feat(client): return empty lists when server lacks capability

.changeset/respect-capability-negotiation.md

@@ -0,0 +1,13 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Respect capability negotiation in list methods by returning empty lists when server lacks capability
+
+The Client now returns empty lists instead of sending requests to servers that don't advertise the corresponding capability:
+- `listPrompts()` returns `{ prompts: [] }` if server lacks prompts capability
+- `listResources()` returns `{ resources: [] }` if server lacks resources capability
+- `listResourceTemplates()` returns `{ resourceTemplates: [] }` if server lacks resources capability
+- `listTools()` returns `{ tools: [] }` if server lacks tools capability
+
+This respects the MCP spec requirement that "Both parties SHOULD respect capability negotiation" and avoids unnecessary server warnings and traffic. The existing `enforceStrictCapabilities` option continues to throw errors when set to `true`.

packages/client/src/client/client.ts

@@ -253,6 +253,7 @@ export class Client<
     private _experimental?: { tasks: ExperimentalClientTasks<RequestT, NotificationT, ResultT> };
     private _listChangedDebounceTimers: Map<string, ReturnType<typeof setTimeout>> = new Map();
     private _pendingListChangedConfig?: ListChangedHandlers;
+    private _enforceStrictCapabilities: boolean;
 
     /**
      * Initializes this client with the given name and version information.
@@ -264,6 +265,7 @@ export class Client<
         super(options);
         this._capabilities = options?.capabilities ?? {};
         this._jsonSchemaValidator = options?.jsonSchemaValidator ?? new AjvJsonSchemaValidator();
+        this._enforceStrictCapabilities = options?.enforceStrictCapabilities ?? false;
 
         // Store list changed config for setup after connection (when we know server capabilities)
         if (options?.listChanged) {
@@ -728,14 +730,31 @@ export class Client<
     }
 
     async listPrompts(params?: ListPromptsRequest['params'], options?: RequestOptions) {
+        if (!this._serverCapabilities?.prompts && !this._enforceStrictCapabilities) {
+            // Respect capability negotiation: server does not support prompts
+            console.debug('Client.listPrompts() called but server does not advertise prompts capability - returning empty list');
+            return { prompts: [] };
+        }
         return this.request({ method: 'prompts/list', params }, ListPromptsResultSchema, options);
     }
 
     async listResources(params?: ListResourcesRequest['params'], options?: RequestOptions) {
+        if (!this._serverCapabilities?.resources && !this._enforceStrictCapabilities) {
+            // Respect capability negotiation: server does not support resources
+            console.debug('Client.listResources() called but server does not advertise resources capability - returning empty list');
+            return { resources: [] };
+        }
         return this.request({ method: 'resources/list', params }, ListResourcesResultSchema, options);
     }
 
     async listResourceTemplates(params?: ListResourceTemplatesRequest['params'], options?: RequestOptions) {
+        if (!this._serverCapabilities?.resources && !this._enforceStrictCapabilities) {
+            // Respect capability negotiation: server does not support resources
+            console.debug(
+                'Client.listResourceTemplates() called but server does not advertise resources capability - returning empty list'
+            );
+            return { resourceTemplates: [] };
+        }
         return this.request({ method: 'resources/templates/list', params }, ListResourceTemplatesResultSchema, options);
     }
 
@@ -860,6 +879,11 @@ export class Client<
     }
 
     async listTools(params?: ListToolsRequest['params'], options?: RequestOptions) {
+        if (!this._serverCapabilities?.tools && !this._enforceStrictCapabilities) {
+            // Respect capability negotiation: server does not support tools
+            console.debug('Client.listTools() called but server does not advertise tools capability - returning empty list');
+            return { tools: [] };
+        }
         const result = await this.request({ method: 'tools/list', params }, ListToolsResultSchema, options);
 
         // Cache the tools and their output schemas for future validation

test/integration/test/client/client.test.ts

@@ -595,6 +595,79 @@ test('should respect server capabilities', async () => {
     ).rejects.toThrow('Server does not support completions');
 });
 
+/***
+ * Test: Return empty lists for missing capabilities (default behavior)
+ * When enforceStrictCapabilities is not set (default), list methods should
+ * return empty lists instead of sending requests to servers that don't
+ * advertise those capabilities.
+ */
+test('should return empty lists for missing capabilities by default', async () => {
+    const server = new Server(
+        {
+            name: 'test server',
+            version: '1.0'
+        },
+        {
+            capabilities: {
+                // Server only supports tools - no prompts or resources
+                tools: {}
+            }
+        }
+    );
+
+    server.setRequestHandler(InitializeRequestSchema, _request => ({
+        protocolVersion: LATEST_PROTOCOL_VERSION,
+        capabilities: {
+            tools: {}
+        },
+        serverInfo: {
+            name: 'test',
+            version: '1.0'
+        }
+    }));
+
+    server.setRequestHandler(ListToolsRequestSchema, () => ({
+        tools: [{ name: 'test-tool', inputSchema: { type: 'object' } }]
+    }));
+
+    const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+    // Client with default settings (enforceStrictCapabilities not set)
+    const client = new Client(
+        {
+            name: 'test client',
+            version: '1.0'
+        },
+        {
+            capabilities: {}
+        }
+    );
+
+    await Promise.all([client.connect(clientTransport), server.connect(serverTransport)]);
+
+    // Server only supports tools
+    expect(client.getServerCapabilities()).toEqual({
+        tools: {}
+    });
+
+    // listTools should work and return actual tools
+    const toolsResult = await client.listTools();
+    expect(toolsResult.tools).toHaveLength(1);
+    expect(toolsResult.tools[0].name).toBe('test-tool');
+
+    // listPrompts should return empty list without sending request
+    const promptsResult = await client.listPrompts();
+    expect(promptsResult.prompts).toEqual([]);
+
+    // listResources should return empty list without sending request
+    const resourcesResult = await client.listResources();
+    expect(resourcesResult.resources).toEqual([]);
+
+    // listResourceTemplates should return empty list without sending request
+    const templatesResult = await client.listResourceTemplates();
+    expect(templatesResult.resourceTemplates).toEqual([]);
+});
+
 /***
  * Test: Respect Client Notification Capabilities
  */
@@ -1885,7 +1958,7 @@ describe('outputSchema validation', () => {
         // Set up server handlers
         server.setRequestHandler(InitializeRequestSchema, async request => ({
             protocolVersion: request.params.protocolVersion,
-            capabilities: {},
+            capabilities: { tools: {} },
             serverInfo: {
                 name: 'test-server',
                 version: '1.0.0'
@@ -1977,7 +2050,7 @@ describe('outputSchema validation', () => {
         // Set up server handlers
         server.setRequestHandler(InitializeRequestSchema, async request => ({
             protocolVersion: request.params.protocolVersion,
-            capabilities: {},
+            capabilities: { tools: {} },
             serverInfo: {
                 name: 'test-server',
                 version: '1.0.0'
@@ -2270,7 +2343,7 @@ describe('outputSchema validation', () => {
         // Set up server handlers
         server.setRequestHandler(InitializeRequestSchema, async request => ({
             protocolVersion: request.params.protocolVersion,
-            capabilities: {},
+            capabilities: { tools: {} },
             serverInfo: {
                 name: 'test-server',
                 version: '1.0.0'