Add extensions field to ClientCapabilities

Addresses PR feedback: extensions need to be on both client and server
capability objects per SEP-2133.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chughtapan

packages/core/src/types/types.ts

@@ -526,7 +526,11 @@ export const ClientCapabilitiesSchema = z.object({
     /**
      * Present if the client supports task creation.
      */
-    tasks: ClientTasksCapabilitySchema.optional()
+    tasks: ClientTasksCapabilitySchema.optional(),
+    /**
+     * Extensions that the client supports. Keys are extension identifiers (vendor-prefix/extension-name).
+     */
+    extensions: z.record(z.string(), AssertObjectSchema).optional()
 });
 
 export const InitializeRequestParamsSchema = BaseRequestParamsSchema.extend({

test/integration/test/server/mcp.test.ts

@@ -342,7 +342,7 @@ describe('Zod v4', () => {
         /***
          * Test: Extensions capability registration
          */
-        test('should register and advertise extensions capability', async () => {
+        test('should register and advertise server extensions capability', async () => {
             const mcpServer = new McpServer({
                 name: 'test server',
                 version: '1.0'
@@ -365,6 +365,33 @@ describe('Zod v4', () => {
             expect(capabilities?.extensions).toBeDefined();
             expect(capabilities?.extensions?.['io.modelcontextprotocol/test-extension']).toEqual({ listChanged: true });
         });
+
+        test('should advertise client extensions capability to server', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+            const client = new Client(
+                {
+                    name: 'test client',
+                    version: '1.0'
+                },
+                {
+                    capabilities: {
+                        extensions: {
+                            'io.modelcontextprotocol/test-extension': { streaming: true }
+                        }
+                    }
+                }
+            );
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
+
+            const capabilities = mcpServer.server.getClientCapabilities();
+            expect(capabilities?.extensions).toBeDefined();
+            expect(capabilities?.extensions?.['io.modelcontextprotocol/test-extension']).toEqual({ streaming: true });
+        });
     });
 
     describe('ResourceTemplate', () => {