conformance in FoundationPreview
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString : Codable {
     public func encode(to encoder: Encoder) throws {
         let conf = AttributeScopeCodableConfiguration(_loadDefaultAttributes())
@@ -145,7 +145,7 @@ extension AttributedString : Codable {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString : CodableWithConfiguration {
 
     private enum CodingKeys : String, CodingKey {
@@ -348,7 +348,7 @@ extension AttributedString : CodableWithConfiguration {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeContainer : CodableWithConfiguration {
     public func encode(to encoder: Encoder, configuration: AttributeScopeCodableConfiguration) throws {
         try AttributedString.encodeAttributeContainer(self.storage, to: encoder, configuration: configuration)
@@ -361,7 +361,7 @@ extension AttributeContainer : CodableWithConfiguration {
 
 #endif // FOUNDATION_FRAMEWORK
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension CodableConfiguration where ConfigurationProvider : AttributeScope {
     public init(wrappedValue: T, from keyPath: KeyPath<AttributeScopes, ConfigurationProvider.Type>) {
         self.wrappedValue = wrappedValue
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedStringProtocol.swift b/Sources/FoundationEssentials/AttributedString/AttributedStringProtocol.swift
index 86471b480..edb0468f9 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedStringProtocol.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedStringProtocol.swift
@@ -18,7 +18,7 @@ internal import _RopeModule
 internal import _FoundationCollections
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public enum AttributeMergePolicy : Sendable {
         case keepNew
@@ -33,7 +33,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol AttributedStringAttributeMutation {
     mutating func setAttributes(_ attributes: AttributeContainer)
     mutating func mergeAttributes(_ attributes: AttributeContainer, mergePolicy: AttributedString.AttributeMergePolicy)
@@ -41,7 +41,7 @@ public protocol AttributedStringAttributeMutation {
 }
 
 @dynamicMemberLookup
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol AttributedStringProtocol
     : AttributedStringAttributeMutation, Hashable, CustomStringConvertible, Sendable
 {
@@ -52,10 +52,10 @@ public protocol AttributedStringProtocol
     var characters : AttributedString.CharacterView { get }
     var unicodeScalars : AttributedString.UnicodeScalarView { get }
     
-    @available(FoundationPreview 6.2, *)
+    @available(FoundationAttributedString 6.2, *)
     var utf8 : AttributedString.UTF8View { get }
     
-    @available(FoundationPreview 6.2, *)
+    @available(FoundationAttributedString 6.2, *)
     var utf16 : AttributedString.UTF16View { get }
 
     @preconcurrency subscript<K: AttributedStringKey>(_: K.Type) -> K.Value? where K.Value : Sendable { get set }
@@ -66,7 +66,7 @@ public protocol AttributedStringProtocol
 }
 
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedStringProtocol {
     var utf8 : AttributedString.UTF8View {
         AttributedString.UTF8View(__guts, in: Range(uncheckedBounds: (startIndex._value, endIndex._value)))
@@ -77,7 +77,7 @@ extension AttributedStringProtocol {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringProtocol {
     public func settingAttributes(_ attributes: AttributeContainer) -> AttributedString {
         var new = AttributedString(self)
@@ -103,7 +103,7 @@ extension AttributedStringProtocol {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringProtocol {
     internal var __guts: AttributedString.Guts {
         if let s = _specializingCast(self, to: AttributedString.self) {
@@ -132,6 +132,7 @@ extension AttributedStringProtocol {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     internal var _baseString: BigString {
         _guts.string
@@ -146,6 +147,7 @@ extension AttributedString {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedSubstring {
     internal var _baseString: BigString {
         _guts.string
@@ -162,14 +164,14 @@ extension AttributedSubstring {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringProtocol { // CustomStringConvertible
     public var description: String {
         __guts.description(in: _stringBounds)
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringProtocol { // Equatable, Hashable
     @_specialize(where Self == AttributedString, RHS == AttributedString)
     @_specialize(where Self == AttributedString, RHS == AttributedSubstring)
@@ -186,7 +188,7 @@ extension AttributedStringProtocol { // Equatable, Hashable
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringProtocol {
     public func index(afterCharacter i: AttributedString.Index) -> AttributedString.Index {
         self.characters.index(after: i)
@@ -262,7 +264,7 @@ extension AttributedStringProtocol {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringProtocol {
     internal func _range<T: StringProtocol>(of stringToFind: T, options: String.CompareOptions = []) -> Range<AttributedString.Index>? {
 
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedSubstring.swift b/Sources/FoundationEssentials/AttributedString/AttributedSubstring.swift
index 4191d9d46..775b0a31d 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedSubstring.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedSubstring.swift
@@ -19,7 +19,7 @@ internal import _FoundationCollections
 #endif
 
 @dynamicMemberLookup
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public struct AttributedSubstring: Sendable {
     /// The guts of the base attributed string.
     internal var _guts: AttributedString.Guts
@@ -49,7 +49,7 @@ public struct AttributedSubstring: Sendable {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedSubstring {
     public var base: AttributedString {
         return AttributedString(_guts)
@@ -64,7 +64,7 @@ extension AttributedSubstring {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedSubstring { // CustomStringConvertible
     public var description: String {
         // FIXME: Why have a custom definition for this if AttributedString falls back
@@ -73,7 +73,7 @@ extension AttributedSubstring { // CustomStringConvertible
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedSubstring { // Equatable
     public static func == (lhs: Self, rhs: Self) -> Bool {
         if lhs._guts === rhs._guts && lhs._range == rhs._range {
@@ -85,7 +85,7 @@ extension AttributedSubstring { // Equatable
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedSubstring : AttributedStringProtocol {
     public var startIndex: AttributedString.Index {
         .init(_range.lowerBound, version: _guts.version)
@@ -159,7 +159,7 @@ extension AttributedSubstring : AttributedStringProtocol {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedSubstring {
     @preconcurrency
     public subscript<K: AttributedStringKey>(_: K.Type) -> K.Value? where K.Value : Sendable {
diff --git a/Sources/FoundationEssentials/AttributedString/Conversion.swift b/Sources/FoundationEssentials/AttributedString/Conversion.swift
index 7b06e0476..6511c7b80 100644
--- a/Sources/FoundationEssentials/AttributedString/Conversion.swift
+++ b/Sources/FoundationEssentials/AttributedString/Conversion.swift
@@ -22,13 +22,13 @@ internal import _FoundationCollections
 #endif
 
 extension String {
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public init(_ characters: Slice<AttributedString.CharacterView>) {
         self.init(characters._characters)
     }
 
     #if false // FIXME: Make this public.
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     @backDeployed(before: macOS 14, iOS 17, tvOS 17, watchOS 10)
     public init(_ characters: AttributedString.CharacterView) {
         if #available(macOS 14, iOS 17, tvOS 17, watchOS 10, *) {
@@ -41,7 +41,7 @@ extension String {
     }
     #endif
 
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     @usableFromInline
     internal init(_characters: AttributedString.CharacterView) {
         self.init(_characters._characters)
@@ -50,7 +50,7 @@ extension String {
 
 #if FOUNDATION_FRAMEWORK
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 public protocol ObjectiveCConvertibleAttributedStringKey : AttributedStringKey {
     associatedtype ObjectiveCValue : NSObject
 
@@ -58,7 +58,7 @@ public protocol ObjectiveCConvertibleAttributedStringKey : AttributedStringKey {
     static func value(for object: ObjectiveCValue) throws -> Value
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension ObjectiveCConvertibleAttributedStringKey where Value : RawRepresentable, Value.RawValue == Int, ObjectiveCValue == NSNumber {
     public static func objectiveCValue(for value: Value) throws -> ObjectiveCValue {
         return NSNumber(value: value.rawValue)
@@ -71,7 +71,7 @@ extension ObjectiveCConvertibleAttributedStringKey where Value : RawRepresentabl
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension ObjectiveCConvertibleAttributedStringKey where Value : RawRepresentable, Value.RawValue == String, ObjectiveCValue == NSString {
     public static func objectiveCValue(for value: Value) throws -> ObjectiveCValue {
         return value.rawValue as NSString
@@ -91,7 +91,7 @@ internal struct _AttributeConversionOptions : OptionSet {
     static let dropThrowingAttributes = Self(rawValue: 1 << 0)
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeContainer {
     public init(_ dictionary: [NSAttributedString.Key : Any]) {
         // Passing .dropThrowingAttributes causes attributes that throw during conversion to be dropped, so it is safe to do try! here
@@ -126,7 +126,7 @@ extension AttributeContainer {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension Dictionary where Key == NSAttributedString.Key, Value == Any {
     public init(_ container: AttributeContainer) {
         // Passing .dropThrowingAttributes causes attributes that throw during conversion to be dropped, so it is safe to do try! here
@@ -161,7 +161,7 @@ extension Dictionary where Key == NSAttributedString.Key, Value == Any {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension NSAttributedString {
     public convenience init(_ attrStr: AttributedString) {
         // Passing .dropThrowingAttributes causes attributes that throw during conversion to be dropped, so it is safe to do try! here
@@ -203,7 +203,7 @@ extension NSAttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     public init(_ nsStr: NSAttributedString) {
         // Passing .dropThrowingAttributes causes attributes that throw during conversion to be dropped, so it is safe to do try! here
@@ -286,7 +286,7 @@ extension AttributedString {
 
 #endif // FOUNDATION_FRAMEWORK
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension String.Index {
     // FIXME: Converting indices between different collection types does not make sense.
     // FIXME: (Indices are meaningless without the collection value to which they belong,
@@ -300,7 +300,7 @@ extension String.Index {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString.Index {
     // FIXME: Converting indices between different collection types does not make sense.
     // FIXME: (Indices are meaningless without the collection value to which they belong,
@@ -322,7 +322,7 @@ extension AttributedString.Index {
 
 #if FOUNDATION_FRAMEWORK
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension NSRange {
     public init<R: RangeExpression, S: AttributedStringProtocol>(
         _ region: R,
@@ -341,7 +341,7 @@ extension NSRange {
         self.init(location: utf16Start - utf16Base, length: utf16Length)
     }
     
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public init?<S: StringProtocol>(_ markdownSourcePosition: AttributedString.MarkdownSourcePosition, in target: S) {
         let startOffsets: AttributedString.MarkdownSourcePosition.Offsets
         let endOffsets: AttributedString.MarkdownSourcePosition.Offsets
@@ -369,6 +369,7 @@ extension NSRange {
 
 #endif // FOUNDATION_FRAMEWORK
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     /// A dummy collection type whose only purpose is to facilitate a `RangeExpression.relative(to:)`
     /// call that takes a range expression with string indices but needs to work on an
@@ -430,7 +431,7 @@ extension BigString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension Range where Bound == AttributedString.Index {
 #if FOUNDATION_FRAMEWORK
     public init?<S: AttributedStringProtocol>(_ range: NSRange, in string: S) {
@@ -491,6 +492,7 @@ extension Range where Bound == AttributedString.Index {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     /// A dummy collection type whose only purpose is to facilitate a `RangeExpression.relative(to:)`
     /// call that takes a range expression with attributed string indices but needs to work on a
@@ -518,7 +520,7 @@ extension AttributedString {
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension Range where Bound == String.Index {
     // FIXME: Converting indices between different collection types does not make sense.
     // FIXME: (Indices are meaningless without the collection value to which they belong,
@@ -560,7 +562,7 @@ extension Range where Bound == String.Index {
 
 #if FOUNDATION_FRAMEWORK
     // TODO: Support AttributedString markdown in FoundationPreview
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public init?<S: StringProtocol>(_ markdownSourcePosition: AttributedString.MarkdownSourcePosition, in target: S) {
         if let start = markdownSourcePosition.startOffsets, let end = markdownSourcePosition.endOffsets {
             self = target.utf8.index(target.startIndex, offsetBy: start.utf8) ..< target.utf8.index(target.startIndex, offsetBy: end.utf8 + 1)
diff --git a/Sources/FoundationEssentials/AttributedString/DiscontiguousAttributedSubstring.swift b/Sources/FoundationEssentials/AttributedString/DiscontiguousAttributedSubstring.swift
index 649ceb730..ae286b8fe 100644
--- a/Sources/FoundationEssentials/AttributedString/DiscontiguousAttributedSubstring.swift
+++ b/Sources/FoundationEssentials/AttributedString/DiscontiguousAttributedSubstring.swift
@@ -20,7 +20,7 @@ internal import _FoundationCollections
 
 /// A discontiguous portion of an attributed string.
 @dynamicMemberLookup
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 public struct DiscontiguousAttributedSubstring: Sendable {
     /// The guts of the base attributed string.
     internal var _guts: AttributedString.Guts
@@ -41,7 +41,7 @@ public struct DiscontiguousAttributedSubstring: Sendable {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension DiscontiguousAttributedSubstring {
     /// The underlying attributed string that the discontiguous attributed substring derives from.
     public var base: AttributedString {
@@ -49,21 +49,21 @@ extension DiscontiguousAttributedSubstring {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension DiscontiguousAttributedSubstring : CustomStringConvertible {
     public var description: String {
         _guts.description(in: _indices)
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension DiscontiguousAttributedSubstring : Equatable {
     public static func == (lhs: Self, rhs: Self) -> Bool {
         AttributedString.Guts._characterwiseIsEqual(lhs.runs, to: rhs.runs)
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension DiscontiguousAttributedSubstring : Hashable {
     public func hash(into hasher: inout Hasher) {
         for range in _indices.ranges {
@@ -72,7 +72,7 @@ extension DiscontiguousAttributedSubstring : Hashable {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension DiscontiguousAttributedSubstring : AttributedStringAttributeMutation {
     internal mutating func ensureUniqueReference() {
         // Note: slices should never discard the data outside their bounds, so we must make a
@@ -155,7 +155,7 @@ extension DiscontiguousAttributedSubstring : AttributedStringAttributeMutation {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension DiscontiguousAttributedSubstring {
     /// The characters of the discontiguous attributed string, as a view into the underlying string.
     public var characters: DiscontiguousSlice<AttributedString.CharacterView> {
@@ -173,7 +173,7 @@ extension DiscontiguousAttributedSubstring {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension DiscontiguousAttributedSubstring {
     /// Returns an attribute value that corresponds to an attributed string key.
     ///
@@ -263,7 +263,7 @@ extension DiscontiguousAttributedSubstring {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString {
     /// Creates an attributed string from a discontiguous attributed substring.
     /// - Parameter substring: A discontiguous attributed substring to create the new attributed string from.
@@ -279,7 +279,7 @@ extension AttributedString {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedStringProtocol {
     /// Returns a discontiguous substring of this attributed string using a set of ranges to indicate the discontiguous substring bounds.
     /// - Parameter indices:  A set of ranges that indicate the bounds of the discontiguous substring to return.
@@ -290,7 +290,7 @@ extension AttributedStringProtocol {
     }
 }
 
-@available(FoundationPreview 6.2, *)
+@available(FoundationAttributedString 6.2, *)
 extension AttributedString {
     /// Returns a discontiguous substring of this discontiguous attributed string using a set of ranges to indicate the discontiguous substring bounds.
     /// - Parameter indices: A set of ranges that indicate the bounds of the discontiguous substring to return.
diff --git a/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift b/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift
index c114f2158..aad294345 100644
--- a/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift
+++ b/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift
@@ -16,7 +16,7 @@
 internal import Foundation_Private.NSAttributedString
 #endif
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes {
     public var foundation: FoundationAttributes.Type { FoundationAttributes.self }
     
@@ -32,39 +32,39 @@ extension AttributeScopes {
         public let measurement: MeasurementAttribute
         public let byteCount: ByteCountAttribute
         
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public let durationField: DurationFieldAttribute
 
         /// The base writing direction of a paragraph.
-        @available(FoundationPreview 6.2, *)
+        @available(FoundationAttributedString 6.2, *)
         public let writingDirection: WritingDirectionAttribute
 
 #if FOUNDATION_FRAMEWORK
-        @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+        @available(FoundationAttributedString 5.9, *)
         public let agreementConcept: AgreementConceptAttribute
-        @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+        @available(FoundationAttributedString 5.9, *)
         public let agreementArgument: AgreementArgumentAttribute
-        @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+        @available(FoundationAttributedString 5.9, *)
         public let referentConcept: ReferentConceptAttribute
-        @available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+        @available(FoundationAttributedString 6.0, *)
         public let localizedNumberFormat: LocalizedNumberFormatAttribute
         
