fix: prevent tools discovery error for prompt-only MCP servers (#10367)

Co-authored-by: Tommaso Sciortino <sciortino@gmail.com>

packages/core/src/tools/mcp-client.test.ts

@@ -43,6 +43,7 @@ describe('mcp-client', () => {
         getStatus: vi.fn(),
         registerCapabilities: vi.fn(),
         setRequestHandler: vi.fn(),
+        getServerCapabilities: vi.fn().mockReturnValue({ tools: {} }),
       };
       vi.mocked(ClientLib.Client).mockReturnValue(
         mockedClient as unknown as ClientLib.Client,
@@ -88,6 +89,7 @@ describe('mcp-client', () => {
         getStatus: vi.fn(),
         registerCapabilities: vi.fn(),
         setRequestHandler: vi.fn(),
+        getServerCapabilities: vi.fn().mockReturnValue({ tools: {} }),
         tool: vi.fn(),
       };
       vi.mocked(ClientLib.Client).mockReturnValue(
@@ -183,6 +185,91 @@ describe('mcp-client', () => {
       );
       consoleErrorSpy.mockRestore();
     });
+
+    it('should not discover tools if server does not support them', async () => {
+      const mockedClient = {
+        connect: vi.fn(),
+        discover: vi.fn(),
+        disconnect: vi.fn(),
+        getStatus: vi.fn(),
+        registerCapabilities: vi.fn(),
+        setRequestHandler: vi.fn(),
+        getServerCapabilities: vi.fn().mockReturnValue({ prompts: {} }),
+        request: vi.fn().mockResolvedValue({ prompts: [] }),
+      };
+      vi.mocked(ClientLib.Client).mockReturnValue(
+        mockedClient as unknown as ClientLib.Client,
+      );
+      vi.spyOn(SdkClientStdioLib, 'StdioClientTransport').mockReturnValue(
+        {} as SdkClientStdioLib.StdioClientTransport,
+      );
+      const mockedMcpToTool = vi.mocked(GenAiLib.mcpToTool);
+      const mockedToolRegistry = {
+        registerTool: vi.fn(),
+      } as unknown as ToolRegistry;
+      const client = new McpClient(
+        'test-server',
+        {
+          command: 'test-command',
+        },
+        mockedToolRegistry,
+        {} as PromptRegistry,
+        {} as WorkspaceContext,
+        false,
+      );
+      await client.connect();
+      await expect(client.discover({} as Config)).rejects.toThrow(
+        'No prompts or tools found on the server.',
+      );
+      expect(mockedMcpToTool).not.toHaveBeenCalled();
+    });
+
+    it('should discover tools if server supports them', async () => {
+      const mockedClient = {
+        connect: vi.fn(),
+        discover: vi.fn(),
+        disconnect: vi.fn(),
+        getStatus: vi.fn(),
+        registerCapabilities: vi.fn(),
+        setRequestHandler: vi.fn(),
+        getServerCapabilities: vi.fn().mockReturnValue({ tools: {} }),
+        request: vi.fn().mockResolvedValue({ prompts: [] }),
+      };
+      vi.mocked(ClientLib.Client).mockReturnValue(
+        mockedClient as unknown as ClientLib.Client,
+      );
+      vi.spyOn(SdkClientStdioLib, 'StdioClientTransport').mockReturnValue(
+        {} as SdkClientStdioLib.StdioClientTransport,
+      );
+      const mockedMcpToTool = vi.mocked(GenAiLib.mcpToTool).mockReturnValue({
+        tool: () =>
+          Promise.resolve({
+            functionDeclarations: [
+              {
+                name: 'testTool',
+                description: 'A test tool',
+              },
+            ],
+          }),
+      } as unknown as GenAiLib.CallableTool);
+      const mockedToolRegistry = {
+        registerTool: vi.fn(),
+      } as unknown as ToolRegistry;
+      const client = new McpClient(
+        'test-server',
+        {
+          command: 'test-command',
+        },
+        mockedToolRegistry,
+        {} as PromptRegistry,
+        {} as WorkspaceContext,
+        false,
+      );
+      await client.connect();
+      await client.discover({} as Config);
+      expect(mockedMcpToTool).toHaveBeenCalledOnce();
+      expect(mockedToolRegistry.registerTool).toHaveBeenCalledOnce();
+    });
   });
   describe('appendMcpServerCommand', () => {
     it('should do nothing if no MCP servers or command are configured', () => {

packages/core/src/tools/mcp-client.ts

@@ -584,6 +584,9 @@ export async function discoverTools(
   cliConfig: Config,
 ): Promise<DiscoveredMCPTool[]> {
   try {
+    // Only request tools if the server supports them.
+    if (mcpClient.getServerCapabilities()?.tools == null) return [];
+
     const mcpCallableTool = mcpToTool(mcpClient, {
       timeout: mcpServerConfig.timeout ?? MCP_DEFAULT_TIMEOUT_MSEC,
     });