feat: add SessionInfoUpdate to SessionUpdate enum

Add a new session_info_update variant to the SessionUpdate discriminated
union, allowing agents to notify clients about session metadata changes
(title, timestamps, custom metadata) in real-time.

Author: Sergey Ignatov

docs/protocol/schema.mdx

@@ -2421,6 +2421,32 @@ See protocol docs: [Session ID](https://agentclientprotocol.com/protocol/session
 
 **Type:** `string`
 
+## <span class="font-mono">SessionInfoUpdate</span>
+
+Update to session metadata. All fields are optional to support partial updates.
+
+Agents send this notification to update session information like title or custom metadata.
+This allows clients to display dynamic session names and track session state changes.
+
+**Type:** Object
+
+**Properties:**
+
+<ResponseField name="_meta" type={"object | null"} >
+  The _meta property is reserved by ACP to allow clients and agents to attach additional
+metadata to their interactions. Implementations MUST NOT make assumptions about values at
+these keys.
+
+See protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/extensibility)
+
+</ResponseField>
+<ResponseField name="title" type={"string | null"} >
+  Human-readable title for the session. Set to null to clear.
+</ResponseField>
+<ResponseField name="updatedAt" type={"string | null"} >
+  ISO 8601 timestamp of last activity. Set to null to clear.
+</ResponseField>
+
 ## <span class="font-mono">SessionMode</span>
 
 A mode the agent can operate in.
@@ -2712,6 +2738,31 @@ See protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/exte
 </Expandable>
 </ResponseField>
 
+<ResponseField name="session_info_update" type="object">
+Session metadata has been updated (title, timestamps, custom metadata)
+
+<Expandable title="Properties">
+
+<ResponseField name="_meta" type={"object | null"} >
+  The _meta property is reserved by ACP to allow clients and agents to attach additional
+metadata to their interactions. Implementations MUST NOT make assumptions about values at
+these keys.
+
+See protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/extensibility)
+
+</ResponseField>
+<ResponseField name="sessionUpdate" type={"string"} required>
+</ResponseField>
+<ResponseField name="title" type={"string | null"} >
+  Human-readable title for the session. Set to null to clear.
+</ResponseField>
+<ResponseField name="updatedAt" type={"string | null"} >
+  ISO 8601 timestamp of last activity. Set to null to clear.
+</ResponseField>
+
+</Expandable>
+</ResponseField>
+
 ## <span class="font-mono">StopReason</span>
 
 Reasons why an agent stops processing a prompt turn.

schema/schema.json

@@ -2067,6 +2067,25 @@
       "description": "A unique identifier for a conversation session between a client and agent.\n\nSessions maintain their own context, conversation history, and state,\nallowing multiple independent interactions with the same agent.\n\nSee protocol docs: [Session ID](https://agentclientprotocol.com/protocol/session-setup#session-id)",
       "type": "string"
     },
+    "SessionInfoUpdate": {
+      "description": "Update to session metadata. All fields are optional to support partial updates.\n\nAgents send this notification to update session information like title or custom metadata.\nThis allows clients to display dynamic session names and track session state changes.",
+      "properties": {
+        "_meta": {
+          "additionalProperties": true,
+          "description": "The _meta property is reserved by ACP to allow clients and agents to attach additional\nmetadata to their interactions. Implementations MUST NOT make assumptions about values at\nthese keys.\n\nSee protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/extensibility)",
+          "type": ["object", "null"]
+        },
+        "title": {
+          "description": "Human-readable title for the session. Set to null to clear.",
+          "type": ["string", "null"]
+        },
+        "updatedAt": {
+          "description": "ISO 8601 timestamp of last activity. Set to null to clear.",
+          "type": ["string", "null"]
+        }
+      },
+      "type": "object"
+    },
     "SessionMode": {
       "description": "A mode the agent can operate in.\n\nSee protocol docs: [Session Modes](https://agentclientprotocol.com/protocol/session-modes)",
       "properties": {
@@ -2282,6 +2301,22 @@
           },
           "required": ["sessionUpdate"],
           "type": "object"
+        },
+        {
+          "allOf": [
+            {
+              "$ref": "#/$defs/SessionInfoUpdate"
+            }
+          ],
+          "description": "Session metadata has been updated (title, timestamps, custom metadata)",
+          "properties": {
+            "sessionUpdate": {
+              "const": "session_info_update",
+              "type": "string"
+            }
+          },
+          "required": ["sessionUpdate"],
+          "type": "object"
         }
       ]
     },

src/client.rs

@@ -90,6 +90,8 @@ pub enum SessionUpdate {
     ///
     /// See protocol docs: [Session Modes](https://agentclientprotocol.com/protocol/session-modes)
     CurrentModeUpdate(CurrentModeUpdate),
+    /// Session metadata has been updated (title, timestamps, custom metadata)
+    SessionInfoUpdate(SessionInfoUpdate),
 }
 
 /// The current mode of the session has changed
@@ -131,6 +133,61 @@ impl CurrentModeUpdate {
     }
 }
 
+/// Update to session metadata. All fields are optional to support partial updates.
+///
+/// Agents send this notification to update session information like title or custom metadata.
+/// This allows clients to display dynamic session names and track session state changes.
+#[derive(Default, Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+#[serde(rename_all = "camelCase")]
+#[non_exhaustive]
+pub struct SessionInfoUpdate {
+    /// Human-readable title for the session. Set to null to clear.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub title: Option<String>,
+    /// ISO 8601 timestamp of last activity. Set to null to clear.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub updated_at: Option<String>,
+    /// The _meta property is reserved by ACP to allow clients and agents to attach additional
+    /// metadata to their interactions. Implementations MUST NOT make assumptions about values at
+    /// these keys.
+    ///
+    /// See protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/extensibility)
+    #[serde(skip_serializing_if = "Option::is_none", rename = "_meta")]
+    pub meta: Option<Meta>,
+}
+
+impl SessionInfoUpdate {
+    #[must_use]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Human-readable title for the session. Set to null to clear.
+    #[must_use]
+    pub fn title(mut self, title: impl IntoOption<String>) -> Self {
+        self.title = title.into_option();
+        self
+    }
+
+    /// ISO 8601 timestamp of last activity. Set to null to clear.
+    #[must_use]
+    pub fn updated_at(mut self, updated_at: impl IntoOption<String>) -> Self {
+        self.updated_at = updated_at.into_option();
+        self
+    }
+
+    /// The _meta property is reserved by ACP to allow clients and agents to attach additional
+    /// metadata to their interactions. Implementations MUST NOT make assumptions about values at
+    /// these keys.
+    ///
+    /// See protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/extensibility)
+    #[must_use]
+    pub fn meta(mut self, meta: impl IntoOption<Meta>) -> Self {
+        self.meta = meta.into_option();
+        self
+    }
+}
+
 /// A streamed item of content
 #[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq)]
 #[serde(rename_all = "camelCase")]