         // TODO: Support AttributedString markdown in FoundationPreview: https://github.com/apple/swift-foundation/issues/44
         public let inlinePresentationIntent: InlinePresentationIntentAttribute
         public let presentationIntent: PresentationIntentAttribute
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public let markdownSourcePosition: MarkdownSourcePositionAttribute
-        @available(FoundationPreview 6.2, *)
+        @available(FoundationAttributedString 6.2, *)
         public let listItemDelimiter: ListItemDelimiterAttribute
         
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public let localizedStringArgumentAttributes: LocalizedStringArgumentAttributes
         
         public let inflectionAlternative: InflectionAlternativeAttribute
         public let morphology: MorphologyAttribute
         public let inflect: InflectionRuleAttribute
         @_spi(AutomaticGrammaticalAgreement)
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public let assumedFallbackInflection: AssumedFallbackInflectionAttribute
 #endif // FOUNDATION_FRAMEWORK
     }
@@ -78,7 +78,7 @@ extension AttributeScopes {
 @available(*, unavailable)
 extension AttributeScopes.FoundationAttributes : Sendable {}
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeDynamicLookup {
     public subscript<T: AttributedStringKey>(dynamicMember keyPath: KeyPath<AttributeScopes.FoundationAttributes, T>) -> T {
         return self[T.self]
@@ -91,7 +91,7 @@ extension AttributeDynamicLookup {
     }
 
 #if FOUNDATION_FRAMEWORK
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public subscript<T: AttributedStringKey>(
         dynamicMember keyPath: KeyPath<AttributeScopes.FoundationAttributes.LocalizedStringArgumentAttributes, T>
     ) -> T {
@@ -102,10 +102,10 @@ extension AttributeDynamicLookup {
 
 // MARK: Attribute Definitions
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes {
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum LinkAttribute : CodableAttributedStringKey {
         public typealias Value = URL
         
@@ -119,7 +119,7 @@ extension AttributeScopes.FoundationAttributes {
 #if FOUNDATION_FRAMEWORK
     
     @frozen
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     public enum ReferentConceptAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
         public typealias Value = Int
         public static let name = NSAttributedString.Key.referentConcept.rawValue
@@ -127,7 +127,7 @@ extension AttributeScopes.FoundationAttributes {
     }
 
     @frozen
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     public enum AgreementConceptAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
         public typealias Value = Int
         public static let name = NSAttributedString.Key.agreeWithConcept.rawValue
@@ -135,7 +135,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 14, iOS 17, tvOS 17, watchOS 10, *)
+    @available(FoundationAttributedString 5.9, *)
     public enum AgreementArgumentAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
         public typealias Value = Int
         public static let name = NSAttributedString.Key.agreeWithArgument.rawValue
@@ -143,7 +143,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum MorphologyAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
         public typealias Value = Morphology
         public static let name = NSAttributedString.Key.morphology.rawValue
@@ -151,7 +151,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum InflectionRuleAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
         public typealias Value = InflectionRule
         public static let name = NSAttributedString.Key.inflectionRule.rawValue
@@ -159,7 +159,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     @_spi(AutomaticGrammaticalAgreement)
     public enum AssumedFallbackInflectionAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
         public typealias Value = Morphology
@@ -168,7 +168,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+    @available(FoundationAttributedString 6.0, *)
     public enum LocalizedNumberFormatAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
         public struct Value: Equatable, Hashable, Codable, Sendable {
             enum Format {
@@ -202,14 +202,14 @@ extension AttributeScopes.FoundationAttributes {
 #endif // FOUNDATION_FRAMEWORK
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum LanguageIdentifierAttribute : CodableAttributedStringKey {
         public typealias Value = String
         public static let name = "NSLanguage"
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum PersonNameComponentAttribute : CodableAttributedStringKey {
         public typealias Value = Component
         public static let name = "NSPersonNameComponentKey"
@@ -219,7 +219,7 @@ extension AttributeScopes.FoundationAttributes {
         }
     }
     
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public struct NumberFormatAttributes: AttributeScope {
         public let numberSymbol: SymbolAttribute
         public let numberPart: NumberPartAttribute
@@ -251,7 +251,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum DateFieldAttribute : CodableAttributedStringKey {
         public enum Field : Hashable, Codable, Sendable {
             case era
@@ -383,7 +383,7 @@ extension AttributeScopes.FoundationAttributes {
 #if FOUNDATION_FRAMEWORK
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum InflectionAlternativeAttribute : CodableAttributedStringKey, MarkdownDecodableAttributedStringKey, ObjectiveCConvertibleAttributedStringKey {
         public typealias Value = AttributedString
         public typealias ObjectiveCValue = NSObject
@@ -410,7 +410,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
 	@frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum InlinePresentationIntentAttribute : CodableAttributedStringKey, ObjectiveCConvertibleAttributedStringKey {
         public typealias Value = InlinePresentationIntent
         public typealias ObjectiveCValue = NSNumber
@@ -426,21 +426,21 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum PresentationIntentAttribute : CodableAttributedStringKey {
         public typealias Value = PresentationIntent
         public static let name = NSAttributedString.Key.presentationIntentAttributeName.rawValue
     }
     
     @frozen
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public enum MarkdownSourcePositionAttribute: CodableAttributedStringKey {
         public static let name = NSAttributedString.Key.markdownSourcePosition.rawValue
         public typealias Value = AttributedString.MarkdownSourcePosition
     }
     
     @frozen
-    @available(FoundationPreview 6.2, *)
+    @available(FoundationAttributedString 6.2, *)
     public enum ListItemDelimiterAttribute : CodableAttributedStringKey, ObjectiveCConvertibleAttributedStringKey {
         public typealias Value = Character
         public typealias ObjectiveCValue = NSString
@@ -480,27 +480,27 @@ extension AttributeScopes.FoundationAttributes {
 #endif // FOUNDATION_FRAMEWORK
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum AlternateDescriptionAttribute : CodableAttributedStringKey {
         public typealias Value = String
         public static let name = "NSAlternateDescription"
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum ImageURLAttribute : CodableAttributedStringKey {
         public typealias Value = URL
         public static let name = "NSImageURL"
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum ReplacementIndexAttribute : CodableAttributedStringKey {
         public typealias Value = Int
         public static let name = "NSReplacementIndex"
     }
     
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public struct MeasurementAttribute : CodableAttributedStringKey {
         public typealias Value = Component
         public static let name = "Foundation.MeasurementAttribute"
@@ -511,7 +511,7 @@ extension AttributeScopes.FoundationAttributes {
     }
     
     @frozen
-    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    @available(FoundationAttributedString 5.5, *)
     public enum ByteCountAttribute : CodableAttributedStringKey {
         public typealias Value = Component
         public static let name = "Foundation.ByteCountAttribute"
@@ -536,7 +536,7 @@ extension AttributeScopes.FoundationAttributes {
     }
 
     @frozen
-    @available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+    @available(FoundationAttributedString 5.7, *)
     public enum DurationFieldAttribute : CodableAttributedStringKey {
         public typealias Value = Field
         public static let name = "Foundation.DurationFieldAttribute"
@@ -553,7 +553,7 @@ extension AttributeScopes.FoundationAttributes {
     }
 
     /// The attribute key for the base writing direction of a paragraph.
-    @available(FoundationPreview 6.2, *)
+    @available(FoundationAttributedString 6.2, *)
     @frozen
     public enum WritingDirectionAttribute: CodableAttributedStringKey {
         public typealias Value = AttributedString.WritingDirection
@@ -564,7 +564,7 @@ extension AttributeScopes.FoundationAttributes {
     }
 
 #if FOUNDATION_FRAMEWORK
-    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    @available(FoundationAttributedString 5.7, *)
     public struct LocalizedStringArgumentAttributes {
     
         // Represents all numeric arguments (i.e. those that use format specifiers such as %d, %f, etc.)
@@ -575,7 +575,7 @@ extension AttributeScopes.FoundationAttributes {
         public let localizedURLArgument: LocalizedURLArgumentAttribute
         
         @frozen
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public enum LocalizedNumericArgumentAttribute : CodableAttributedStringKey {
             public static let name = "Foundation.LocalizedNumericArgumentAttribute"
             public enum Value : Hashable, Codable, Sendable {
@@ -587,21 +587,21 @@ extension AttributeScopes.FoundationAttributes {
         }
         
         @frozen
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public enum LocalizedDateArgumentAttribute : CodableAttributedStringKey {
             public typealias Value = Date
             public static let name = "Foundation.LocalizedDateArgumentAttribute"
         }
         
         @frozen
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public enum LocalizedDateIntervalArgumentAttribute : CodableAttributedStringKey {
             public typealias Value = Range<Date>
             public static let name = "Foundation.LocalizedDateIntervalArgumentAttribute"
         }
         
         @frozen
-        @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+        @available(FoundationAttributedString 5.7, *)
         public enum LocalizedURLArgumentAttribute : CodableAttributedStringKey {
             public typealias Value = URL
             public static let name = "Foundation.LocalizedURLArgumentAttribute"
@@ -820,7 +820,7 @@ extension AttributeScopes.FoundationAttributes.LocalizedStringArgumentAttributes
 @available(*, unavailable)
 extension AttributeScopes.FoundationAttributes.LocalizedStringArgumentAttributes.LocalizedDateIntervalArgumentAttribute : Sendable {}
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.LinkAttribute : ObjectiveCConvertibleAttributedStringKey {
     public typealias ObjectiveCValue = NSObject // NSURL or NSString
     
@@ -841,17 +841,17 @@ extension AttributeScopes.FoundationAttributes.LinkAttribute : ObjectiveCConvert
     }
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.LanguageIdentifierAttribute : MarkdownDecodableAttributedStringKey {
     public static let markdownName = "languageIdentifier"
 }
 
-@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.PersonNameComponentAttribute : ObjectiveCConvertibleAttributedStringKey {
     public typealias ObjectiveCValue = NSString
 }
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension AttributeScopes.FoundationAttributes.LocalizedNumberFormatAttribute.Value: _ObjectiveCBridgeable {
     public func _bridgeToObjectiveC() -> __NSLocalizedNumberFormatRule {
         switch self.format {
@@ -878,6 +878,7 @@ extension AttributeScopes.FoundationAttributes.LocalizedNumberFormatAttribute.Va
 
 #endif // FOUNDATION_FRAMEWORK
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     /// The writing direction of a piece of text.
     ///
@@ -927,7 +928,7 @@ extension AttributedString {
     /// direction. In vertical scripts, a writing direction of ``leftToRight``
     /// is interpreted as top-to-bottom and a writing direction of
     /// ``rightToLeft`` is interpreted as bottom-to-top.
-    @available(FoundationPreview 6.2, *)
+    @available(FoundationAttributedString 6.2, *)
     @frozen
     public enum WritingDirection: Codable, Hashable, CaseIterable, Sendable {
         /// A left-to-right writing direction in horizontal script.
diff --git a/Sources/FoundationInternationalization/Formatting/ByteCountFormatStyle.swift b/Sources/FoundationInternationalization/Formatting/ByteCountFormatStyle.swift
index 0ab8cf921..ad31dcfee 100644
--- a/Sources/FoundationInternationalization/Formatting/ByteCountFormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/ByteCountFormatStyle.swift
@@ -16,7 +16,7 @@ import FoundationEssentials
 
 internal import _FoundationICU
 
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public struct ByteCountFormatStyle: FormatStyle, Sendable {
     public var style: Style { get { attributed.style } set { attributed.style = newValue} }
     public var allowedUnits: Units { get { attributed.allowedUnits } set { attributed.allowedUnits = newValue} }
@@ -247,7 +247,7 @@ public struct ByteCountFormatStyle: FormatStyle, Sendable {
     }
 }
 
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public extension FormatStyle where Self == ByteCountFormatStyle {
     static func byteCount(style: ByteCountFormatStyle.Style, allowedUnits: ByteCountFormatStyle.Units = .all, spellsOutZero: Bool = true, includesActualByteCount: Bool = false) -> Self {
         return ByteCountFormatStyle(style: style, allowedUnits: allowedUnits, spellsOutZero: spellsOutZero, includesActualByteCount: includesActualByteCount)
@@ -284,6 +284,7 @@ private func localizedParens(locale: Locale) -> (String, String) {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.ByteCountAttribute.Component {
     internal init?(unumberFormatField: UNumberFormatFields, unit: ByteCountFormatStyle.Unit) {
         switch unumberFormatField {
@@ -309,6 +310,7 @@ extension AttributeScopes.FoundationAttributes.ByteCountAttribute.Component {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.ByteCountAttribute.Unit {
     internal init(_ unit: ByteCountFormatStyle.Unit) {
         switch unit {
diff --git a/Sources/FoundationInternationalization/Formatting/Date/Date+VerbatimFormatStyle.swift b/Sources/FoundationInternationalization/Formatting/Date/Date+VerbatimFormatStyle.swift
index 26698f3b8..5fe39d255 100644
--- a/Sources/FoundationInternationalization/Formatting/Date/Date+VerbatimFormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/Date/Date+VerbatimFormatStyle.swift
@@ -35,10 +35,17 @@ extension Date {
         }
 
         /// Returns a type erased attributed variant of this style.
+        #if FOUNDATION_FRAMEWORK
         @available(macOS, deprecated: 15, introduced: 12, message: "Use attributedStyle instead")
         @available(iOS, deprecated: 18, introduced: 15, message: "Use attributedStyle instead")
         @available(tvOS, deprecated: 18, introduced: 15, message: "Use attributedStyle instead")
         @available(watchOS, deprecated: 11, introduced: 8, message: "Use attributedStyle instead")
+        #else
+        @available(macOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        @available(iOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        @available(tvOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        @available(watchOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        #endif
         public var attributed: AttributedStyle {
             .init(style: .verbatimFormatStyle(self))
         }
@@ -78,7 +85,7 @@ extension Date.VerbatimFormatStyle: ParseableFormatStyle {
 
 // MARK: Typed Attributed Style
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Date.VerbatimFormatStyle {
     /// The type preserving attributed variant of this style.
     ///
@@ -176,7 +183,7 @@ extension Date.VerbatimFormatStyle : DiscreteFormatStyle {
     }
 }
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Date.VerbatimFormatStyle.Attributed : DiscreteFormatStyle {
     public func discreteInput(before input: Date) -> Date? {
         base.discreteInput(before: input)
diff --git a/Sources/FoundationInternationalization/Formatting/Date/DateFormatStyle.swift b/Sources/FoundationInternationalization/Formatting/Date/DateFormatStyle.swift
index 5a25b2548..057cf5021 100644
--- a/Sources/FoundationInternationalization/Formatting/Date/DateFormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/Date/DateFormatStyle.swift
@@ -241,10 +241,17 @@ extension Date {
         public var capitalizationContext: FormatStyleCapitalizationContext
 
         /// Returns a type erased attributed variant of this style.
+        #if FOUNDATION_FRAMEWORK
         @available(macOS, deprecated: 15, introduced: 12, message: "Use attributedStyle instead")
         @available(iOS, deprecated: 18, introduced: 15, message: "Use attributedStyle instead")
         @available(tvOS, deprecated: 18, introduced: 15, message: "Use attributedStyle instead")
         @available(watchOS, deprecated: 11, introduced: 8, message: "Use attributedStyle instead")
+        #else
+        @available(macOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        @available(iOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        @available(tvOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        @available(watchOS, deprecated: 26, introduced: 26, message: "Use attributedStyle instead")
+        #endif
         public var attributed: AttributedStyle {
             .init(style: .formatStyle(self))
         }
@@ -288,10 +295,17 @@ extension Date {
 
     // MARK: Type-Erased AttributedStyle
 
+    #if FOUNDATION_FRAMEWORK
     @available(macOS, deprecated: 15, introduced: 12, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
     @available(iOS, deprecated: 18, introduced: 15, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
     @available(tvOS, deprecated: 18, introduced: 15, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
     @available(watchOS, deprecated: 11, introduced: 8, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+    #else
+    @available(macOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+    @available(iOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+    @available(tvOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+    @available(watchOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+    #endif
     public struct AttributedStyle : Sendable {
 
         enum InnerStyle: Codable, Hashable {
@@ -338,15 +352,22 @@ extension Date {
     }
 }
 
+#if FOUNDATION_FRAMEWORK
 @available(macOS, deprecated: 15, introduced: 12, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
 @available(iOS, deprecated: 18, introduced: 15, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
 @available(tvOS, deprecated: 18, introduced: 15, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
 @available(watchOS, deprecated: 11, introduced: 8, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+#else
+@available(macOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+@available(iOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+@available(tvOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+@available(watchOS, deprecated: 26, introduced: 26, message: "Use Date.FormatStyle.Attributed or Date.VerbatimFormatStyle.Attributed instead")
+#endif
 extension Date.AttributedStyle : FormatStyle {}
 
