Stdio MCP server cwd field silently ignored on Windows (2.1.123)

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):

``json
{
"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"
}
}
}
``

1. Run DEBUG=true CLAUDE_CODE_DEBUG_LOGS_DIR=/tmp/log claude mcp list from

the repo root (NOT the server dir).

1. 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