fix(core): show clear error when MCP server cwd does not exist (#3192)

* fix(core): show clear error when MCP server cwd does not exist

Validate that the configured cwd directory exists before spawning the
MCP server process. Previously, a non-existent cwd caused Node.js to
emit "spawn <cmd> ENOENT" — indistinguishable from the command binary
being missing. Now throws a descriptive error naming the server and
the missing path.

Fixes #3163

* test(core): add test for MCP stdio transport without cwd

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

@@ -23,6 +23,11 @@ import {
 } from './mcp-client.js';
 import type { ToolRegistry } from './tool-registry.js';
 
+const mockExistsSync = vi.hoisted(() => vi.fn(() => true));
+
+vi.mock('node:fs', () => ({
+  existsSync: mockExistsSync,
+}));
 vi.mock('@modelcontextprotocol/sdk/client/stdio.js');
 vi.mock('@modelcontextprotocol/sdk/client/index.js');
 vi.mock('@google/genai');
@@ -289,6 +294,46 @@ describe('mcp-client', () => {
       });
     });
 
+    it('should connect via command without cwd', async () => {
+      const mockedTransport = vi
+        .spyOn(SdkClientStdioLib, 'StdioClientTransport')
+        .mockReturnValue({} as SdkClientStdioLib.StdioClientTransport);
+
+      await createTransport(
+        'test-server',
+        {
+          command: 'test-command',
+          args: ['--foo', 'bar'],
+        },
+        false,
+      );
+
+      expect(mockedTransport).toHaveBeenCalledWith({
+        command: 'test-command',
+        args: ['--foo', 'bar'],
+        cwd: undefined,
+        env: expect.any(Object),
+        stderr: 'pipe',
+      });
+    });
+
+    it('should throw if cwd does not exist', async () => {
+      mockExistsSync.mockReturnValueOnce(false);
+
+      await expect(
+        createTransport(
+          'test-server',
+          {
+            command: 'test-command',
+            cwd: '/nonexistent/path',
+          },
+          false,
+        ),
+      ).rejects.toThrow(
+        "MCP server 'test-server': configured cwd does not exist: /nonexistent/path",
+      );
+    });
+
     describe('useGoogleCredentialProvider', () => {
       it('should use GoogleCredentialProvider when specified', async () => {
         const transport = await createTransport(

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

@@ -34,6 +34,7 @@ import { SdkControlClientTransport } from './sdk-control-client-transport.js';
 
 import type { FunctionDeclaration } from '@google/genai';
 import { mcpToTool } from '@google/genai';
+import { existsSync } from 'node:fs';
 import { basename } from 'node:path';
 import { pathToFileURL } from 'node:url';
 import { MCPOAuthProvider } from '../mcp/oauth-provider.js';
@@ -1402,6 +1403,12 @@ export async function createTransport(
   }
 
   if (mcpServerConfig.command) {
+    if (mcpServerConfig.cwd && !existsSync(mcpServerConfig.cwd)) {
+      throw new Error(
+        `MCP server '${mcpServerName}': configured cwd does not exist: ${mcpServerConfig.cwd}`,
+      );
+    }
+
     const transport = new StdioClientTransport({
       command: mcpServerConfig.command,
       args: mcpServerConfig.args || [],