 // MARK: Typed Attributed Style
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Date.FormatStyle {
     /// The type preserving attributed variant of this style.
     ///
@@ -557,7 +578,7 @@ extension Date.FormatStyle {
 
 // MARK: Symbol Modifiers Attributed Style
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Date.FormatStyle.Attributed {
     /// Change the representation of the era in the format.
     ///
@@ -854,7 +875,7 @@ extension Date.FormatStyle : DiscreteFormatStyle {
     }
 }
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Date.FormatStyle.Attributed : DiscreteFormatStyle {
     public func discreteInput(before input: Date) -> Date? {
         base.discreteInput(before: input)
@@ -1000,6 +1021,7 @@ extension Calendar {
 
 // MARK: Utils
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.DateFieldAttribute.Field {
     init?(udateFormatField: UDateFormatField) {
         switch udateFormatField {
@@ -1096,6 +1118,7 @@ extension Date.FormatStyle : CustomConsumingRegexComponent {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension String {
     func _attributedStringFromPositions(_ positions: [ICUDateFormatter.AttributePosition]) -> AttributedString {
         typealias DateFieldAttribute = AttributeScopes.FoundationAttributes.DateFieldAttribute.Field
diff --git a/Sources/FoundationInternationalization/Formatting/Duration+Formatting.swift b/Sources/FoundationInternationalization/Formatting/Duration+Formatting.swift
index cf2ad5b6c..a228ecf4d 100644
--- a/Sources/FoundationInternationalization/Formatting/Duration+Formatting.swift
+++ b/Sources/FoundationInternationalization/Formatting/Duration+Formatting.swift
@@ -37,7 +37,7 @@ extension Duration {
 
     /// Formats `self` using the hour-minute-second time pattern
     /// - Returns: A formatted string to describe the duration, such as "1:30:56" for a duration of 1 hour, 30 minutes, and 56 seconds
-    @available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
+    @available(FoundationAttributedString 5.7, *)
     public func formatted() -> String {
         return Self.TimeFormatStyle(pattern: .hourMinuteSecond).format(self)
     }
diff --git a/Sources/FoundationInternationalization/Formatting/Duration+TimeFormatStyle.swift b/Sources/FoundationInternationalization/Formatting/Duration+TimeFormatStyle.swift
index 2578c39d1..846273f04 100644
--- a/Sources/FoundationInternationalization/Formatting/Duration+TimeFormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/Duration+TimeFormatStyle.swift
@@ -21,13 +21,13 @@ let minuteSymbol: Character = "m"
 let secondSymbol: Character = "s"
 let quoteSymbol: Character = "'"
 
-@available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+@available(FoundationAttributedString 5.7, *)
 extension Duration {
 
     /// Format style to format a `Duration` in a localized positional format.
     /// For example, one hour and ten minutes is displayed as “1:10:00” in
     /// the U.S. English locale, or “1.10.00” in the Finnish locale.
-    @available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
+    @available(FoundationAttributedString 5.7, *)
     public struct TimeFormatStyle : FormatStyle, Sendable {
 
         /// The units to display a Duration with and configurations for the units.
@@ -152,7 +152,7 @@ extension Duration {
     internal typealias _TimeFormatStyle = TimeFormatStyle
 }
 
-@available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+@available(FoundationAttributedString 5.7, *)
 extension FormatStyle where Self == Duration.TimeFormatStyle {
     /// A factory variable to create a time format style to format a duration.
     /// - Parameter pattern: A `Pattern` to specify the units to include in the displayed string and the behavior of the units.
@@ -165,7 +165,7 @@ extension FormatStyle where Self == Duration.TimeFormatStyle {
 // MARK: - Attributed style
 
 
-@available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+@available(FoundationAttributedString 5.7, *)
 extension Duration.TimeFormatStyle {
 
     /// Formats a duration as an attributed string with the `durationField` attribute key and `FoundationAttributes.DurationFieldAttribute` attribute.
@@ -178,7 +178,7 @@ extension Duration.TimeFormatStyle {
     /// : { nil }
     /// 26.25 { durationField: .seconds }
     /// ```
-    @available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+    @available(FoundationAttributedString 5.7, *)
     @dynamicMemberLookup
     public struct Attributed : FormatStyle, Sendable {
 
@@ -423,7 +423,7 @@ extension Duration.TimeFormatStyle {
     }
 }
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Duration.TimeFormatStyle {
     /// Returns a modified style that applies the given `grouping` rule to the highest field in the
     /// pattern.
@@ -440,7 +440,7 @@ extension Duration.TimeFormatStyle {
     }
 }
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Duration.TimeFormatStyle.Attributed {
     /// Returns a modified style that applies the given `grouping` rule to the highest field in the
     /// pattern.
@@ -453,7 +453,7 @@ extension Duration.TimeFormatStyle.Attributed {
 
 // MARK: Dynamic Member Lookup
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Duration.TimeFormatStyle.Attributed {
     private var innerStyle: Duration.TimeFormatStyle {
         get {
@@ -480,7 +480,7 @@ extension Duration.TimeFormatStyle.Attributed {
 
 // MARK: DiscreteFormatStyle Conformance
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Duration.TimeFormatStyle.Attributed : DiscreteFormatStyle {
     public func discreteInput(before input: Duration) -> Duration? {
         Duration.TimeFormatStyle(pattern: pattern, locale: locale).discreteInput(before: input)
@@ -491,7 +491,7 @@ extension Duration.TimeFormatStyle.Attributed : DiscreteFormatStyle {
     }
 }
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Duration.TimeFormatStyle : DiscreteFormatStyle {
     public func discreteInput(before input: Duration) -> Duration? {
         let (bound, isIncluded) = Duration.bound(for: input, in: interval(for: input), countingDown: true, roundingRule: self.pattern.roundingRule)
diff --git a/Sources/FoundationInternationalization/Formatting/Duration+UnitsFormatStyle.swift b/Sources/FoundationInternationalization/Formatting/Duration+UnitsFormatStyle.swift
index f17ce7548..e64543567 100644
--- a/Sources/FoundationInternationalization/Formatting/Duration+UnitsFormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/Duration+UnitsFormatStyle.swift
@@ -491,6 +491,7 @@ extension Duration {
         /// 26.25 { durationField: .seconds, component: .value }
         /// seconds { durationField: .seconds, component: .unit }
         /// ```
+        @available(FoundationAttributedString 5.7, *)
         public var attributed: Attributed {
             Attributed(innerStyle: self)
         }
@@ -532,7 +533,7 @@ extension FormatStyle where Self == Duration.UnitsFormatStyle {
 
 // MARK: - Attributed style
 
-@available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+@available(FoundationAttributedString 5.7, *)
 extension Duration.UnitsFormatStyle {
 
     /// A format style to format a duration as an attributed string. Units in the string are annotated with the `durationField` and `measurement` attribute keys and the `DurationFieldAttribute` and `MeasurementAttribute` attribute values.
@@ -550,7 +551,7 @@ extension Duration.UnitsFormatStyle {
     /// 26.25 { durationField: .seconds, component: .value }
     /// seconds { durationField: .seconds, component: .unit }
     /// ```
-    @available(macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0, *)
+    @available(FoundationAttributedString 5.7, *)
     @dynamicMemberLookup
     public struct Attributed : FormatStyle, Sendable {
 
@@ -632,7 +633,7 @@ extension Duration.UnitsFormatStyle {
 
 // MARK: Dynamic Member Lookup
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Duration.UnitsFormatStyle.Attributed {
     public subscript<T>(dynamicMember key: KeyPath<Duration.UnitsFormatStyle, T>) -> T {
         innerStyle[keyPath: key]
@@ -650,7 +651,7 @@ extension Duration.UnitsFormatStyle.Attributed {
 
 // MARK: DiscreteFormatStyle Conformance
 
-@available(macOS 15, iOS 18, tvOS 18, watchOS 11, *)
+@available(FoundationAttributedString 6.0, *)
 extension Duration.UnitsFormatStyle.Attributed : DiscreteFormatStyle {
     public func discreteInput(before input: Duration) -> Duration? {
         self.innerStyle.discreteInput(before: input)
diff --git a/Sources/FoundationInternationalization/Formatting/Number/Decimal+FormatStyle.swift b/Sources/FoundationInternationalization/Formatting/Number/Decimal+FormatStyle.swift
index cb2be15f3..4b1a2c869 100644
--- a/Sources/FoundationInternationalization/Formatting/Number/Decimal+FormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/Number/Decimal+FormatStyle.swift
@@ -22,6 +22,7 @@ extension Decimal {
             self.locale = locale
         }
 
+        @available(FoundationAttributedString 5.5, *)
         public var attributed: Attributed {
             return Attributed(style: self)
         }
@@ -107,6 +108,7 @@ extension Decimal.FormatStyle {
             self.locale = locale
         }
 
+        @available(FoundationAttributedString 5.5, *)
         public var attributed: Attributed {
             return Attributed(style: self)
         }
@@ -186,6 +188,7 @@ extension Decimal.FormatStyle {
             self.collection = Configuration.Collection(presentation: .standard)
         }
 
+        @available(FoundationAttributedString 5.5, *)
         public var attributed: Attributed {
             return Attributed(style: self)
         }
@@ -262,6 +265,7 @@ extension Decimal.FormatStyle {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     public struct Attributed : Sendable {
         enum Style : Hashable, Codable, Sendable {
             case decimal(Decimal.FormatStyle)
@@ -330,7 +334,7 @@ extension Decimal.FormatStyle.Percent : FormatStyle {}
 @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
 extension Decimal.FormatStyle.Currency : FormatStyle {}
 
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 extension Decimal.FormatStyle.Attributed : FormatStyle {}
 
 @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
diff --git a/Sources/FoundationInternationalization/Formatting/Number/FloatingPointFormatStyle.swift b/Sources/FoundationInternationalization/Formatting/Number/FloatingPointFormatStyle.swift
index 249259e5d..96c5a6ef1 100644
--- a/Sources/FoundationInternationalization/Formatting/Number/FloatingPointFormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/Number/FloatingPointFormatStyle.swift
@@ -23,6 +23,7 @@ public struct FloatingPointFormatStyle<Value: BinaryFloatingPoint>: Codable, Has
         self.locale = locale
     }
 
+    @available(FoundationAttributedString 5.5, *)
     public var attributed: FloatingPointFormatStyle.Attributed {
         return FloatingPointFormatStyle.Attributed(style: self)
     }
@@ -85,6 +86,7 @@ extension FloatingPointFormatStyle {
             self.locale = locale
         }
 
+        @available(FoundationAttributedString 5.5, *)
         public var attributed: FloatingPointFormatStyle.Attributed {
             return FloatingPointFormatStyle.Attributed(style: self)
         }
@@ -153,6 +155,7 @@ extension FloatingPointFormatStyle {
             self.collection = Configuration.Collection(presentation: .standard)
         }
 
+        @available(FoundationAttributedString 5.5, *)
         public var attributed: FloatingPointFormatStyle.Attributed {
             return FloatingPointFormatStyle.Attributed(style: self)
         }
@@ -337,7 +340,7 @@ public extension FormatStyle where Self == FloatingPointFormatStyle<Float16>.Per
 
 // MARK: - Attributed string
 
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 extension FloatingPointFormatStyle {
     public struct Attributed : Codable, Hashable, FormatStyle, Sendable {
         enum Style : Codable, Hashable, Sendable {
diff --git a/Sources/FoundationInternationalization/Formatting/Number/ICUNumberFormatter.swift b/Sources/FoundationInternationalization/Formatting/Number/ICUNumberFormatter.swift
index ed288d189..ed180022b 100644
--- a/Sources/FoundationInternationalization/Formatting/Number/ICUNumberFormatter.swift
+++ b/Sources/FoundationInternationalization/Formatting/Number/ICUNumberFormatter.swift
@@ -77,6 +77,7 @@ internal class ICUNumberFormatterBase : @unchecked Sendable {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func attributedStringFromPositions(_ positions: [ICUNumberFormatter.AttributePosition], string: String) -> AttributedString {
         typealias NumberPartAttribute = AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute.NumberPart
         typealias NumberSymbolAttribute = AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttribute.Symbol
@@ -246,6 +247,7 @@ final class ICUNumberFormatter : ICUNumberFormatterBase, @unchecked Sendable {
         _create(with: .init(skeleton: style.collection.skeleton, localeIdentifier: style.locale.identifierCapturingPreferences))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func attributedFormat(_ v: Value) -> AttributedString {
         guard let (str, attributes) = attributedFormatPositions(v) else {
             return AttributedString(v.fallbackDescription)
@@ -294,6 +296,7 @@ final class ICUCurrencyNumberFormatter : ICUNumberFormatterBase, @unchecked Send
         _create(with: .init(skeleton: style.collection.skeleton, currencyCode: style.currencyCode, localeIdentifier: style.locale.identifierCapturingPreferences))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func attributedFormat(_ v: Value) -> AttributedString {
         guard let (str, attributes) = attributedFormatPositions(v) else {
             return AttributedString(v.fallbackDescription)
@@ -339,6 +342,7 @@ final class ICUPercentNumberFormatter : ICUNumberFormatterBase, @unchecked Senda
         _create(with: .init(skeleton: style.collection.skeleton, localeIdentifier: style.locale.identifierCapturingPreferences))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func attributedFormat(_ v: Value) -> AttributedString {
         guard let (str, attributes) = attributedFormatPositions(v) else {
             return AttributedString(v.fallbackDescription)
@@ -364,6 +368,7 @@ final class ICUByteCountNumberFormatter : ICUNumberFormatterBase, @unchecked Sen
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func attributedFormat(_ v: Value, unit: ByteCountFormatStyle.Unit) -> AttributedString {
         guard let (str, attributes) = attributedFormatPositions(v) else {
             return AttributedString(v.fallbackDescription)
@@ -371,6 +376,7 @@ final class ICUByteCountNumberFormatter : ICUNumberFormatterBase, @unchecked Sen
         return attributedStringFromPositions(attributes, string: str, unit: unit)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     private func attributedStringFromPositions(_ positions: [ICUNumberFormatter.AttributePosition], string: String, unit: ByteCountFormatStyle.Unit) -> AttributedString {
         typealias NumberPartAttribute = AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute.NumberPart
         typealias NumberSymbolAttribute = AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttribute.Symbol
@@ -422,6 +428,7 @@ final class ICUMeasurementNumberFormatter : ICUNumberFormatterBase, @unchecked S
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func attributedFormat(_ v: Value) -> AttributedString {
         guard let (str, attributes) = attributedFormatPositions(v) else {
             return AttributedString(v.fallbackDescription)
@@ -430,6 +437,7 @@ final class ICUMeasurementNumberFormatter : ICUNumberFormatterBase, @unchecked S
     }
 
     /// Overrides superclass implementation to add the `MeasurementAttribute` property.
+    @available(FoundationAttributedString 5.5, *)
     override func attributedStringFromPositions(_ positions: [ICUNumberFormatter.AttributePosition], string: String) -> AttributedString {
         typealias NumberPartAttribute = AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute.NumberPart
         typealias NumberSymbolAttribute = AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttribute.Symbol
diff --git a/Sources/FoundationInternationalization/Formatting/Number/IntegerFormatStyle.swift b/Sources/FoundationInternationalization/Formatting/Number/IntegerFormatStyle.swift
index 16f1d0142..216713089 100644
--- a/Sources/FoundationInternationalization/Formatting/Number/IntegerFormatStyle.swift
+++ b/Sources/FoundationInternationalization/Formatting/Number/IntegerFormatStyle.swift
@@ -25,6 +25,7 @@ public struct IntegerFormatStyle<Value: BinaryInteger>: Codable, Hashable, Senda
         self.locale = locale
     }
 
+    @available(FoundationAttributedString 5.5, *)
     public var attributed: IntegerFormatStyle.Attributed {
         return IntegerFormatStyle.Attributed(style: self)
     }
@@ -89,6 +90,7 @@ extension IntegerFormatStyle {
             self.locale = locale
         }
 
+        @available(FoundationAttributedString 5.5, *)
         public var attributed: IntegerFormatStyle.Attributed {
             return IntegerFormatStyle.Attributed(style: self)
         }
@@ -153,6 +155,7 @@ extension IntegerFormatStyle {
             self.collection = Configuration.Collection(presentation: .standard)
         }
 
+        @available(FoundationAttributedString 5.5, *)
         public var attributed: IntegerFormatStyle.Attributed {
             return IntegerFormatStyle.Attributed(style: self)
         }
@@ -485,7 +488,7 @@ public extension FormatStyle {
 
