fix(client): add missing Windows env vars to DEFAULT_INHERITED_ENV_VARS

.changeset/fix-default-inherited-env-vars-windows.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Add missing Windows environment variables to `DEFAULT_INHERITED_ENV_VARS`: `PATHEXT`, `COMSPEC`, `PROGRAMFILES(X86)`, `PROGRAMW6432`, and `WINDIR`. Without `PATHEXT`, spawning common tools like `npm` or `git` from a stdio MCP server fails with `ENOENT` on Windows because Node can't resolve the `.cmd`/`.exe` extension.

packages/client/src/client/stdio.ts

@@ -47,17 +47,22 @@ export const DEFAULT_INHERITED_ENV_VARS =
     process.platform === 'win32'
         ? [
               'APPDATA',
+              'COMSPEC',
               'HOMEDRIVE',
               'HOMEPATH',
               'LOCALAPPDATA',
               'PATH',
+              'PATHEXT',
               'PROCESSOR_ARCHITECTURE',
+              'PROGRAMFILES',
+              'PROGRAMFILES(X86)',
+              'PROGRAMW6432',
               'SYSTEMDRIVE',
               'SYSTEMROOT',
               'TEMP',
               'USERNAME',
               'USERPROFILE',
-              'PROGRAMFILES'
+              'WINDIR'
           ]
         : /* list inspired by the default env inheritance of sudo */
           ['HOME', 'LOGNAME', 'PATH', 'SHELL', 'TERM', 'USER'];

packages/client/test/client/stdio.test.ts

@@ -1,7 +1,7 @@
 import type { JSONRPCMessage } from '@modelcontextprotocol/core';
 
 import type { StdioServerParameters } from '../../src/client/stdio.js';
-import { StdioClientTransport } from '../../src/client/stdio.js';
+import { DEFAULT_INHERITED_ENV_VARS, StdioClientTransport } from '../../src/client/stdio.js';
 
 // Configure default server parameters based on OS
 // Uses 'more' command for Windows and 'tee' command for Unix/Linux
@@ -77,3 +77,33 @@ test('should return child process pid', async () => {
     await client.close();
     expect(client.pid).toBeNull();
 });
+
+test('DEFAULT_INHERITED_ENV_VARS matches the host platform', () => {
+    if (process.platform === 'win32') {
+        // Variables Windows tooling needs to resolve executables and shells.
+        // Missing PATHEXT or COMSPEC causes spawn ENOENT for npm/git/etc.
+        expect(DEFAULT_INHERITED_ENV_VARS).toEqual(
+            expect.arrayContaining([
+                'APPDATA',
+                'COMSPEC',
+                'HOMEDRIVE',
+                'HOMEPATH',
+                'LOCALAPPDATA',
+                'PATH',
+                'PATHEXT',
+                'PROCESSOR_ARCHITECTURE',
+                'PROGRAMFILES',
+                'PROGRAMFILES(X86)',
+                'PROGRAMW6432',
+                'SYSTEMDRIVE',
+                'SYSTEMROOT',
+                'TEMP',
+                'USERNAME',
+                'USERPROFILE',
+                'WINDIR'
+            ])
+        );
+    } else {
+        expect(DEFAULT_INHERITED_ENV_VARS).toEqual(['HOME', 'LOGNAME', 'PATH', 'SHELL', 'TERM', 'USER']);
+    }
+});