I think this would be the best approach once we shouldn't change the public dap.

Author: thaystg

src/adapter/debugAdapter.ts

@@ -39,9 +39,6 @@ import { VariableStore } from './variableStore';
 
 const localize = nls.loadMessageBundle();
 
-export interface DapCapabilitiesExtended extends Dap.Capabilities {
-  supportsDebuggerProperties?: boolean;
-}
 // This class collects configuration issued before "launch" request,
 // to be applied after launch.
 export class DebugAdapter implements IDisposable {
@@ -168,7 +165,7 @@ export class DebugAdapter implements IDisposable {
     return capabilities;
   }
 
-  static capabilities(): DapCapabilitiesExtended {
+  static capabilities(): Dap.CapabilitiesExtended {
     return {
       supportsConfigurationDoneRequest: true,
       supportsFunctionBreakpoints: false,

src/build/dapCustom.ts

@@ -578,6 +578,34 @@ const dapCustom: JSONSchema4 = {
       description:
         'Arguments for "setDebuggerProperty" request. Properties are determined by debugger.',
     },
+
+    ...makeRequest(
+      'capabilitiesExtended',
+      'The event indicates that one or more capabilities have changed.',
+      {
+        properties: {
+          params: {
+            $ref: '#/definitions/CapabilitiesExtended',
+          },
+        },
+        required: ['params'],
+      },
+    ),
+
+    CapabilitiesExtended: {
+      allOf: [
+        { $ref: '#/definitions/Capabilities' },
+        {
+          type: 'object',
+          description: 'Extension of Capabilities defined in public DAP',
+          properties: {
+            supportsDebuggerProperties: {
+              type: 'boolean',
+            },
+          },
+        },
+      ],
+    },
   },
 };
 

src/dap/api.d.ts

@@ -1113,6 +1113,20 @@ export namespace Dap {
     setDebuggerPropertyRequest(
       params: SetDebuggerPropertyParams,
     ): Promise<SetDebuggerPropertyResult>;
+
+    /**
+     * The event indicates that one or more capabilities have changed.
+     */
+    on(
+      request: 'capabilitiesExtended',
+      handler: (params: CapabilitiesExtendedParams) => Promise<CapabilitiesExtendedResult | Error>,
+    ): () => void;
+    /**
+     * The event indicates that one or more capabilities have changed.
+     */
+    capabilitiesExtendedRequest(
+      params: CapabilitiesExtendedParams,
+    ): Promise<CapabilitiesExtendedResult>;
   }
 
   export interface TestApi {
@@ -1858,6 +1872,11 @@ export namespace Dap {
      * Sets debugger properties.
      */
     setDebuggerProperty(params: SetDebuggerPropertyParams): Promise<SetDebuggerPropertyResult>;
+
+    /**
+     * The event indicates that one or more capabilities have changed.
+     */
+    capabilitiesExtended(params: CapabilitiesExtendedParams): Promise<CapabilitiesExtendedResult>;
   }
 
   export interface AttachParams {
@@ -1940,6 +1959,12 @@ export namespace Dap {
     capabilities: Capabilities;
   }
 
+  export interface CapabilitiesExtendedParams {
+    params: CapabilitiesExtended;
+  }
+
+  export interface CapabilitiesExtendedResult {}
+
   export interface CompletionsParams {
     /**
      * Returns completions in the scope of this stack frame. If not specified, the completions are returned for the global scope.
@@ -4611,6 +4636,10 @@ export namespace Dap {
    */
   export type CompletionItemType = string;
 
+  export interface CapabilitiesExtended extends Capabilities {
+    supportsDebuggerProperties?: boolean;
+  }
+
   /**
    * Information about the capabilities of a debug adapter.
    */

src/dap/telemetryClassification.d.ts

@@ -132,4 +132,6 @@ interface IDAPOperationClassification {
   '!setsourcemapstepping.errors': { classification: 'CallstackOrException'; purpose: 'PerformanceAndHealth' };
   setdebuggerproperty: { classification: 'SystemMetaData'; purpose: 'PerformanceAndHealth' };
   '!setdebuggerproperty.errors': { classification: 'CallstackOrException'; purpose: 'PerformanceAndHealth' };
+  capabilitiesextended: { classification: 'SystemMetaData'; purpose: 'PerformanceAndHealth' };
+  '!capabilitiesextended.errors': { classification: 'CallstackOrException'; purpose: 'PerformanceAndHealth' };
 }