 // MARK: - Attributed string
 
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 extension IntegerFormatStyle {
     public struct Attributed : Codable, Hashable, FormatStyle, Sendable {
         enum Style : Codable, Hashable {
diff --git a/Sources/FoundationInternationalization/Formatting/Number/NumberAttributedFormat.swift b/Sources/FoundationInternationalization/Formatting/Number/NumberAttributedFormat.swift
index 4c9b32f0c..acd789cca 100644
--- a/Sources/FoundationInternationalization/Formatting/Number/NumberAttributedFormat.swift
+++ b/Sources/FoundationInternationalization/Formatting/Number/NumberAttributedFormat.swift
@@ -16,6 +16,7 @@ import FoundationEssentials
 
 internal import _FoundationICU
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttribute.Symbol {
     init?(unumberFormatField: UNumberFormatFields) {
         switch unumberFormatField {
@@ -35,6 +36,7 @@ extension AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttr
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute.NumberPart {
     init?(unumberFormatField: UNumberFormatFields) {
         switch unumberFormatField {
@@ -48,6 +50,7 @@ extension AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPart
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.FoundationAttributes.MeasurementAttribute.Component {
     init?(unumberFormatField: UNumberFormatFields) {
         switch unumberFormatField {
diff --git a/Sources/TestSupport/TestSupport.swift b/Sources/TestSupport/TestSupport.swift
index a8847500f..5031ba81a 100644
--- a/Sources/TestSupport/TestSupport.swift
+++ b/Sources/TestSupport/TestSupport.swift
@@ -137,7 +137,7 @@ public typealias PropertyListDecoder = FoundationEssentials.PropertyListDecoder
 
 @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
 public typealias FormatStyle = FoundationEssentials.FormatStyle
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias ByteCountFormatStyle = FoundationInternationalization.ByteCountFormatStyle
 @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
 public typealias ListFormatStyle = FoundationInternationalization.ListFormatStyle
@@ -158,31 +158,31 @@ public typealias DiscreteFormatStyle = FoundationEssentials.DiscreteFormatStyle
 @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
 public typealias StringStyle = FoundationInternationalization.StringStyle
 
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributedString = FoundationEssentials.AttributedString
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributeScope = FoundationEssentials.AttributeScope
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributeContainer = FoundationEssentials.AttributeContainer
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributeDynamicLookup = FoundationEssentials.AttributeDynamicLookup
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributeScopes = FoundationEssentials.AttributeScopes
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributedStringAttributeMutation = FoundationEssentials.AttributedStringAttributeMutation
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributedStringKey = FoundationEssentials.AttributedStringKey
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributedStringProtocol = FoundationEssentials.AttributedStringProtocol
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias AttributedSubstring = FoundationEssentials.AttributedSubstring
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias ScopedAttributeContainer = FoundationEssentials.ScopedAttributeContainer
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias CodableAttributedStringKey = FoundationEssentials.CodableAttributedStringKey
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias EncodableAttributedStringKey = FoundationEssentials.EncodableAttributedStringKey
-@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+@available(FoundationAttributedString 5.5, *)
 public typealias DecodableAttributedStringKey = FoundationEssentials.DecodableAttributedStringKey
 
 @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
diff --git a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringCOWTests.swift b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringCOWTests.swift
index d31a835c4..f4a39ce21 100644
--- a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringCOWTests.swift
+++ b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringCOWTests.swift
@@ -18,6 +18,7 @@ import Testing
 @testable import Foundation
 #endif
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedStringProtocol {
     fileprivate mutating func genericSetAttribute() {
         self.testInt = 3
@@ -29,7 +30,7 @@ extension AttributedStringProtocol {
 private struct AttributedStringCOWTests {
     
     // MARK: - Utility Functions
-    
+    @available(FoundationAttributedString 5.5, *)
     func createAttributedString() -> AttributedString {
         var str = AttributedString("Hello", attributes: container)
         str += AttributedString(" ")
@@ -37,6 +38,7 @@ private struct AttributedStringCOWTests {
         return str
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func assertCOWCopy(sourceLocation: SourceLocation = #_sourceLocation, _ operation: (inout AttributedString) -> Void) {
         let str = createAttributedString()
         var copy = str
@@ -44,6 +46,7 @@ private struct AttributedStringCOWTests {
         #expect(str != copy, "Mutation operation did not copy when multiple references exist", sourceLocation: sourceLocation)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func assertCOWCopyManual(sourceLocation: SourceLocation = #_sourceLocation, _ operation: (inout AttributedString) -> Void) {
         var str = createAttributedString()
         let gutsPtr = Unmanaged.passUnretained(str._guts)
@@ -52,6 +55,7 @@ private struct AttributedStringCOWTests {
         #expect(gutsPtr.toOpaque() != newGutsPtr.toOpaque(), "Mutation operation with manual copy did not perform copy", sourceLocation: sourceLocation)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func assertCOWNoCopy(sourceLocation: SourceLocation = #_sourceLocation, _ operation: (inout AttributedString) -> Void) {
         var str = createAttributedString()
         let gutsPtr = Unmanaged.passUnretained(str._guts)
@@ -60,37 +64,42 @@ private struct AttributedStringCOWTests {
         #expect(gutsPtr.toOpaque() == newGutsPtr.toOpaque(), "Mutation operation copied when only one reference exists", sourceLocation: sourceLocation)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func assertCOWBehavior(sourceLocation: SourceLocation = #_sourceLocation, _ operation: (inout AttributedString) -> Void) {
         assertCOWCopy(sourceLocation: sourceLocation, operation)
         assertCOWNoCopy(sourceLocation: sourceLocation, operation)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func makeSubrange(_ str: AttributedString) -> Range<AttributedString.Index> {
         return str.characters.index(str.startIndex, offsetBy: 2)..<str.characters.index(str.endIndex, offsetBy: -2)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func makeSubranges(_ str: AttributedString) -> RangeSet<AttributedString.Index> {
         let rangeA = str.characters.index(str.startIndex, offsetBy: 2)..<str.characters.index(str.startIndex, offsetBy: 4)
         let rangeB = str.characters.index(str.endIndex, offsetBy: -4)..<str.characters.index(str.endIndex, offsetBy: -2)
         return RangeSet([rangeA, rangeB])
     }
     
-    let container: AttributeContainer = {
+    @available(FoundationAttributedString 5.5, *)
+    var container: AttributeContainer {
         var container = AttributeContainer()
         container.testInt = 2
         return container
-    }()
+    }
     
-    let containerB: AttributeContainer = {
+    @available(FoundationAttributedString 5.5, *)
+    var containerB: AttributeContainer {
         var container = AttributeContainer()
         container.testBool = true
         return container
-    }()
+    }
     
     // MARK: - Tests
     
-    @Test
-    func topLevelType() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func topLevelType() {
         assertCOWBehavior { (str) in
             str.setAttributes(container)
         }
@@ -126,8 +135,8 @@ private struct AttributedStringCOWTests {
         }
     }
     
-    @Test
-    func substring() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func substring() {
         assertCOWBehavior { (str) in
             str[makeSubrange(str)].setAttributes(container)
         }
@@ -148,8 +157,8 @@ private struct AttributedStringCOWTests {
         }
     }
     
-    @Test
-    func discontiguousSubstring() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func discontiguousSubstring() {
         assertCOWBehavior { (str) in
             str[makeSubranges(str)].setAttributes(container)
         }
@@ -174,8 +183,8 @@ private struct AttributedStringCOWTests {
         }
     }
     
-    @Test
-    func characters() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func characters() {
         let char: Character = "a"
         
         assertCOWBehavior { (str) in
@@ -198,8 +207,8 @@ private struct AttributedStringCOWTests {
         }
     }
     
-    @Test
-    func unicodeScalars() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func unicodeScalars() {
         let scalar: UnicodeScalar = "a"
         
         assertCOWBehavior { (str) in
@@ -207,8 +216,8 @@ private struct AttributedStringCOWTests {
         }
     }
     
-    @Test
-    func genericProtocol() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func genericProtocol() {
         assertCOWBehavior {
             $0.genericSetAttribute()
         }
@@ -217,8 +226,8 @@ private struct AttributedStringCOWTests {
         }
     }
     
-    @Test
-    func indexTracking() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func indexTracking() {
         assertCOWBehavior {
             _ = $0.transform(updating: $0.startIndex ..< $0.endIndex) {
                 $0.testInt = 2
diff --git a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringConstrainingBehaviorTests.swift b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringConstrainingBehaviorTests.swift
index f3f41d360..030e5fea7 100644
--- a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringConstrainingBehaviorTests.swift
+++ b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringConstrainingBehaviorTests.swift
@@ -21,6 +21,7 @@ import Foundation
 @Suite("AttributedString Constraining Behavior")
 private struct AttributedStringConstrainingBehaviorTests {
     
+    @available(FoundationAttributedString 5.5, *)
     func verify<K: AttributedStringKey>(
         string: AttributedString,
         matches expected: [(String, K.Value?)],
@@ -43,6 +44,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func verify<K: AttributedStringKey, K2: AttributedStringKey>(string: AttributedString, matches expected: [(String, K.Value?, K2.Value?)], for key: KeyPath<AttributeDynamicLookup, K>, _ key2: KeyPath<AttributeDynamicLookup, K2>, sourceLocation: SourceLocation = #_sourceLocation)
     where K.Value : Sendable, K2.Value : Sendable
     {
@@ -60,6 +62,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func verify<K: AttributedStringKey, K2: AttributedStringKey, K3: AttributedStringKey>(string: AttributedString, matches expected: [(String, K.Value?, K2.Value?, K3.Value?)], for key: KeyPath<AttributeDynamicLookup, K>, _ key2: KeyPath<AttributeDynamicLookup, K2>, _ key3: KeyPath<AttributeDynamicLookup, K3>, sourceLocation: SourceLocation = #_sourceLocation)
     where K.Value : Sendable, K2.Value : Sendable, K3.Value : Sendable
     {
@@ -81,6 +84,7 @@ private struct AttributedStringConstrainingBehaviorTests {
     
     // MARK: Extending Run Tests
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func extendingRunAddCharacters() {
         let str = AttributedString("Hello, world", attributes: .init().testInt(2).testNonExtended(1))
         
@@ -108,6 +112,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: result, matches: [("He", 2, 1), ("Hi!", 2, nil), ("rld", 2, 1)], for: \.testInt, \.testNonExtended)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func extendingRunAddUnicodeScalars() {
         let str = AttributedString("Hello, world", attributes: .init().testInt(2).testNonExtended(1))
         let scalarsStr = "A\u{0301}B"
@@ -132,6 +137,7 @@ private struct AttributedStringConstrainingBehaviorTests {
     
     // MARK: - Paragraph Constrained Tests
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphAttributeExpanding() {
         var str = AttributedString("Hello, world\nNext Paragraph")
         var range = str.index(afterCharacter: str.startIndex) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -153,6 +159,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: str, matches: [("Hello, world\n", 4), ("Next Paragraph", 4)], for: \.testParagraphConstrained)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphAttributeRemoval() {
         var str = AttributedString("Hello, world\nNext Paragraph", attributes: .init().testParagraphConstrained(2))
         var range = str.index(afterCharacter: str.startIndex) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -172,6 +179,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: str, matches: [("Hello, world\n", nil), ("Next Paragraph", nil)], for: \.testParagraphConstrained)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphAttributeContainerApplying() {
         var container = AttributeContainer.testParagraphConstrained(2).testString("Hello")
         var str = AttributedString("Hello, world\nNext Paragraph")
@@ -200,6 +208,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: str, matches: [("H", 4, nil, 1), ("el", 4, "Hello", 1), ("lo, w", 4, nil, 1), ("orld\n", 4, nil, 2), ("N", 4, nil, 2), ("ext Paragrap", 4, nil, 1), ("h", 4, "Hello", 2)], for: \.testParagraphConstrained, \.testString, \.testInt)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphAttributeContainerReplacing() {
         var str = AttributedString("Hello, world\nNext Paragraph")
         let range = str.index(afterCharacter: str.startIndex) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -221,6 +230,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: result, matches: [("H", 3, 2, nil), ("el", 3, nil, true), ("lo, world\n", 3, 2, nil), ("Next Paragraph", nil, 2, nil)], for: \.testParagraphConstrained, \.testInt, \.testBool)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphTextMutation() {
         let str = AttributedString("Hello, world\n", attributes: .init().testParagraphConstrained(1)) + AttributedString("Next Paragraph", attributes: .init().testParagraphConstrained(2))
         
@@ -265,6 +275,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: result, matches: [("Hello, wTest\n", 1), ("Replacementxt Paragraph", 1)], for: \.testParagraphConstrained)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphAttributedTextMutation() {
         let str = AttributedString("Hello, world\n", attributes: .init().testParagraphConstrained(1)) + AttributedString("Next Paragraph", attributes: .init().testParagraphConstrained(2))
         let singleReplacement = AttributedString("Test", attributes: .init().testParagraphConstrained(5).testSecondParagraphConstrained(6).testBool(true))
@@ -316,6 +327,7 @@ private struct AttributedStringConstrainingBehaviorTests {
     }
     
 #if FOUNDATION_FRAMEWORK
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphFromUntrustedRuns() throws {
         let str = NSMutableAttributedString(string: "Hello ", attributes: [.testParagraphConstrained : NSNumber(2)])
         str.append(NSAttributedString(string: "World", attributes: [.testParagraphConstrained : NSNumber(3), .testSecondParagraphConstrained : NSNumber(4)]))
@@ -325,6 +337,7 @@ private struct AttributedStringConstrainingBehaviorTests {
     }
 #endif // FOUNDATION_FRAMEWORK
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func paragraphFromReplacedSubrange() {
         let str = AttributedString("Before\nHello, world\nNext Paragraph\nAfter", attributes: .init().testParagraphConstrained(1))
         
@@ -349,6 +362,7 @@ private struct AttributedStringConstrainingBehaviorTests {
     
     // MARK: - Character Constrained Tests
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func characterAttributeApply() {
         let str = AttributedString("*__*__**__*")
         
@@ -367,6 +381,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: result, matches: [("*", nil, 1), ("__", nil, 1), ("*", nil, 1), ("__", nil, 1), ("*", nil, 1), ("*", nil, 1), ("__", nil, 1), ("*", 3, 1)], for: \.testCharacterConstrained, \.testInt)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func characterAttributeSubCharacterApply() {
         let str = AttributedString("ABC \u{FFFD} DEF")
 
@@ -399,6 +414,7 @@ private struct AttributedStringConstrainingBehaviorTests {
 
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func characterAttributeContainerReplacing() {
         var str = AttributedString("*__*__**__*")
         let range = str.index(afterCharacter: str.startIndex) ..< str.index(str.startIndex, offsetByCharacters: 4)
@@ -420,6 +436,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: result, matches: [("*", nil, 2, nil), ("__", nil, nil, true), ("*", 3, nil, true), ("__", nil, 2, nil), ("*", nil, 2, nil), ("*", nil, 2, nil), ("__", nil, 2, nil), ("*", nil, 2, nil)], for: \.testCharacterConstrained, \.testInt, \.testBool)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func characterTextMutation() {
         let str = AttributedString("*__*__**__*", attributes: .init().testCharacterConstrained(2))
         
@@ -449,6 +466,7 @@ private struct AttributedStringConstrainingBehaviorTests {
     }
     
 #if FOUNDATION_FRAMEWORK
+    @available(FoundationAttributedString 5.5, *)
     @Test func characterFromUntrustedRuns() throws {
         let str = NSMutableAttributedString(string: "*__*__**__*", attributes: [.testCharacterConstrained : NSNumber(2)])
         str.append(NSAttributedString(string: "_*"))
@@ -460,6 +478,7 @@ private struct AttributedStringConstrainingBehaviorTests {
     
     // MARK: Invalidation Tests
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func invalidationAttributeChange() {
         let str = AttributedString("Hello, world", attributes: .init().testInt(1).testAttributeDependent(2))
         
@@ -494,6 +513,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: result, matches: [("Hello, world", 2, nil)], for: \.testInt, \.testAttributeDependent)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func invalidationCharacterChange() {
         let str = AttributedString("Hello, world", attributes: .init().testInt(1).testCharacterDependent(2))
         
