claude-code - 💡(How to fix) Fix Stdio MCP server cwd field silently ignored on Windows (2.1.123) [1 participants]

Description

On Windows, the cwd field in a stdio MCP server config (.mcp.json or user-scope mcpServers in ~/.claude.json) is silently ignored when Claude Code spawns the server. The subprocess inherits the parent process's cwd instead of the configured value. This breaks any server that uses cwd-relative paths to locate its script file or its data directories.

The failure presents as ✗ Failed to connect in claude mcp list with no surface-level explanation. The real cause is only visible by running with DEBUG=true and inspecting the debug log, which shows the spawned Python interpreter reporting it cannot find server.py because it's looking in the wrong directory.

A second, compounding failure mode (covered partially by #46360): when working around the first by using absolute paths for both the interpreter AND the script, the recommended cmd /c <python> <script> Windows wrapper hits cmd.exe's "old-behavior" quote-stripping rule (≥2 quoted args triggers it), producing 'c:\Users\noahr\code' is not recognized as an internal or external command. The wrapper that's supposed to make Windows work makes Windows worse once both args have spaces. Dropping the wrapper (using direct interpreter + absolute script path with no cmd /c) is what works — same workaround as #46360 Bug 2, but the diagnostic path is different.

Steps to Reproduce

1. Windows 11 (10.0.26200), Claude Code 2.1.123 (CLI binary AND VS Code extension bundle).
2. Create a Python MCP server at C:\some\path with spaces\my-server\server.py.
3. Add to .mcp.json in some other directory (e.g. a repo root):

   {
     "mcpServers": {
       "demo": {
         "type": "stdio",
         "command": "cmd",
         "args": ["/c", "C:\some\path with spaces\my-server\.venv\Scripts\python.exe", "server.py"],
         "cwd": "C:\some\path with spaces\my-server"
       }
     }
   }

4. Run DEBUG=true CLAUDE_CODE_DEBUG_LOGS_DIR=/tmp/log claude mcp list from the repo root (NOT the server dir).
5. Observe ✗ Failed to connect and, in the debug log:

   Server stderr: python.exe: can't open file 'C:\<repo-root>\server.py': [Errno 2] No such file or directory

Expected Behavior

Claude Code should set the spawned subprocess's working directory to the cwd value from the config, so that relative server.py (and any cwd-relative data paths the server uses) resolve correctly. This matches how vanilla child_process.spawn(cmd, args, {cwd}) in Node behaves.

Actual Behavior

The subprocess inherits the parent's cwd. The cwd field in the config has no effect.

Workaround

Drop the cmd /c wrapper, use bare interpreter + absolute path to the script:

{
  "command": "C:\full\path\python.exe",
  "args": ["C:\full\path\server.py"]
}

And ensure each server itself uses os.path.dirname(os.path.abspath(__file__)) for any data lookups, since cwd still won't be set on the spawned process.

Environment

• OS: Windows 11 Home 10.0.26200 (win32-x64)
• Claude Code CLI: 2.1.123 (both ~/.local/bin/claude.exe and ~/.vscode/extensions/anthropic.claude-code-2.1.123-win32-x64/resources/native-binary/claude.exe)
• VS Code extension: 2.1.123
• Affected: every stdio MCP server defined with relative script path + cwd
• Diagnostic-visibility prerequisite: DEBUG=true env var; otherwise claude mcp list reports only ✗ Failed to connect

Related

• #46360 — Windows claude mcp add /c flag mangling + workaround (different root cause, same workaround shape)
• #49133 — Silent-failure UX gap meta-issue (macOS-flagged but applies on Windows too — confirms the diagnostic-discoverability problem)

Suggested Labels

bug, area:mcp, platform:windows, has repro