Windows + Claude Code: Playwright MCP fails with npx/cmd.exe — working solution

[varanakonda]

Problem

Playwright MCP server fails to start in Claude Code on Windows when configured with npx (either directly or via cmd.exe /c). The stdio transport pipes don't connect, and the server appears as disconnected with no error output.

Environment

• Windows 11
• Claude Code CLI (native, not VS Code extension)
• Edge running with --remote-debugging-port=9222
• @playwright/mcp versions tested: 0.0.41, 0.0.68

What doesn't work

1. claude mcp add with cmd /c npx:

claude mcp add playwright -- cmd /c npx -y @playwright/mcp@0.0.41 --cdp-endpoint http://127.0.0.1:9222

Result: Claude Code parses /c as a path (C:/), producing broken config:

{ "command": "cmd", "args": ["C:/", "npx", ...] }

2. Manual config with cmd.exe /c npx:

{
  "command": "cmd.exe",
  "args": ["/c", "npx", "-y", "@playwright/mcp@0.0.68", "--cdp-endpoint", "http://127.0.0.1:9222"]
}

Result: Process starts but stdin/stdout pipes don't connect. MCP stdio transport hangs silently.

Root cause: On Windows, npx is actually npx.cmd (a batch wrapper). When spawned via cmd.exe /c, the stdin/stdout pipes used by MCP's stdio transport don't connect properly to the actual Node.js process.

3. Playwright plugin (playwright@claude-plugins-official):
The official Claude Code plugin doesn't support --cdp-endpoint and has namespace conflicts (plugin:playwright:playwright).

Working solution

Use node directly with the absolute path to cli.js:

In ~/.claude.json (under the project's mcpServers):

"playwright": {
  "type": "stdio",
  "command": "node",
  "args": [
    "/absolute/path/to/project/node_modules/@playwright/mcp/cli.js",
    "--cdp-endpoint",
    "http://127.0.0.1:9222"
  ]
}

Steps:

1. Install the package: npm add -D @playwright/mcp
2. Find cli.js: node_modules/@playwright/mcp/cli.js
3. Add the config above to ~/.claude.json under your project's mcpServers section
4. Restart Claude Code

This bypasses the .cmd batch wrapper entirely and connects stdin/stdout correctly for MCP stdio transport.

Verification

You can test the MCP handshake manually:

echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"capabilities":{},"clientInfo":{"name":"test","version":"1.0"},"protocolVersion":"2024-11-05"}}' | node node_modules/@playwright/mcp/cli.js --cdp-endpoint http://127.0.0.1:9222

Should return: {"jsonrpc":"2.0","id":1,"result":{"serverInfo":{"name":"Playwright","version":"..."},...}}

Suggestion

Consider documenting the node cli.js approach for Windows users in the README, or fixing npx stdio pipe handling on Windows.

Related issues: #1234, #1359

[pavelfeldman]

I'd say this is something for Claude Code client to address. Did you check if they have it filed?

[varanakonda]

@pavelfeldman yes, filed at anthropics/claude-code#46360 with full repro and the working node cli.js workaround.

For scope clarification — this issue surfaces two distinct bugs:

1. claude mcp add mangles /c → C:/ — purely Claude Code's argument parser, fully Anthropic's to fix.

2. Even with corrected config, cmd.exe /c npx breaks MCP stdio pipes on Windows — npx.cmd is a batch wrapper that doesn't properly inherit stdin/stdout when spawned via cmd.exe. This is arguably half-Anthropic (spawn strategy) and half here (the README examples use npx, which is npx.cmd on Windows).

The node node_modules/@playwright/mcp/cli.js form in the issue body bypasses both bugs. Might be worth a note in the README for Windows users, since it works regardless of how the host AI client spawns the process.