@@ -580,6 +600,7 @@ private struct AttributedStringConstrainingBehaviorTests {
         verify(string: result, matches: [("H", nil, nil, "Hello"), ("ello, world", 1, nil, nil)], for: \.testInt, \.testCharacterDependent, \.testString)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func invalidationCharacterInsertionBetweenRuns() {
         var str = AttributedString("Hello", attributes: .init().testInt(1).testCharacterDependent(2))
         str += AttributedString("World", attributes: .init().testInt(1).testCharacterDependent(3))
diff --git a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringDiscontiguousTests.swift b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringDiscontiguousTests.swift
index ae247167b..524eb0348 100644
--- a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringDiscontiguousTests.swift
+++ b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringDiscontiguousTests.swift
@@ -20,8 +20,8 @@ import Foundation
 
 @Suite("Discontiguous AttributedString")
 private struct AttributedStringDiscontiguousTests {
-    @Test
-    func emptySlice() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func emptySlice() {
         let str = AttributedString()
         let slice = str[RangeSet()]
         #expect(slice.runs.isEmpty)
@@ -44,8 +44,8 @@ private struct AttributedStringDiscontiguousTests {
         }
     }
     
-    @Test
-    func characters() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func characters() {
         let str = AttributedString("abcdefgabc")
         let fullSlice = str[str.startIndex ..< str.endIndex].characters
         let fullDiscontiguousSlice = str[RangeSet(str.startIndex ..< str.endIndex)].characters
@@ -58,8 +58,8 @@ private struct AttributedStringDiscontiguousTests {
         #expect(Array(slice) == ["a", "b", "c", "a", "b", "c"])
     }
     
-    @Test
-    func unicodeScalars() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func unicodeScalars() {
         let str = AttributedString("abcdefgabc")
         let fullSlice = str[str.startIndex ..< str.endIndex].unicodeScalars
         let fullDiscontiguousSlice = str[RangeSet(str.startIndex ..< str.endIndex)].unicodeScalars
@@ -72,8 +72,8 @@ private struct AttributedStringDiscontiguousTests {
         #expect(Array(slice) == ["a", "b", "c", "a", "b", "c"])
     }
     
-    @Test
-    func attributes() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func attributes() {
         let str = AttributedString("abcdefg")
         let rangeA = str.startIndex ..< str.index(str.startIndex, offsetByCharacters: 1)
         let rangeB = str.index(str.startIndex, offsetByCharacters: 2) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -179,8 +179,8 @@ private struct AttributedStringDiscontiguousTests {
         }
     }
     
-    @Test
-    func reinitialization() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func reinitialization() {
         var str = AttributedString("abcdefg")
         let rangeA = str.startIndex ..< str.index(str.startIndex, offsetByCharacters: 1)
         let rangeB = str.index(str.startIndex, offsetByCharacters: 2) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -192,8 +192,8 @@ private struct AttributedStringDiscontiguousTests {
         #expect(reinitialized == AttributedString("ace", attributes: AttributeContainer.testInt(2)))
     }
     
-    @Test
-    func reslicing() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func reslicing() {
         var str = AttributedString("abcdefg")
         let rangeA = str.startIndex ..< str.index(str.startIndex, offsetByCharacters: 1)
         let rangeB = str.index(str.startIndex, offsetByCharacters: 2) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -208,8 +208,8 @@ private struct AttributedStringDiscontiguousTests {
         #expect(str[RangeSet()][RangeSet()] == str[RangeSet()])
     }
     
-    @Test
-    func runs() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func runs() {
         var str = AttributedString("AAA", attributes: AttributeContainer.testInt(2))
         str += AttributedString("BBB", attributes: AttributeContainer.testInt(3).testString("foo"))
         str += AttributedString("CC", attributes: AttributeContainer.testInt(3).testString("bar"))
@@ -234,8 +234,8 @@ private struct AttributedStringDiscontiguousTests {
         #expect(runs.reversed().map(\.range) == expectedRanges.reversed())
     }
     
-    @Test
-    func coalescedRuns() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func coalescedRuns() {
         struct EquatableBox<T: Equatable, U: Equatable>: Equatable, CustomStringConvertible {
             let t: T
             let u: U
@@ -281,8 +281,8 @@ private struct AttributedStringDiscontiguousTests {
         #expect(runs[\.testString].reversed().map(EquatableBox.init) == testStringExpectation.reversed())
     }
     
-    @Test
-    func removeSubranges() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func removeSubranges() {
         var str = AttributedString("abcdefg")
         let rangeA = str.startIndex ..< str.index(str.startIndex, offsetByCharacters: 1)
         let rangeB = str.index(str.startIndex, offsetByCharacters: 2) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -297,8 +297,8 @@ private struct AttributedStringDiscontiguousTests {
         #expect(str == result)
     }
     
-    @Test
-    func sliceSetter() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func sliceSetter() {
         var str = AttributedString("abcdefg")
         let rangeA = str.startIndex ..< str.index(str.startIndex, offsetByCharacters: 1)
         let rangeB = str.index(str.startIndex, offsetByCharacters: 2) ..< str.index(str.startIndex, offsetByCharacters: 3)
@@ -332,8 +332,8 @@ private struct AttributedStringDiscontiguousTests {
         }
     }
     
-    @Test
-    func graphemesAcrossDiscontiguousRanges() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func graphemesAcrossDiscontiguousRanges() {
         let str = "a\n\u{301}"
         let attrStr = AttributedString(str)
         let strRangeA = str.startIndex ..< str.index(after: str.startIndex) // Range of 'a'
diff --git a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexTrackingTests.swift b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexTrackingTests.swift
index e89f1a4a0..17069e4b4 100644
--- a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexTrackingTests.swift
+++ b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexTrackingTests.swift
@@ -20,8 +20,8 @@ import Foundation
 
 @Suite("AttributedString Index Tracking")
 private struct AttributedStringIndexTrackingTests {
-    @Test
-    func basics() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func basics() throws {
         var text = AttributedString("ABC. Hello, world!")
         let original = text
         let helloRange = try #require(text.range(of: "Hello"))
@@ -36,8 +36,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(text[updatedRanges[1]] == original[worldRange])
     }
     
-    @Test
-    func insertionWithinRange() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func insertionWithinRange() throws {
         var text = AttributedString("Hello, world")
         var helloRange = try #require(text.range(of: "Hello"))
         
@@ -48,8 +48,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(String(text[helloRange].characters) == "Hel_Goodbye_lo")
     }
     
-    @Test
-    func insertionAtStartOfRange() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func insertionAtStartOfRange() throws {
         var text = AttributedString("Hello, world")
         let helloRange = try #require(text.range(of: "llo"))
         
@@ -60,8 +60,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(String(text[updatedHelloRange].characters) == "llo")
     }
     
-    @Test
-    func insertionAtEndOfRange() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func insertionAtEndOfRange() throws {
         var text = AttributedString("Hello, world")
         let helloRange = try #require(text.range(of: "llo"))
         
@@ -72,8 +72,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(String(text[updatedHelloRange].characters) == "llo")
     }
     
-    @Test
-    func insertionAtEmptyRange() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func insertionAtEmptyRange() throws {
         var text = AttributedString("ABCDE")
         let idx = text.index(text.startIndex, offsetByCharacters: 3)
         
@@ -85,8 +85,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(text.characters[updatedRange.lowerBound] == "D")
     }
     
-    @Test
-    func removalWithinRange() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func removalWithinRange() throws {
         var text = AttributedString("Hello, world")
         var helloRange = try #require(text.range(of: "Hello"))
         
@@ -97,8 +97,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(String(text[helloRange].characters) == "Heo")
     }
     
-    @Test
-    func fullCollapse() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func fullCollapse() throws {
         do {
             var text = AttributedString("Hello, world")
             var helloRange = try #require(text.range(of: "Hello"))
@@ -135,8 +135,8 @@ private struct AttributedStringIndexTrackingTests {
         }
     }
     
-    @Test
-    func collapseLeft() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func collapseLeft() throws {
         var text = AttributedString("Hello, world")
         var helloRange = try #require(text.range(of: "Hello"))
         
@@ -147,8 +147,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(String(text[helloRange].characters) == "He")
     }
     
-    @Test
-    func collapseRight() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func collapseRight() throws {
         var text = AttributedString("Hello, world")
         var worldRange = try #require(text.range(of: "world"))
         
@@ -159,8 +159,8 @@ private struct AttributedStringIndexTrackingTests {
         #expect(String(text[worldRange].characters) == "rld")
     }
     
-    @Test
-    func nesting() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func nesting() throws {
         var text = AttributedString("Hello, world")
         var helloRange = try #require(text.range(of: "Hello"))
         try text.transform(updating: &helloRange) {
@@ -174,8 +174,8 @@ private struct AttributedStringIndexTrackingTests {
     }
     
     #if FOUNDATION_EXIT_TESTS
-    @Test
-    func trackingLostPreconditions() async {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func trackingLostPreconditions() async {
         await #expect(processExitsWith: .failure) {
             var text = AttributedString("Hello, world")
             var helloRange = try #require(text.range(of: "Hello"))
@@ -210,8 +210,8 @@ private struct AttributedStringIndexTrackingTests {
     }
     #endif
     
-    @Test
-    func trackingLost() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func trackingLost() throws {
         let text = AttributedString("Hello, world")
         let helloRange = try #require(text.range(of: "Hello"))
         
@@ -247,8 +247,8 @@ private struct AttributedStringIndexTrackingTests {
         }
     }
     
-    @Test
-    func attributeMutation() throws {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func attributeMutation() throws {
         var text = AttributedString("Hello, world!")
         let original = text
         let helloRange = try #require(text.range(of: "Hello"))
@@ -264,8 +264,8 @@ private struct AttributedStringIndexTrackingTests {
     }
     
     #if FOUNDATION_EXIT_TESTS
-    @Test
-    func invalidInputRanges() async {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func invalidInputRanges() async {
         await #expect(processExitsWith: .failure) {
             var text = AttributedString("Hello, world")
             let other = text + AttributedString("Extra text")
diff --git a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexValidityTests.swift b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexValidityTests.swift
index 5369467ea..b0fcd485b 100644
--- a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexValidityTests.swift
+++ b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringIndexValidityTests.swift
@@ -20,8 +20,8 @@ import Foundation
 
 @Suite("AttributedString Index Validity")
 private struct AttributedStringIndexValidityTests {
-    @Test
-    public func startEndRange() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func startEndRange() {
         let str = AttributedString("Hello, world")
         
         #expect(str.startIndex.isValid(within: str))
@@ -94,8 +94,8 @@ private struct AttributedStringIndexValidityTests {
         }
     }
     
-    @Test
-    public func exhaustiveIndices() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func exhaustiveIndices() {
         let str = AttributedString("Hello Cafe\u{301} 👍🏻🇺🇸 World")
         for idx in str.characters.indices {
             #expect(idx.isValid(within: str))
@@ -111,8 +111,8 @@ private struct AttributedStringIndexValidityTests {
         }
     }
     
-    @Test
-    public func outOfBoundsContiguous() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func outOfBoundsContiguous() {
         let str = AttributedString("Hello, world")
         let subStart = str.index(afterCharacter: str.startIndex)
         let subEnd = str.index(beforeCharacter: str.endIndex)
@@ -127,8 +127,8 @@ private struct AttributedStringIndexValidityTests {
         #expect(!(str.endIndex ..< str.endIndex).isValid(within: substr))
     }
     
-    @Test
-    public func outOfBoundsDiscontiguous() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func outOfBoundsDiscontiguous() {
         let str = AttributedString("Hello, world")
         let idxA = str.index(afterCharacter: str.startIndex)
         let idxB = str.index(afterCharacter: idxA)
@@ -148,8 +148,8 @@ private struct AttributedStringIndexValidityTests {
         #expect(!(str.endIndex ..< str.endIndex).isValid(within: substr))
     }
     
-    @Test
-    public func mutationInvalidation() {
+    @available(FoundationAttributedString 5.5, *)
+    @Test func mutationInvalidation() {
         func checkInPlace(_ mutation: (inout AttributedString) -> (), sourceLocation: SourceLocation = #_sourceLocation) {
             var str = AttributedString("Hello World")
             let idxA = str.startIndex
diff --git a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTestSupport.swift b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTestSupport.swift
index df8f443d0..c45661dbc 100644
--- a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTestSupport.swift
+++ b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTestSupport.swift
@@ -26,6 +26,7 @@ extension NSAttributedString.Key {
 }
 #endif
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes.TestAttributes {
 
     enum TestIntAttribute: CodableAttributedStringKey {
@@ -123,6 +124,7 @@ struct NonCodableType : Hashable {
     var inner : Int
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeScopes {
     var test: TestAttributes.Type { TestAttributes.self }
     
@@ -141,6 +143,7 @@ extension AttributeScopes {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributeDynamicLookup {
     subscript<T: AttributedStringKey>(dynamicMember keyPath: KeyPath<AttributeScopes.TestAttributes, T>) -> T {
         get { self[T.self] }
diff --git a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTests.swift b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTests.swift
index 36befc4e9..c33f303e9 100644
--- a/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTests.swift
+++ b/Tests/FoundationEssentialsTests/AttributedString/AttributedStringTests.swift
@@ -38,6 +38,7 @@ import AppKit
 private struct  AttributedStringTests {
     // MARK: - Enumeration Tests
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func emptyEnumeration() {
         for _ in AttributedString().runs {
             Issue.record("Empty AttributedString should not enumerate any attributes")
@@ -59,6 +60,7 @@ private struct  AttributedStringTests {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func verifyAttributes<T>(_ runs: AttributedString.Runs.AttributesSlice1<T>, string: AttributedString, expectation: [(String, T.Value?)], sourceLocation: SourceLocation = #_sourceLocation) where T.Value : Sendable {
         // Test that the attribute is correct when iterating through attribute runs
         var expectIterator = expectation.makeIterator()
@@ -85,6 +87,7 @@ private struct  AttributedStringTests {
         #expect(expectIterator.next() == nil, "Additional runs expected but not found", sourceLocation: sourceLocation)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func verifyAttributes<T, U>(_ runs: AttributedString.Runs.AttributesSlice2<T, U>, string: AttributedString, expectation: [(String, T.Value?, U.Value?)], sourceLocation: SourceLocation = #_sourceLocation) where T.Value : Sendable, U.Value : Sendable {
         // Test that the attributes are correct when iterating through attribute runs
         var expectIterator = expectation.makeIterator()
@@ -141,6 +144,7 @@ private struct  AttributedStringTests {
     }
 #endif // FOUNDATION_FRAMEWORK
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func simpleEnumeration() throws {
         var attrStr = AttributedString("Hello", attributes: AttributeContainer().testInt(1))
         attrStr += " "
@@ -180,6 +184,7 @@ private struct  AttributedStringTests {
         verifyAttributes(attrView[\.testInt, \.testDouble], string: attrStr, expectation: [("Hello", 1, nil), (" ", nil, nil), ("World", nil, 2.0)])
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func sliceEnumeration() throws {
         var attrStr = AttributedString("Hello", attributes: AttributeContainer().testInt(1))
         attrStr += AttributedString(" ")
@@ -222,6 +227,7 @@ private struct  AttributedStringTests {
     }
     
 #if FOUNDATION_FRAMEWORK
+    @available(FoundationAttributedString 5.5, *)
     @Test func nsSliceEnumeration() {
         var attrStr = AttributedString("Hello", attributes: AttributeContainer().testInt(1))
         attrStr += AttributedString(" ")
@@ -250,6 +256,7 @@ private struct  AttributedStringTests {
 
     // MARK: - Attribute Tests
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func simpleAttribute() {
         let attrStr = AttributedString("Foo", attributes: AttributeContainer().testInt(42))
         let (value, range) = attrStr.runs[\.testInt][attrStr.startIndex]
@@ -257,6 +264,7 @@ private struct  AttributedStringTests {
         #expect(range == attrStr.startIndex ..< attrStr.endIndex)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func constructorAttribute() {
         // TODO: Re-evaluate whether we want these.
         let attrStr = AttributedString("Hello", attributes: AttributeContainer().testString("Helvetica").testInt(2))
@@ -266,6 +274,7 @@ private struct  AttributedStringTests {
         #expect(attrStr == expected)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func addAndRemoveAttribute() {
         let attr : Int = 42
         let attr2 : Double = 1.0
@@ -282,6 +291,7 @@ private struct  AttributedStringTests {
         #expect(attrStr == expected2)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func addingAndRemovingAttribute() {
         let container = AttributeContainer().testInt(1).testDouble(2.2)
         let attrStr = AttributedString("Test").mergingAttributes(container)
@@ -292,6 +302,7 @@ private struct  AttributedStringTests {
         #expect(doubleRemoved == AttributedString("Test", attributes: AttributeContainer().testInt(1)))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func scopedAttributes() {
         var str = AttributedString("Hello, world", attributes: AttributeContainer().testInt(2).testDouble(3.4))
         #expect(str.test.testInt == 2)
@@ -308,6 +319,7 @@ private struct  AttributedStringTests {
         #expect(str[range].test.testBool == true)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func runAttributes() {
         var str = AttributedString("String", attributes: .init().testString("test1"))
         str += "None"
@@ -322,6 +334,7 @@ private struct  AttributedStringTests {
 
