From 6a72cd064bccb5fda4618671c2da63c4e22c1ef9 Mon Sep 17 00:00:00 2001
From: Jacob MacDonald <jakemac@google.com>
Date: Tue, 5 Aug 2025 15:50:30 -0700
Subject: [PATCH] check for the prompt capability before listing prompts from
 MCP servers (#5616)

Co-authored-by: Jacob Richman <jacob314@gmail.com>
Co-authored-by: Sandy Tao <sandytao520@icloud.com>
---
 packages/core/src/tools/mcp-client.test.ts | 51 ++++++++++++++++++----
 packages/core/src/tools/mcp-client.ts      |  3 ++
 2 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/packages/core/src/tools/mcp-client.test.ts b/packages/core/src/tools/mcp-client.test.ts
index a8289d3b..9997d60e 100644
--- a/packages/core/src/tools/mcp-client.test.ts
+++ b/packages/core/src/tools/mcp-client.test.ts
@@ -58,9 +58,7 @@ describe('mcp-client', () => {
       const mockedClient = {} as unknown as ClientLib.Client;
       const consoleErrorSpy = vi
         .spyOn(console, 'error')
-        .mockImplementation(() => {
-          // no-op
-        });
+        .mockImplementation(() => {});
 
       const testError = new Error('Invalid tool name');
       vi.mocked(DiscoveredMCPTool).mockImplementation(
@@ -113,12 +111,17 @@ describe('mcp-client', () => {
           { name: 'prompt2' },
         ],
       });
+      const mockGetServerCapabilities = vi.fn().mockReturnValue({
+        prompts: {},
+      });
       const mockedClient = {
+        getServerCapabilities: mockGetServerCapabilities,
         request: mockRequest,
       } as unknown as ClientLib.Client;
 
       await discoverPrompts('test-server', mockedClient, mockedPromptRegistry);
 
+      expect(mockGetServerCapabilities).toHaveBeenCalledOnce();
       expect(mockRequest).toHaveBeenCalledWith(
         { method: 'prompts/list', params: {} },
         expect.anything(),
@@ -129,37 +132,67 @@ describe('mcp-client', () => {
       const mockRequest = vi.fn().mockResolvedValue({
         prompts: [],
       });
+      const mockGetServerCapabilities = vi.fn().mockReturnValue({
+        prompts: {},
+      });
+
       const mockedClient = {
+        getServerCapabilities: mockGetServerCapabilities,
         request: mockRequest,
       } as unknown as ClientLib.Client;
 
       const consoleLogSpy = vi
         .spyOn(console, 'debug')
-        .mockImplementation(() => {
-          // no-op
-        });
+        .mockImplementation(() => {});
 
       await discoverPrompts('test-server', mockedClient, mockedPromptRegistry);
 
+      expect(mockGetServerCapabilities).toHaveBeenCalledOnce();
       expect(mockRequest).toHaveBeenCalledOnce();
       expect(consoleLogSpy).not.toHaveBeenCalled();
 
       consoleLogSpy.mockRestore();
     });
 
+    it('should do nothing if the server has no prompt support', async () => {
+      const mockRequest = vi.fn().mockResolvedValue({
+        prompts: [],
+      });
+      const mockGetServerCapabilities = vi.fn().mockReturnValue({});
+
+      const mockedClient = {
+        getServerCapabilities: mockGetServerCapabilities,
+        request: mockRequest,
+      } as unknown as ClientLib.Client;
+
+      const consoleLogSpy = vi
+        .spyOn(console, 'debug')
+        .mockImplementation(() => {});
+
+      await discoverPrompts('test-server', mockedClient, mockedPromptRegistry);
+
+      expect(mockGetServerCapabilities).toHaveBeenCalledOnce();
+      expect(mockRequest).not.toHaveBeenCalled();
+      expect(consoleLogSpy).not.toHaveBeenCalled();
+
+      consoleLogSpy.mockRestore();
+    });
+
     it('should log an error if discovery fails', async () => {
       const testError = new Error('test error');
       testError.message = 'test error';
       const mockRequest = vi.fn().mockRejectedValue(testError);
+      const mockGetServerCapabilities = vi.fn().mockReturnValue({
+        prompts: {},
+      });
       const mockedClient = {
+        getServerCapabilities: mockGetServerCapabilities,
         request: mockRequest,
       } as unknown as ClientLib.Client;
 
       const consoleErrorSpy = vi
         .spyOn(console, 'error')
-        .mockImplementation(() => {
-          // no-op
-        });
+        .mockImplementation(() => {});
 
       await discoverPrompts('test-server', mockedClient, mockedPromptRegistry);
 
diff --git a/packages/core/src/tools/mcp-client.ts b/packages/core/src/tools/mcp-client.ts
index 00f2197a..26244d9e 100644
--- a/packages/core/src/tools/mcp-client.ts
+++ b/packages/core/src/tools/mcp-client.ts
@@ -496,6 +496,9 @@ export async function discoverPrompts(
   promptRegistry: PromptRegistry,
 ): Promise<Prompt[]> {
   try {
+    // Only request prompts if the server supports them.
+    if (mcpClient.getServerCapabilities()?.prompts == null) return [];
+
     const response = await mcpClient.request(
       { method: 'prompts/list', params: {} },
       ListPromptsResultSchema,