[tanishqbaweja]

Backport of #2043 to v1.x. Fixes #2037 for v1.

Motivation and Context

#2037 reported that the Windows branch of DEFAULT_INHERITED_ENV_VARS drops variables that Windows tools need. #2043 fixed it on main (shipped in @modelcontextprotocol/client 2.1.0) and closed the issue, but v1.x still has the old list as of 1.32.2. The triage note on #2037 says v1.x needs the same change.

When StdioClientTransport gets no env, or an env that is merged over getDefaultEnvironment(), a Windows server process starts without PATHEXT, COMSPEC or PROGRAMDATA. A server that shells out to PowerShell then can't resolve native executables, and Windows OpenSSH exits 255. Both failures were reported on #2043.

This PR makes the Windows list identical to main's: the same six names (COMSPEC, PATHEXT, PROGRAMDATA, PROGRAMFILES(X86), PROGRAMW6432, WINDIR) in the same sorted order, so later cherry-picks stay clean. It adds a test and a patch changeset.

How Has This Been Tested?

On Windows 11, Node 24.20:

• npx vitest run test/client/stdio.test.ts -t "Windows variables" fails on unchanged v1.x (missing COMSPEC, PATHEXT, PROGRAMDATA, PROGRAMFILES(X86), PROGRAMW6432, WINDIR) and passes with this change. The test sets process.platform to win32 and re-imports the module, so it checks the Windows list on Linux CI too. The test on main only checks the host platform's list.
• npm run check (typecheck, eslint, prettier) passes.
• npm run build passes for ESM and CJS.
• npm test: 56 files, 1859 tests pass (1858 before, plus the new one). On Windows the run also reports 2 unhandled Unexpected end of JSON input errors from the existing more-based tests in test/client/stdio.test.ts. The same 2 errors appear on unchanged v1.x. CI runs on Linux, where those tests use tee.
• npx vitest run test/e2e/scenarios/stdio.test.ts: 6 of 7 pass, including transport:stdio:default-env-safelist. transport:stdio:shutdown-escalation fails on Windows with or without this change.
• End to end: a stdio McpServer started by StdioClientTransport with no env option. Its tool runs PowerShell Get-Command node and ssh.exe -V:
  • unchanged v1.x: the server has no PATHEXT, COMSPEC or PROGRAMDATA. PowerShell fails with The term 'node' is not recognized ..., and ssh.exe -V exits 255 with no output.
  • this branch: PowerShell resolves C:\Program Files\nodejs\node.exe, and ssh.exe -V exits 0 (OpenSSH_for_Windows_9.5p1).
  • Each fix comes from one variable: adding only PATHEXT fixes PowerShell, and adding only PROGRAMDATA fixes OpenSSH. COMSPEC alone fixes neither.

Breaking Changes

None. On Windows, child processes that use the default environment now also get these six system variables. None of them holds secrets. Non-Windows platforms are unchanged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed (patch changeset)

Additional context

No error handling is involved: the change only adds names to a list.

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