diff --git a/packages/core/src/types/spec.types.ts b/packages/core/src/types/spec.types.ts
index f36434bef..ba849c546 100644
--- a/packages/core/src/types/spec.types.ts
+++ b/packages/core/src/types/spec.types.ts
@@ -586,7 +586,7 @@ export interface ClientCapabilities {
      * @example Extensions — UI extension with MIME type support
      * {@includeCode ./examples/ClientCapabilities/extensions-ui-mime-types.json}
      */
-    extensions?: { [key: string]: object };
+    extensions?: { [key: string]: JSONObject };
 }
 
 /**
@@ -703,7 +703,7 @@ export interface ServerCapabilities {
      * @example Extensions — UI extension support
      * {@includeCode ./examples/ServerCapabilities/extensions-ui.json}
      */
-    extensions?: { [key: string]: object };
+    extensions?: { [key: string]: JSONObject };
 }
 
 /**
diff --git a/packages/core/src/types/types.ts b/packages/core/src/types/types.ts
index 98216a4e0..967092d3f 100644
--- a/packages/core/src/types/types.ts
+++ b/packages/core/src/types/types.ts
@@ -556,7 +556,11 @@ 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(), JSONObjectSchema).optional()
 });
 
 export const InitializeRequestParamsSchema = BaseRequestParamsSchema.extend({
@@ -634,7 +638,11 @@ export const ServerCapabilitiesSchema = z.object({
     /**
      * Present if the server supports task creation.
      */
-    tasks: ServerTasksCapabilitySchema.optional()
+    tasks: ServerTasksCapabilitySchema.optional(),
+    /**
+     * Extensions that the server supports. Keys are extension identifiers (vendor-prefix/extension-name).
+     */
+    extensions: z.record(z.string(), JSONObjectSchema).optional()
 });
 
 /**
diff --git a/test/integration/test/server/mcp.test.ts b/test/integration/test/server/mcp.test.ts
index 4ea04beae..b58149716 100644
--- a/test/integration/test/server/mcp.test.ts
+++ b/test/integration/test/server/mcp.test.ts
@@ -338,6 +338,60 @@ describe('Zod v4', () => {
                 message: 'Completed step 3 of 3'
             });
         });
+
+        /***
+         * Test: Extensions capability registration
+         */
+        test('should register and advertise server extensions capability', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+            const client = new Client({
+                name: 'test client',
+                version: '1.0'
+            });
+
+            mcpServer.server.registerCapabilities({
+                extensions: {
+                    'io.modelcontextprotocol/test-extension': { listChanged: true }
+                }
+            });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
+
+            const capabilities = client.getServerCapabilities();
+            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', () => {