     // MARK: - Comparison Tests
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributedStringEquality() {
         #expect(AttributedString() == AttributedString())
         #expect(AttributedString("abc") == AttributedString("abc"))
@@ -360,6 +373,7 @@ private struct  AttributedStringTests {
         #expect(a2.characters.elementsEqual(a3.characters))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributedSubstringEquality() {
         let emptyStr = AttributedString("01234567890123456789")
 
@@ -390,6 +404,7 @@ private struct  AttributedStringTests {
         #expect(emptyStr[index0 ..< index5] == AttributedString("01234"))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func runEquality() {
         var attrStr = AttributedString("Hello", attributes: AttributeContainer().testInt(1))
         attrStr += AttributedString(" ")
@@ -427,6 +442,7 @@ private struct  AttributedStringTests {
         #expect(attrStr.runs != attrStr3.runs)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func substringRunEquality() {
         var attrStr = AttributedString("Hello", attributes: AttributeContainer().testInt(1))
         attrStr += AttributedString(" ")
@@ -458,6 +474,7 @@ private struct  AttributedStringTests {
 
     // MARK: - Mutation Tests
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func directMutationCopyOnWrite() {
         var attrStr = AttributedString("ABC")
         let copy = attrStr
@@ -467,6 +484,7 @@ private struct  AttributedStringTests {
         #expect(attrStr != copy)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributeMutationCopyOnWrite() {
         var attrStr = AttributedString("ABC")
         let copy = attrStr
@@ -475,6 +493,7 @@ private struct  AttributedStringTests {
         #expect(attrStr != copy)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func sliceAttributeMutation() {
         let attr : Int = 42
         let attr2 : Double = 1.0
@@ -494,6 +513,7 @@ private struct  AttributedStringTests {
         #expect(copy != attrStr)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func enumerationAttributeMutation() {
         var attrStr = AttributedString("A", attributes: AttributeContainer().testInt(1))
         attrStr += AttributedString("B", attributes: AttributeContainer().testDouble(2.0))
@@ -511,6 +531,7 @@ private struct  AttributedStringTests {
         #expect(expected == attrStr)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func mutateMultipleAttributes() {
         var attrStr = AttributedString("A", attributes: AttributeContainer().testInt(1).testBool(true))
         attrStr += AttributedString("B", attributes: AttributeContainer().testInt(1).testDouble(2))
@@ -578,6 +599,7 @@ private struct  AttributedStringTests {
         #expect(changeRange1expected == changeRange1)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func mutateAttributes() {
         var attrStr = AttributedString("A", attributes: AttributeContainer().testInt(1).testBool(true))
         attrStr += AttributedString("B", attributes: AttributeContainer().testInt(1).testDouble(2))
@@ -639,6 +661,7 @@ private struct  AttributedStringTests {
         #expect(changeRange2expected == changeRange2)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func replaceAttributes() {
         var attrStr = AttributedString("A", attributes: AttributeContainer().testInt(1).testBool(true))
         attrStr += AttributedString("B", attributes: AttributeContainer().testInt(1).testDouble(2))
@@ -679,6 +702,7 @@ private struct  AttributedStringTests {
     }
  
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func sliceMutation() {
         var attrStr = AttributedString("Hello World", attributes: AttributeContainer().testInt(1))
         let start = attrStr.characters.index(attrStr.startIndex, offsetBy: 6)
@@ -690,6 +714,7 @@ private struct  AttributedStringTests {
         #expect(attrStr != AttributedString("Hello Goodbye", attributes: AttributeContainer().testInt(1)))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func overlappingSliceMutation() throws {
         var attrStr = AttributedString("Hello, world!")
         attrStr[try #require(attrStr.range(of: "Hello"))].testInt = 1
@@ -705,6 +730,7 @@ private struct  AttributedStringTests {
         #expect(attrStr == expected)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func characters_replaceSubrange() throws {
         var attrStr = AttributedString("Hello World", attributes: AttributeContainer().testInt(1))
         attrStr.characters.replaceSubrange(try #require(attrStr.range(of: " ")), with: " Good ")
@@ -713,6 +739,7 @@ private struct  AttributedStringTests {
         #expect(expected == attrStr)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func charactersMutation_append() {
         var attrStr = AttributedString("Hello World", attributes: AttributeContainer().testInt(1))
         attrStr.characters.append(contentsOf: " Goodbye")
@@ -721,6 +748,7 @@ private struct  AttributedStringTests {
         #expect(expected == attrStr)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func unicodeScalars_replaceSubrange() {
         var attrStr = AttributedString("La Cafe\u{301}", attributes: AttributeContainer().testInt(1))
         let unicode = attrStr.unicodeScalars
@@ -730,6 +758,7 @@ private struct  AttributedStringTests {
         #expect(expected == attrStr)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func unicodeScalarsMutation_append() {
         var attrStr = AttributedString("Cafe", attributes: AttributeContainer().testInt(1))
         attrStr.unicodeScalars.append("\u{301}")
@@ -738,6 +767,7 @@ private struct  AttributedStringTests {
         #expect(expected == attrStr)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func subCharacterAttributeSetting() {
         var attrStr = AttributedString("Cafe\u{301}", attributes: AttributeContainer().testInt(1))
         let cafRange = attrStr.characters.startIndex ..< attrStr.characters.index(attrStr.characters.startIndex, offsetBy: 3)
@@ -753,6 +783,7 @@ private struct  AttributedStringTests {
         #expect(expected == attrStr)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func replaceSubrange_rangeExpression() {
         var attrStr = AttributedString("Hello World", attributes: AttributeContainer().testInt(1))
         
@@ -765,6 +796,7 @@ private struct  AttributedStringTests {
         #expect(attrStr == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func settingAttributes() {
         var attrStr = AttributedString("Hello World", attributes: .init().testInt(1))
         attrStr += AttributedString(". My name is Foundation!", attributes: .init().testBool(true))
@@ -775,6 +807,7 @@ private struct  AttributedStringTests {
         #expect(result == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func addAttributedString() {
         let attrStr = AttributedString("Hello ", attributes: .init().testInt(1))
         let attrStr2 = AttributedString("World", attributes: .init().testInt(2))
@@ -799,6 +832,7 @@ private struct  AttributedStringTests {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func replaceSubrangeWithSubstrings() {
         let baseString = AttributedString("A", attributes: .init().testInt(1))
         + AttributedString("B", attributes: .init().testInt(2))
@@ -829,6 +863,7 @@ private struct  AttributedStringTests {
         #expect(targetString == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     func assertStringIsCoalesced(_ str: AttributedString) {
         var prev: AttributedString.Runs.Run?
         for run in str.runs {
@@ -839,6 +874,7 @@ private struct  AttributedStringTests {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func coalescing() {
         let str = AttributedString("Hello", attributes: .init().testInt(1))
         let appendSame = str + AttributedString("World", attributes: .init().testInt(1))
@@ -899,6 +935,7 @@ private struct  AttributedStringTests {
         #expect(str10.runs.count == 1)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func replaceWithEmptyElements() {
         var str = AttributedString("Hello, world")
         let range = str.startIndex ..< str.characters.index(str.startIndex, offsetBy: 5)
@@ -907,6 +944,7 @@ private struct  AttributedStringTests {
         #expect(str == AttributedString(", world"))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func description() {
         let string = AttributedString("A", attributes: .init().testInt(1))
         + AttributedString("B", attributes: .init().testInt(2))
@@ -938,6 +976,7 @@ E {
         #expect(runsDesc == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func containerDescription() {
         let cont = AttributeContainer().testBool(false).testInt(1).testDouble(2.0).testString("3")
         
@@ -952,6 +991,7 @@ E {
         #expect(desc.contains("\tTestBool = false\n"))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func runAndSubstringDescription() {
         let string = AttributedString("A", attributes: .init().testInt(1))
         + AttributedString("B", attributes: .init().testInt(2))
@@ -987,6 +1027,7 @@ E {
         #expect(subDescs == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func replacingAttributes() {
         var str = AttributedString("Hello", attributes: .init().testInt(2))
         str += AttributedString("World", attributes: .init().testString("Test"))
@@ -1000,6 +1041,7 @@ E {
         #expect(result == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func scopedAttributeContainer() {
         var str = AttributedString("Hello, world")
         
@@ -1036,6 +1078,7 @@ E {
         #expect(str[otherRange].testBool == true)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func mergeAttributes() {
         let originalAttributes = AttributeContainer.testInt(2).testBool(true)
         let newAttributes = AttributeContainer.testString("foo")
@@ -1048,6 +1091,7 @@ E {
         #expect(str.mergingAttributes(overlappingAttributes, mergePolicy: .keepCurrent) == AttributedString("Hello, world", attributes: originalAttributes.testDouble(4.3)))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func mergeAttributeContainers() {
         let originalAttributes = AttributeContainer.testInt(2).testBool(true)
         let newAttributes = AttributeContainer.testString("foo")
@@ -1059,6 +1103,7 @@ E {
         #expect(originalAttributes.merging(overlappingAttributes, mergePolicy: .keepCurrent) == originalAttributes.testDouble(4.3))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func changingSingleCharacterUTF8Length() throws {
         var attrstr = AttributedString("\u{1F3BA}\u{1F3BA}") // UTF-8 Length of 8
         attrstr.characters[attrstr.startIndex] = "A" // Changes UTF-8 Length to 5
@@ -1070,6 +1115,7 @@ E {
     
     // MARK: - Substring Tests
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func substringBase() {
         let str = AttributedString("Hello World", attributes: .init().testInt(1))
         var substr = str[str.startIndex ..< str.characters.index(str.startIndex, offsetBy: 5)]
@@ -1084,6 +1130,7 @@ E {
         #expect(str2[range].base == str2)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func substringGetAttribute() {
         let str = AttributedString("Hello World", attributes: .init().testInt(1))
         let range = str.startIndex ..< str.characters.index(str.startIndex, offsetBy: 5)
@@ -1097,6 +1144,7 @@ E {
         #expect(str2[range2].testBool == nil)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func substringDescription() {
         var str = AttributedString("Hello", attributes: .init().testInt(2))
         str += " "
@@ -1108,6 +1156,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func substringReplaceAttributes() {
         var str = AttributedString("Hello", attributes: .init().testInt(2).testString("Foundation"))
         str += " "
@@ -1123,6 +1172,7 @@ E {
         #expect(str == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func substringEquality() {
         let str = AttributedString("")
         let range = str.startIndex ..< str.endIndex
@@ -1136,6 +1186,7 @@ E {
         #expect(substringB == substringB)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func initializationFromSubstring() throws {
         var attrStr = AttributedString("yolo^+1 result<:s>^", attributes: AttributeContainer.testInt(2).testString("Hello"))
         attrStr.replaceSubrange(try #require(attrStr.range(of: "<:s>")), with: AttributedString(""))
@@ -1166,6 +1217,7 @@ E {
         var attributedString = AttributedString()
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func jsonEncoding() throws {
         let encoder = JSONEncoder()
         var attrStr = AttributedString("Hello", attributes: AttributeContainer().testBool(true).testString("blue").testInt(1))
@@ -1179,6 +1231,7 @@ E {
         #expect(decoded.attributedString == attrStr)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func decodingThenConvertingToNSAttributedString() throws {
         let encoder = JSONEncoder()
         var attrStr = AttributedString("Hello", attributes: AttributeContainer().testBool(true))
@@ -1193,6 +1246,7 @@ E {
         #expect(ns == decodedns)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func customAttributeCoding() throws {
         struct MyAttributes : AttributeScope {
             var customCodable : AttributeScopes.TestAttributes.CustomCodableAttribute
@@ -1215,6 +1269,7 @@ E {
         #expect(decoded.attributedString == attrStr)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func customCodableTypeWithCodableAttributedString() throws {
         struct MyType : Codable, Equatable {
             var other: NonCodableType
@@ -1254,6 +1309,7 @@ E {
         #expect(type == decoded)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func codingErrorsPropagateUpToCallSite() {
         enum CustomAttribute : CodableAttributedStringKey {
             typealias Value = String
@@ -1284,6 +1340,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func encodeWithPartiallyCodableScope() throws {
         enum NonCodableAttribute : AttributedStringKey {
             typealias Value = Int
@@ -1311,6 +1368,7 @@ E {
         #expect(decoded.str == expected)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func automaticCoding() throws {
         struct Obj : Codable, Equatable {
             @CodableConfiguration(from: AttributeScopes.TestAttributes.self) var attrStr = AttributedString()
@@ -1359,6 +1417,7 @@ E {
     }
 
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func manualCoding() throws {
         struct Obj : Codable, Equatable {
             var attrStr : AttributedString
@@ -1452,6 +1511,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func codableRawRepresentableAttribute() throws {
         struct Attribute : CodableAttributedStringKey {
             static let name = "MyAttribute"
@@ -1480,6 +1540,7 @@ E {
         #expect(decoded.str[Attribute.self] == .two)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func containerEncoding() throws {
         struct ContainerContainer : Codable {
             @CodableConfiguration(from: AttributeScopes.TestAttributes.self) var container = AttributeContainer()
@@ -1493,6 +1554,7 @@ E {
         #expect(obj.container == decoded.container)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func defaultAttributesCoding() throws {
         struct DefaultContainer : Codable, Equatable {
             var str : AttributedString
@@ -1506,6 +1568,7 @@ E {
         #expect(cont == decoded)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func decodingMultibyteCharacters() throws {
         let json = "{\"str\": [\"🎺ABC\", {\"TestInt\": 2}]}"
         struct Object : Codable {
@@ -1521,6 +1584,7 @@ E {
     
     // MARK: - Conversion Tests
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionToObjC() throws {
         var ourString = AttributedString("Hello", attributes: AttributeContainer().testInt(2))
         ourString += AttributedString(" ")
@@ -1532,6 +1596,7 @@ E {
         #expect(theirString == ourObjCString)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionFromObjC() throws {
         let nsString = NSMutableAttributedString(string: "Hello!")
         let rangeA = NSMakeRange(0, 3)
@@ -1545,6 +1610,7 @@ E {
         #expect(string == convertedString)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func roundTripConversion_boxed() throws {
         struct MyCustomType : Hashable {
             var num: Int
@@ -1569,6 +1635,7 @@ E {
         #expect(converted[MyCustomAttribute.self] == customVal)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func roundTripConversion_customConversion() throws {
         struct MyCustomType : Hashable { }
 
@@ -1595,6 +1662,7 @@ E {
         #expect(converted[MyCustomAttribute.self] == customVal)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func incompleteConversionFromObjC() throws {
         struct TestStringAttributeOnly : AttributeScope {
             var testString: AttributeScopes.TestAttributes.TestStringAttribute // Missing TestBoolAttribute
@@ -1612,6 +1680,7 @@ E {
         #expect(converted == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func incompleteConversionToObjC() throws {
         struct TestStringAttributeOnly : AttributeScope {
             var testString: AttributeScopes.TestAttributes.TestStringAttribute // Missing TestBoolAttribute
@@ -1625,6 +1694,7 @@ E {
         #expect(!attrs.keys.contains(.testBool))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionNestedScope() throws {
         struct SuperScope : AttributeScope {
             var subscope : SubScope
@@ -1647,6 +1717,7 @@ E {
         #expect(converted == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionAttributeContainers() throws {
         let container = AttributeContainer.testInt(2).testDouble(3.1).testString("Hello")
         
@@ -1665,6 +1736,7 @@ E {
         #expect(container == container2)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionFromInvalidObjectiveCValueTypes() throws {
         let nsStr = NSAttributedString(string: "Hello", attributes: [.testInt : "I am not an Int"])
         #expect(throws: (any Error).self) {
@@ -1696,6 +1768,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionToUTF16() throws {
         // Ensure that we're correctly using UTF16 offsets with NSAS and UTF8 offsets with AS without mixing the two
         let multiByteCharacters = ["\u{2029}", "\u{1D11E}", "\u{1D122}", "\u{1F91A}\u{1F3FB}"]
@@ -1713,6 +1786,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionWithoutScope() throws {
         // Ensure simple conversion works (no errors when loading AppKit/UIKit/SwiftUI)
         let attrStr = AttributedString()
@@ -1757,6 +1831,7 @@ E {
     }
     
