Fix: Return empty Arrays for List methods when no handlers registered (#929)

packages/core/src/shared/protocol.ts

@@ -603,6 +603,17 @@ export abstract class Protocol<SendRequestT extends Request, SendNotificationT e
         }
     }
 
+    private _getDefaultListResponse(method: string): SendResultT | null {
+        const listMethodResponses: Record<string, string> = {
+            'prompts/list': 'prompts',
+            'resources/list': 'resources',
+            'resources/templates/list': 'resourceTemplates',
+            'tools/list': 'tools'
+        };
+        const responseKey = listMethodResponses[method];
+        return responseKey ? ({ [responseKey]: [] } as SendResultT) : null;
+    }
+
     /**
      * Attaches to the given transport, starts it, and starts listening for messages.
      *
@@ -684,6 +695,32 @@ export abstract class Protocol<SendRequestT extends Request, SendNotificationT e
         const relatedTaskId = request.params?._meta?.[RELATED_TASK_META_KEY]?.taskId;
 
         if (handler === undefined) {
+            const defaultResponse = this._getDefaultListResponse(request.method);
+            if (defaultResponse !== null) {
+                const successResponse: JSONRPCResultResponse = {
+                    jsonrpc: '2.0',
+                    id: request.id,
+                    result: defaultResponse
+                };
+
+                if (relatedTaskId && this._taskMessageQueue) {
+                    this._enqueueTaskMessage(
+                        relatedTaskId,
+                        {
+                            type: 'response',
+                            message: successResponse,
+                            timestamp: Date.now()
+                        },
+                        capturedTransport?.sessionId
+                    ).catch(error => this._onerror(new Error(`Failed to enqueue default list response: ${error}`)));
+                } else {
+                    capturedTransport
+                        ?.send(successResponse)
+                        .catch(error => this._onerror(new Error(`Failed to send default list response: ${error}`)));
+                }
+                return;
+            }
+
             const errorResponse: JSONRPCErrorResponse = {
                 jsonrpc: '2.0',
                 id: request.id,

test/integration/test/server.test.ts

@@ -3281,3 +3281,21 @@ test('should respect client task capabilities', async () => {
 
     clientTaskStore.cleanup();
 });
+
+test('should return empty arrays for list methods when server has capabilities but no handlers', async () => {
+    const server = new Server({ name: 'test server', version: '1.0' }, { capabilities: { prompts: {}, resources: {}, tools: {} } });
+
+    const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+    const client = new Client({ name: 'test client', version: '1.0' }, { capabilities: { sampling: {} } });
+
+    await Promise.all([client.connect(clientTransport), server.connect(serverTransport)]);
+
+    expect((await client.listPrompts()).prompts).toEqual([]);
+    expect((await client.listResources()).resources).toEqual([]);
+    expect((await client.listResourceTemplates()).resourceTemplates).toEqual([]);
+    expect((await client.listTools()).tools).toEqual([]);
+
+    await client.close();
+    await server.close();
+});