From 212edf31ed7835c240f73c9f65d04cc05d2786fc Mon Sep 17 00:00:00 2001
From: Jack Wotherspoon <jackwoth@google.com>
Date: Tue, 14 Apr 2026 13:24:21 -0400
Subject: [PATCH] chore(mcp): check MCP error code over brittle string match
 (#25381)

---
 packages/core/src/tools/mcp-client.test.ts | 22 ++++++++++++++++++++++
 packages/core/src/tools/mcp-client.ts      | 16 +++++++++-------
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/packages/core/src/tools/mcp-client.test.ts b/packages/core/src/tools/mcp-client.test.ts
index 4a14b671a0..50b17aa735 100644
--- a/packages/core/src/tools/mcp-client.test.ts
+++ b/packages/core/src/tools/mcp-client.test.ts
@@ -20,6 +20,8 @@ import { MCPOAuthTokenStorage } from '../mcp/oauth-token-storage.js';
 import { OAuthUtils } from '../mcp/oauth-utils.js';
 import type { PromptRegistry } from '../prompts/prompt-registry.js';
 import {
+  ErrorCode,
+  McpError,
   PromptListChangedNotificationSchema,
   ResourceListChangedNotificationSchema,
   ToolListChangedNotificationSchema,
@@ -35,6 +37,7 @@ import {
   isEnabled,
   McpClient,
   populateMcpServerCommand,
+  discoverPrompts,
   type McpContext,
 } from './mcp-client.js';
 import type { ToolRegistry } from './tool-registry.js';
@@ -320,6 +323,25 @@ describe('mcp-client', () => {
       );
     });
 
+    it('should return empty array for discoverPrompts on MethodNotFound error without diagnostic', async () => {
+      const mockedClient = {
+        getServerCapabilities: vi.fn().mockReturnValue({ prompts: {} }),
+        listPrompts: vi
+          .fn()
+          .mockRejectedValue(
+            new McpError(ErrorCode.MethodNotFound, 'Method not supported'),
+          ),
+      };
+      const result = await discoverPrompts(
+        'test-server',
+        mockedClient as unknown as ClientLib.Client,
+        MOCK_CONTEXT,
+      );
+      expect(result).toEqual([]);
+      // MethodNotFound errors should be silently ignored regardless of message text
+      expect(MOCK_CONTEXT.emitMcpDiagnostic).not.toHaveBeenCalled();
+    });
+
     it('should not discover tools if server does not support them', async () => {
       const mockedClient = {
         connect: vi.fn(),
diff --git a/packages/core/src/tools/mcp-client.ts b/packages/core/src/tools/mcp-client.ts
index a7852050fc..0441063f81 100644
--- a/packages/core/src/tools/mcp-client.ts
+++ b/packages/core/src/tools/mcp-client.ts
@@ -27,6 +27,8 @@ import {
   ReadResourceResultSchema,
   ResourceListChangedNotificationSchema,
   ToolListChangedNotificationSchema,
+  ErrorCode,
+  McpError,
   PromptListChangedNotificationSchema,
   ProgressNotificationSchema,
   type GetPromptResult,
@@ -1250,6 +1252,10 @@ export async function connectAndDiscover(
   }
 }
 
+function isMcpMethodNotFoundError(error: unknown): boolean {
+  return error instanceof McpError && error.code === ErrorCode.MethodNotFound;
+}
+
 /**
  * Discovers and sanitizes tools from a connected MCP client.
  * It retrieves function declarations from the client, filters out disabled tools,
@@ -1329,10 +1335,7 @@ export async function discoverTools(
     }
     return discoveredTools;
   } catch (error) {
-    if (
-      error instanceof Error &&
-      !error.message?.includes('Method not found')
-    ) {
+    if (!isMcpMethodNotFoundError(error)) {
       cliConfig.emitMcpDiagnostic(
         'error',
         `Error discovering tools from ${mcpServerName}: ${getErrorMessage(
@@ -1456,8 +1459,7 @@ export async function discoverPrompts(
         ),
     }));
   } catch (error) {
-    // It's okay if the method is not found, which is a common case.
-    if (error instanceof Error && error.message?.includes('Method not found')) {
+    if (isMcpMethodNotFoundError(error)) {
       return [];
     }
     cliConfig.emitMcpDiagnostic(
@@ -1505,7 +1507,7 @@ async function listResources(
       cursor = response.nextCursor ?? undefined;
     } while (cursor);
   } catch (error) {
-    if (error instanceof Error && error.message?.includes('Method not found')) {
+    if (isMcpMethodNotFoundError(error)) {
       return [];
     }
     cliConfig.emitMcpDiagnostic(