     #if canImport(Accessibility)
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionWithoutScope_Accessibility() throws {
         let attributedString = AttributedString("Hello", attributes: .init().accessibilityTextCustom(["ABC"]))
         let nsAttributedString = NSAttributedString(attributedString)
@@ -1772,6 +1847,7 @@ E {
     #endif
     
     #if canImport(AppKit)
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionWithoutScope_AppKit() throws {
         var container = AttributeContainer()
         container.appKit.kern = 2.3
@@ -1784,6 +1860,7 @@ E {
     #endif
     
     #if canImport(UIKit)
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionWithoutScope_UIKit() throws {
         var container = AttributeContainer()
         container.uiKit.kern = 2.3
@@ -1796,6 +1873,7 @@ E {
     #endif
     
     #if canImport(SwiftUI)
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionWithoutScope_SwiftUI() throws {
         var container = AttributeContainer()
         container.swiftUI.kern = 2.3
@@ -1807,6 +1885,7 @@ E {
     }
     #endif
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func conversionCoalescing() throws {
         let nsStr = NSMutableAttributedString("Hello, world")
         nsStr.setAttributes([.link : NSURL(string: "http://apple.com")!, .testInt : NSNumber(integerLiteral: 2)], range: NSRange(location: 0, length: 6))
@@ -1818,6 +1897,7 @@ E {
         #expect(attrStr.link == nil)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func unalignedConversion() throws {
         let tests: [(NSRange, Int)] = [
             (NSRange(location: 0, length: 12), 1),
@@ -1842,6 +1922,7 @@ E {
 
     // MARK: - View Tests
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func charViewIndexing_backwardsFromEndIndex() {
         let testString = AttributedString("abcdefghi")
         let testChars = testString.characters
@@ -1849,6 +1930,7 @@ E {
         #expect(testChars[testIndex] == "i")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attrViewIndexing() {
         var attrStr = AttributedString("A")
         attrStr += "B"
@@ -1867,6 +1949,7 @@ E {
         #expect(attrStrRuns.count == 1)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func unicodeScalarsViewIndexing() {
         let attrStr = AttributedString("Cafe\u{301}", attributes: AttributeContainer().testInt(1))
         let unicode = attrStr.unicodeScalars
@@ -1874,6 +1957,7 @@ E {
         #expect(unicode[unicode.index(unicode.endIndex, offsetBy: -2)] == "e")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func characterSlicing() {
         let a: AttributedString = "\u{1f1fa}\u{1f1f8}" // Regional indicators U & S
         let i = a.unicodeScalars.index(after: a.startIndex)
@@ -1884,6 +1968,7 @@ E {
         #expect(b.count == 0)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func characterSlicing_RangeExpressions() {
         // Make sure `AttributedString` and `String` produce consistent results when slicing,
         // for every range expression, whether or not the bounds fall on `Character` boundaries.
@@ -1941,6 +2026,7 @@ E {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func unicodeScalarsSlicing() {
         let attrStr = AttributedString("Cafe\u{301}", attributes: AttributeContainer().testInt(1))
         let range = attrStr.startIndex ..< attrStr.endIndex
@@ -1962,6 +2048,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func protocolRunIndexing() {
         var str = AttributedString("Foo", attributes: .init().testInt(1))
         str += AttributedString("Bar", attributes: .init().testInt(2))
@@ -1984,6 +2071,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func runSliceSubscripting() {
         var str = AttributedString("Foo", attributes: .init().testInt(1))
         str += AttributedString("Bar", attributes: .init().testInt(2))
@@ -2014,6 +2102,7 @@ E {
 
     // MARK: - Other Tests
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func initWithSequence() {
         let expected = AttributedString("Hello World", attributes: AttributeContainer().testInt(2))
         let sequence: [Character] = ["H", "e", "l", "l", "o", " ", "W", "o", "r", "l", "d"]
@@ -2029,6 +2118,7 @@ E {
         #expect(attrStr3 == expected)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func longestEffectiveRangeOfAttribute() {
         var str = AttributedString("Abc")
         str += AttributedString("def", attributes: AttributeContainer.testInt(2).testString("World"))
@@ -2044,6 +2134,7 @@ E {
         #expect(range == expectedRange)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributeContainer() {
         var container = AttributeContainer().testBool(true).testInt(1)
         #expect(container.testBool == true)
@@ -2060,6 +2151,7 @@ E {
         #expect(container.testBool == nil)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributeContainerEquality() {
         let containerA = AttributeContainer().testInt(2).testString("test")
         let containerB = AttributeContainer().testInt(2).testString("test")
@@ -2074,6 +2166,7 @@ E {
         #expect(containerD == containerE)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributeContainerSetOnSubstring() {
         let container = AttributeContainer().testBool(true).testInt(1)
 
@@ -2088,6 +2181,7 @@ E {
         #expect(run.testString == "yellow")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func slice() {
         let attrStr = AttributedString("Hello World")
         let chars = attrStr.characters
@@ -2096,6 +2190,7 @@ E {
         #expect(AttributedString(slice) == AttributedString("World"))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func createStringsFromCharactersWithUnicodeScalarIndexes() {
         var attrStr = AttributedString("Caf", attributes: AttributeContainer().testString("a"))
         attrStr += AttributedString("e", attributes: AttributeContainer().testString("b"))
@@ -2112,6 +2207,7 @@ E {
         #expect(strs2 == ["Caf", "", "e\u{301}"])
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func settingAttributeOnSlice() throws {
         var attrString = AttributedString("This is a string.")
         var range = attrString.startIndex ..< attrString.characters.index(attrString.startIndex, offsetBy: 1)
@@ -2145,6 +2241,7 @@ E {
         #expect(attrString.characters[startIndex] == "D")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func expressibleByStringLiteral() {
         let variable : AttributedString = "Test"
         #expect(variable == AttributedString("Test"))
@@ -2155,6 +2252,7 @@ E {
         takesAttrStr("Test")
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func hashing() {
         let attrStr = AttributedString("Hello, world.", attributes: .init().testInt(2).testBool(false))
         let attrStr2 = AttributedString("Hello, world.", attributes: .init().testInt(2).testBool(false))
@@ -2170,6 +2268,7 @@ E {
         #expect(dictionary[attrStr2] == 456)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func hashingSubstring() {
         let a: AttributedString = "aXa"
         let b: AttributedString = "bXb"
@@ -2192,6 +2291,7 @@ E {
         #expect(hasherA.finalize() == hasherB.finalize())
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func hashingContainer() {
         let containerA = AttributeContainer.testInt(2).testBool(false)
         let containerB = AttributeContainer.testInt(2).testBool(false)
@@ -2207,6 +2307,7 @@ E {
         #expect(dictionary[containerB] == 456)
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func utf16String() {
         let multiByteCharacters = ["\u{2029}", "\u{1D11E}", "\u{1D122}", "\u{1F91A}\u{1F3FB}"]
         
@@ -2219,6 +2320,7 @@ E {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func plusOperators() {
         let ab = AttributedString("a") + AttributedString("b")
         #expect(ab == AttributedString("ab"))
@@ -2242,6 +2344,7 @@ E {
         #expect(abc_lit == abc)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func search() throws {
         let testString = AttributedString("abcdefghi")
         #expect(testString.range(of: "baba") == nil)
@@ -2287,6 +2390,7 @@ E {
         #expect(testString.range(of: "abc", options: [.anchored, .backwards]) == nil)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func substringSearch() throws {
         let fullString = AttributedString("___abcdefghi___")
         let testString = fullString[try #require(fullString.range(of: "abcdefghi"))]
@@ -2333,6 +2437,7 @@ E {
         #expect(testString.range(of: "abc", options: [.anchored, .backwards]) == nil)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func indexConversion() throws {
         let attrStr = AttributedString("ABCDE")
         let str = "ABCDE"
@@ -2349,6 +2454,7 @@ E {
     
 #if FOUNDATION_FRAMEWORK
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func rangeConversion() throws {
         let attrStr = AttributedString("ABCDE")
         let nsAS = NSAttributedString("ABCDE")
@@ -2367,6 +2473,7 @@ E {
         #expect(String(attrStr[attrStrR_reconverted2].characters) == "BCD")
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func unalignedRangeConversion() {
         do {
             // U+0301 Combining Acute Accent (one unicode scalar, one UTF-16)
@@ -2418,6 +2525,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func nsRangeConversionOnSlice() throws {
         let str = AttributedString("012345")
         let slice = str[str.index(str.startIndex, offsetByCharacters: 3) ..< str.endIndex]
@@ -2428,6 +2536,7 @@ E {
     
 #endif // FOUNDATION_FRAMEWORK
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func oobRangeConversion() {
         let attrStr = AttributedString("")
         let str = "Hello"
@@ -2437,6 +2546,7 @@ E {
     
 #if FOUNDATION_FRAMEWORK
     // TODO: Support scope-specific AttributedString initialization in FoundationPreview
+    @available(FoundationAttributedString 5.5, *)
     @Test func scopedCopy() {
         var str = AttributedString("A")
         str += AttributedString("B", attributes: .init().testInt(2))
@@ -2474,6 +2584,7 @@ E {
         #expect(AttributedString(str[range], including: None.self) == AttributedString("BC"))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func scopeIterationAPI() {
         struct TestScope : AttributeScope {
             let testInt: AttributeScopes.TestAttributes.TestIntAttribute
@@ -2492,6 +2603,7 @@ E {
     }
 #endif // FOUNDATION_FRAMEWORK
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func assignDifferentSubstring() {
         var attrStr1 = AttributedString("ABCDE")
         let attrStr2 = AttributedString("XYZ")
@@ -2501,6 +2613,7 @@ E {
         #expect(attrStr1 == "AXE")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func cowDuringSubstringMutation() {
         func frobnicate(_ sub: inout AttributedSubstring) {
             var new = sub
@@ -2516,6 +2629,7 @@ E {
     }
 
     #if FOUNDATION_EXIT_TESTS
+    @available(FoundationAttributedString 5.5, *)
     @Test func reassignmentDuringMutation() async {
         await #expect(processExitsWith: .failure) {
             func frobnicate(_ sub: inout AttributedSubstring) {
@@ -2528,6 +2642,7 @@ E {
     }
     #endif
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func assignDifferentCharacterView() {
         var attrStr1 = AttributedString("ABC", attributes: .init().testInt(1)) + AttributedString("DE", attributes: .init().testInt(3))
         let attrStr2 = AttributedString("XYZ", attributes: .init().testInt(2))
@@ -2536,6 +2651,7 @@ E {
         #expect(attrStr1 == AttributedString("XYZ", attributes: .init().testInt(1)))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func cowDuringCharactersMutation() {
         func frobnicate(_ chars: inout AttributedString.CharacterView) {
             var new = chars
@@ -2548,6 +2664,7 @@ E {
         #expect(attrStr == AttributedString("XYZ", attributes: .init().testInt(1)))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func assignDifferentUnicodeScalarView() {
         var attrStr1 = AttributedString("ABC", attributes: .init().testInt(1)) + AttributedString("DE", attributes: .init().testInt(3))
         let attrStr2 = AttributedString("XYZ", attributes: .init().testInt(2))
@@ -2556,6 +2673,7 @@ E {
         #expect(attrStr1 == AttributedString("XYZ", attributes: .init().testInt(1)))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func cowDuringUnicodeScalarsMutation() {
         func frobnicate(_ chars: inout AttributedString.CharacterView) {
             var new = chars
@@ -2568,6 +2686,7 @@ E {
         #expect(attrStr == AttributedString("XYZ", attributes: .init().testInt(1)))
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func utf88View() {
         let testStrings = [
             "Hello, world",
@@ -2597,6 +2716,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func utf16View() {
         let testStrings = [
             "Hello, world",
@@ -2626,6 +2746,7 @@ E {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributeContainerFiltering() {
         #expect(AttributeContainer().filter(runBoundaries: nil) == AttributeContainer())
         #expect(AttributeContainer().filter(runBoundaries: .paragraph) == AttributeContainer())
@@ -2647,6 +2768,7 @@ E {
         #expect(testConstrainedContainer.filter(inheritedByAddedText: false) == AttributeContainer.testNonExtended(5))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func writingDirectionBehavior() throws {
         // Indicate that this sentence is primarily right to left, because the English term "Swift" is embedded into an Arabic sentence.
         var string = AttributedString("Swift مذهل!", attributes: .init().writingDirection(.rightToLeft))
diff --git a/Tests/FoundationInternationalizationTests/Formatting/ByteCountFormatStyleTests.swift b/Tests/FoundationInternationalizationTests/Formatting/ByteCountFormatStyleTests.swift
index 40bea6531..14db109a7 100644
--- a/Tests/FoundationInternationalizationTests/Formatting/ByteCountFormatStyleTests.swift
+++ b/Tests/FoundationInternationalizationTests/Formatting/ByteCountFormatStyleTests.swift
@@ -20,6 +20,7 @@ private struct ByteCountFormatStyleTests {
     static let locales = [Locale(identifier: "en_US"), .init(identifier: "fr_FR"), .init(identifier: "zh_TW"), .init(identifier: "zh_CN"), .init(identifier: "ar")]
 
     @Test(arguments: locales)
+    @available(FoundationAttributedString 5.5, *)
     func zeroSpelledOutKb(locale: Locale) {
         let localizedZerosSpelledOutKb: [Locale: String] = [
             Locale(identifier: "en_US"): "Zero kB",
@@ -33,6 +34,7 @@ private struct ByteCountFormatStyleTests {
     }
 
     @Test(arguments: locales)
+    @available(FoundationAttributedString 5.5, *)
     func zeroSpelledOutBytes(locale: Locale) {
         let localizedZerosSpelledOutBytes: [Locale: String] = [
             Locale(identifier: "en_US"): "Zero bytes",
@@ -90,6 +92,7 @@ private struct ByteCountFormatStyleTests {
 
 #if FIXED_86386674
     @Test(arguments: locales)
+    @available(FoundationAttributedString 5.5, *)
     func singularUnitsBinary(locale: Locale) {
         for i in 0...5 {
             let value: Int64 = (1 << (i*10))
@@ -100,6 +103,7 @@ private struct ByteCountFormatStyleTests {
 
 #if FIXED_86386684
     @Test(arguments: locales)
+    @available(FoundationAttributedString 5.5, *)
     func singularUnitsDecimal(locale: Locale) {
         for i in 0...5 {
             #expect(Int64(pow(10.0, Double(i*3))).formatted(.byteCount(style: .file).locale(locale)) == localizedSingular[locale]![i])
@@ -107,19 +111,23 @@ private struct ByteCountFormatStyleTests {
     }
 #endif
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func localizedParens() {
         #expect(1024.formatted(.byteCount(style: ByteCountFormatStyle.Style.binary, includesActualByteCount: true).locale(.init(identifier: "zh_TW"))) == "1 kB（1,024 byte）")
         #expect(1024.formatted(.byteCount(style: ByteCountFormatStyle.Style.binary, includesActualByteCount: true).locale(.init(identifier: "en_US"))) == "1 kB (1,024 bytes)")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func actualByteCount() {
         #expect(1024.formatted(.byteCount(style: ByteCountFormatStyle.Style.file, includesActualByteCount: true).locale(.init(identifier: "en_US"))) == "1 kB (1,024 bytes)")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func rtl() {
         #expect(1024.formatted(.byteCount(style: ByteCountFormatStyle.Style.binary, includesActualByteCount: true).locale(.init(identifier: "ar_SA"))) == "١ كيلوبايت (١٬٠٢٤ بايت)")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributed() {
         var expected: [Segment]
 
@@ -187,6 +195,7 @@ private struct ByteCountFormatStyleTests {
     }
 
 #if !_pointerBitWidth(_32)
+    @available(FoundationAttributedString 5.5, *)
     @Test func testEveryAllowedUnit() {
         // 84270854: The largest unit supported currently is pb
         let expectations: [ByteCountFormatStyle.Units: String] = [
@@ -208,6 +217,7 @@ private struct ByteCountFormatStyleTests {
 #endif
 }
 
+@available(FoundationAttributedString 5.5, *)
 fileprivate struct Segment {
     let string: String
     let number: AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute.NumberPart?
@@ -228,6 +238,7 @@ fileprivate struct Segment {
 
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension Sequence where Element == Segment {
     var attributedString: AttributedString {
         self.map { segment in
diff --git a/Tests/FoundationInternationalizationTests/Formatting/DateFormatStyleTests.swift b/Tests/FoundationInternationalizationTests/Formatting/DateFormatStyleTests.swift
index 83dabb19f..babc46741 100644
--- a/Tests/FoundationInternationalizationTests/Formatting/DateFormatStyleTests.swift
+++ b/Tests/FoundationInternationalizationTests/Formatting/DateFormatStyleTests.swift
@@ -597,6 +597,7 @@ private struct DateFormatStyleTests {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension Sequence where Element == (String, AttributeScopes.FoundationAttributes.DateFieldAttribute.Field?) {
     var attributedString: AttributedString {
         self.map { pair in
@@ -609,8 +610,11 @@ extension Sequence where Element == (String, AttributeScopes.FoundationAttribute
 private struct DateAttributedFormatStyleTests {
     var enUSLocale = Locale(identifier: "en_US")
     var gmtTimeZone = TimeZone(secondsFromGMT: 0)!
-
+    
+    @available(FoundationAttributedString 5.5, *)
     typealias Segment = (String, AttributeScopes.FoundationAttributes.DateFieldAttribute.Field?)
+    
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributedFormatStyle() throws {
         let baseStyle = Date.FormatStyle(locale: enUSLocale, timeZone: gmtTimeZone)
         // dateFormatter.date(from: "2021-04-12 15:04:32")!
@@ -634,6 +638,7 @@ private struct DateAttributedFormatStyleTests {
         }
     }
     
+    @available(FoundationAttributedString 5.5, *)
     @Test func individualFields() throws {
         let baseStyle = Date.FormatStyle(locale: enUSLocale, timeZone: gmtTimeZone)
         // dateFormatter.date(from: "2021-04-12 15:04:32")!
@@ -660,6 +665,7 @@ private struct DateAttributedFormatStyleTests {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func codable() throws {
         let encoder = JSONEncoder()
         let decoder = JSONDecoder()
@@ -673,6 +679,7 @@ private struct DateAttributedFormatStyleTests {
         }
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func settingLocale() throws {
         // dateFormatter.date(from: "2021-04-12 15:04:32")!
         let date = Date(timeIntervalSinceReferenceDate: 639932672.0)
@@ -693,6 +700,7 @@ private struct DateAttributedFormatStyleTests {
     }
 
 #if FOUNDATION_FRAMEWORK
+    @available(FoundationAttributedString 5.5, *)
     @Test func formattingWithPrefsOverride() {
         let date = Date(timeIntervalSince1970: 0)
         let enUS = "en_US"
@@ -855,6 +863,7 @@ private struct DateAttributedFormatStyleTests {
 private struct DateVerbatimFormatStyleTests {
     var utcTimeZone = TimeZone(identifier: "UTC")!
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func formats() throws {
         // dateFormatter.date(from: "2021-01-23 14:51:20")!
         let date = Date(timeIntervalSinceReferenceDate: 633106280.0)
@@ -877,6 +886,7 @@ private struct DateVerbatimFormatStyleTests {
         verify("\(hour: .defaultDigits(clock: .twelveHour, hourCycle: .zeroBased)) heures et \(minute: .twoDigits) minutes", expected: "2 heures et 51 minutes")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func parseable() throws {
         // dateFormatter.date(from: "2021-01-23 14:51:20")!
         let date = Date(timeIntervalSinceReferenceDate: 633106280.0)
@@ -898,6 +908,7 @@ private struct DateVerbatimFormatStyleTests {
     }
 
     // Test parsing strings containing `abbreviated` names
+    @available(FoundationAttributedString 5.5, *)
     @Test func nonLenientParsingAbbreviatedNames() throws {
 
         // dateFormatter.date(from: "1970-01-01 00:00:00")!
@@ -941,6 +952,7 @@ private struct DateVerbatimFormatStyleTests {
 #endif // FIXED_ICU_74_DAYPERIOD
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func issue95845290() throws {
         let formatString: Date.FormatString = "\(weekday: .abbreviated) \(month: .abbreviated) \(day: .twoDigits) \(hour: .twoDigits(clock: .twentyFourHour, hourCycle: .zeroBased)):\(minute: .twoDigits):\(second: .twoDigits) \(timeZone: .iso8601(.short)) \(year: .defaultDigits)"
         let enGB = Locale(identifier: "en_GB")
@@ -958,9 +970,11 @@ private struct DateVerbatimFormatStyleTests {
             #expect(date == Date(timeIntervalSinceReferenceDate: 677261400.0))
         }
     }
-
+    
+    @available(FoundationAttributedString 5.5, *)
     typealias Segment = (String, AttributeScopes.FoundationAttributes.DateFieldAttribute.Field?)
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributedString() throws {
         // dateFormatter.date(from: "2021-01-23 14:51:20")!
         let date = Date(timeIntervalSinceReferenceDate: 633106280.0)
@@ -984,11 +998,13 @@ private struct DateVerbatimFormatStyleTests {
                  ("20", .second)])
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func storedVar() {
         _ = Date.FormatStyle.dateTime
         _ = Date.ISO8601FormatStyle.iso8601
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func allIndividualFields() {
         // dateFormatter.date(from: "2021-01-23 14:51:20")!
         let date = Date(timeIntervalSinceReferenceDate: 633106280.0)
diff --git a/Tests/FoundationInternationalizationTests/Formatting/DurationTimeFormatStyleTests.swift b/Tests/FoundationInternationalizationTests/Formatting/DurationTimeFormatStyleTests.swift
index b161b68bd..ab80b41bb 100644
--- a/Tests/FoundationInternationalizationTests/Formatting/DurationTimeFormatStyleTests.swift
+++ b/Tests/FoundationInternationalizationTests/Formatting/DurationTimeFormatStyleTests.swift
@@ -298,6 +298,7 @@ private struct DurationToMeasurementAdditionTests {
 private struct TestDurationTimeFormatStyle {
     let enUS = Locale(identifier: "en_US")
 
+    @available(FoundationAttributedString 5.5, *)
     func assertFormattedWithPattern(seconds: Int, milliseconds: Int = 0, pattern: Duration._TimeFormatStyle.Pattern, grouping: NumberFormatStyleConfiguration.Grouping? = nil, expected: String, sourceLocation: SourceLocation = #_sourceLocation) {
         var style = Duration.TimeFormatStyle(pattern: pattern).locale(enUS)
         if let grouping {
@@ -306,6 +307,7 @@ private struct TestDurationTimeFormatStyle {
         #expect(Duration(seconds: Int64(seconds), milliseconds: Int64(milliseconds)).formatted(style) == expected, sourceLocation: sourceLocation)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func durationPatternStyle() {
         assertFormattedWithPattern(seconds: 3695, pattern: .hourMinute, expected: "1:02")
         assertFormattedWithPattern(seconds: 3695, pattern: .hourMinute(padHourToLength: 1, roundSeconds: .down), expected: "1:01")
@@ -318,6 +320,7 @@ private struct TestDurationTimeFormatStyle {
         assertFormattedWithPattern(seconds: 3695, milliseconds: 350, pattern: .minuteSecond(padMinuteToLength: 2, fractionalSecondsLength: 2), expected: "61:35.35")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func durationPatternPadding() {
         assertFormattedWithPattern(seconds: 3695, pattern: .hourMinute(padHourToLength: 2), expected: "01:02")
         assertFormattedWithPattern(seconds: 3695, pattern: .hourMinuteSecond(padHourToLength: 2), expected: "01:01:35")
@@ -325,12 +328,14 @@ private struct TestDurationTimeFormatStyle {
         assertFormattedWithPattern(seconds: 3695, milliseconds: 500, pattern: .hourMinuteSecond(padHourToLength: 2, fractionalSecondsLength: 2), expected: "01:01:35.50")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func durationPatternGrouping() {
         assertFormattedWithPattern(seconds: 36950000, pattern: .hourMinute(padHourToLength: 2), grouping: nil, expected: "10,263:53")
         assertFormattedWithPattern(seconds: 36950000, pattern: .hourMinute(padHourToLength: 2), grouping: .automatic, expected: "10,263:53")
         assertFormattedWithPattern(seconds: 36950000, pattern: .hourMinute(padHourToLength: 2), grouping: .never, expected: "10263:53")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func noFractionParts() {
 
         // minutes, seconds
@@ -401,6 +406,7 @@ private struct TestDurationTimeFormatStyle {
         assertFormattedWithPattern(seconds: 5399, milliseconds: 500, pattern: .hourMinute, expected: "1:30")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func showFractionalSeconds() {
 
         // minutes, seconds
@@ -445,6 +451,7 @@ private struct TestDurationTimeFormatStyle {
         assertFormattedWithPattern(seconds: 7199, milliseconds: 995, pattern: .hourMinuteSecond(padHourToLength: 2, fractionalSecondsLength: 2), expected: "02:00:00.00")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func negativeValues() {
         assertFormattedWithPattern(seconds: 0, milliseconds: -499, pattern: .hourMinuteSecond, expected: "0:00:00")
         assertFormattedWithPattern(seconds: 0, milliseconds: -500, pattern: .hourMinuteSecond, expected: "0:00:00")
@@ -471,6 +478,7 @@ private struct TestDurationTimeFormatStyle {
 
 // MARK: - Attributed string test
 
+@available(FoundationAttributedString 5.5, *)
 extension Sequence where Element == DurationTimeAttributedStyleTests.Segment {
     var attributedString: AttributedString {
         self.map { tuple in
@@ -487,13 +495,17 @@ extension Sequence where Element == DurationTimeAttributedStyleTests.Segment {
 @Suite("Duration.TimeFormatStyle.Attributed")
 private struct DurationTimeAttributedStyleTests {
 
+    @available(FoundationAttributedString 5.5, *)
     typealias Segment = (String, AttributeScopes.FoundationAttributes.DurationFieldAttribute.Field?)
+    
     let enUS = Locale(identifier: "en_US")
 
+    @available(FoundationAttributedString 5.5, *)
     func assertWithPattern(seconds: Int, milliseconds: Int = 0, pattern: Duration._TimeFormatStyle.Pattern, expected: [Segment], locale: Locale = Locale(identifier: "en_US"), sourceLocation: SourceLocation = #_sourceLocation) {
         #expect(Duration(seconds: Int64(seconds), milliseconds: Int64(milliseconds)).formatted(.time(pattern: pattern).locale(locale).attributed) == expected.attributedString, sourceLocation: sourceLocation)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributedStyle_enUS() {
         assertWithPattern(seconds: 3695, pattern: .hourMinute, expected: [
             ("1", .hours),
@@ -558,6 +570,7 @@ private struct DurationTimeAttributedStyleTests {
 
 @Suite("Duration.TimeFormatStyle Discrete Conformance")
 private struct TestDurationTimeDiscreteConformance {
+    @available(FoundationAttributedString 5.5, *)
     @Test func basics() throws {
         var style: Duration.TimeFormatStyle
         style = .init(pattern: .minuteSecond(padMinuteToLength: 0, roundFractionalSeconds: .down)).locale(Locale(identifier: "en_US"))
@@ -642,6 +655,7 @@ private struct TestDurationTimeDiscreteConformance {
         #expect(style.discreteInput(before: .seconds(-1)) == .milliseconds(-1500))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func regressions() throws {
         var style: Duration._TimeFormatStyle
 
@@ -650,6 +664,7 @@ private struct TestDurationTimeDiscreteConformance {
         #expect(try #require(style.discreteInput(after: Duration(secondsComponent: -8, attosecondsComponent: -531546586433266880))) <= Duration(secondsComponent: 30, attosecondsComponent: 0))
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test(arguments:
         [FloatingPointRoundingRule.up, .down, .towardZero, .awayFromZero, .toNearestOrAwayFromZero, .toNearestOrEven].flatMap { roundingRule in
             [
diff --git a/Tests/FoundationInternationalizationTests/Formatting/DurationUnitsFormatStyleTests.swift b/Tests/FoundationInternationalizationTests/Formatting/DurationUnitsFormatStyleTests.swift
index c3bd0b1b6..979ed68c0 100644
--- a/Tests/FoundationInternationalizationTests/Formatting/DurationUnitsFormatStyleTests.swift
+++ b/Tests/FoundationInternationalizationTests/Formatting/DurationUnitsFormatStyleTests.swift
@@ -902,6 +902,7 @@ private struct DurationUnitsFormatStyleTests {
 
 // MARK: - Attributed string test
 
+@available(FoundationAttributedString 5.5, *)
 extension Sequence where Element == DurationUnitAttributedFormatStyleTests.Segment {
     var attributedString: AttributedString {
         self.map { tuple in
@@ -920,10 +921,13 @@ extension Sequence where Element == DurationUnitAttributedFormatStyleTests.Segme
 
 @Suite("Duration.UnitsFormatStyle.Attributed")
 private struct DurationUnitAttributedFormatStyleTests {
+    @available(FoundationAttributedString 5.5, *)
     typealias Segment = (String, AttributeScopes.FoundationAttributes.DurationFieldAttribute.Field?, AttributeScopes.FoundationAttributes.MeasurementAttribute.Component?)
+    
     let enUS = Locale(identifier: "en_US")
     let frFR = Locale(identifier: "fr_FR")
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func attributedStyle_enUS() {
         let d1 = Duration.seconds(2 * 3600 + 43 * 60 + 24) // 2hr 43min 24s
         let d2 = Duration.seconds(43 * 60 + 24) // 43min 24s
@@ -1091,6 +1095,7 @@ private struct DurationUnitAttributedFormatStyleTests {
                        ].attributedString)
     }
 
+    @available(FoundationAttributedString 5.5, *)
     @Test func testAttributedStyle_frFR() {
         let d1 = Duration.seconds(2 * 3600 + 43 * 60 + 24) // 2hr 43min 24s
         let d0 = Duration.seconds(0)
diff --git a/Tests/FoundationInternationalizationTests/Formatting/NumberFormatStyleTests.swift b/Tests/FoundationInternationalizationTests/Formatting/NumberFormatStyleTests.swift
index 2623c40cf..6325a8a69 100644
--- a/Tests/FoundationInternationalizationTests/Formatting/NumberFormatStyleTests.swift
+++ b/Tests/FoundationInternationalizationTests/Formatting/NumberFormatStyleTests.swift
@@ -261,7 +261,12 @@ final class NumberFormatStyleTests: XCTestCase {
         XCTAssertEqual(Float64.nan.formatted(.currency(code: "USD").locale(Locale(identifier: "uz_Cyrl"))), "ҳақиқий сон эмас US$")
     }
 
+    @available(FoundationAttributedString 5.5, *)
     func testFormattedAttributedLeadingDotSyntax() throws {
+        if #unavailable(FoundationAttributedString 5.5) {
+            throw XCTSkip("This test is not available on this OS version")
+        }
+        
         let int = 42
         XCTAssertEqual(int.formatted(.number.attributed), IntegerFormatStyle().attributed.format(int))
         XCTAssertEqual(int.formatted(.percent.attributed), IntegerFormatStyle.Percent().attributed.format(int))
@@ -1634,8 +1639,10 @@ final class IntegerFormatStyleExhaustiveTests: XCTestCase {
 
 // MARK: - Attributed string
 
+@available(FoundationAttributedString 5.5, *)
 fileprivate typealias Segment = (String, AttributeScopes.FoundationAttributes.NumberFormatAttributes.NumberPartAttribute.NumberPart?, AttributeScopes.FoundationAttributes.NumberFormatAttributes.SymbolAttribute.Symbol?)
 
+@available(FoundationAttributedString 5.5, *)
 extension Sequence where Element == Segment {
     var attributedString: AttributedString {
         self.map { tuple in
@@ -1652,15 +1659,23 @@ extension Sequence where Element == Segment {
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 extension AttributedString {
     fileprivate var string: String {
         String(self._guts.string)
     }
 }
 
+@available(FoundationAttributedString 5.5, *)
 class TestNumberAttributeFormatStyle: XCTestCase {
     let enUS = Locale(identifier: "en_US")
     let frFR = Locale(identifier: "fr_FR")
+    
+    override func setUpWithError() throws {
+        if #unavailable(FoundationAttributedString 5.5) {
+            throw XCTSkip("This test is not available on this OS version")
+        }
+    }
 
     func testIntegerStyle() throws {
         let style: IntegerFormatStyle<Int> = .init(locale: enUS)
@@ -2203,7 +2218,11 @@ extension NumberFormatStyleTests {
 
 extension NumberFormatStyleTests {
 
+    @available(FoundationAttributedString 5.5, *)
     func testIntegerFormatStyleBigNumberNoCrash() throws {
+        if #unavailable(FoundationAttributedString 5.5) {
+            throw XCTSkip("This test is not available on this OS version")
+        }
         let uint64Style: IntegerFormatStyle<UInt64> = .init(locale: enUSLocale)
         XCTAssertEqual(uint64Style.format(UInt64.max), "18,446,744,073,709,551,615")
         XCTAssertEqual(UInt64.max.formatted(.number.locale(enUSLocale)), "18,446,744,073,709,551,615")