MCP Config Fix Kit: templates and a checker for Claude Desktop, Claude Code, Cursor and VS Code (Windows and macOS)

Config templates, a file location table, a one page troubleshooting guide keyed on error text like "spawn npx ENOENT", and an offline Python script that reads your MCP config and flags known mistakes. Formats checked against each client's official docs on 2026-10-12. The Windows `cmd /c npx` form comes from the Node.js docs, not the client docs.

MCP servers often fail to start because of small config problems: a bare `npx` on Windows (the checker only warns on this, because the official Claude Desktop example uses it), a `.cmd` file that Node.js can reject with `spawn EINVAL`, the wrong top-level key, a trailing comma, a single backslash in a Windows path, or a file in the wrong folder. This kit collects those known causes in one place.

What is inside:

- 40 JSON templates. Claude Desktop, Claude Code `.mcp.json`, Cursor global, Cursor project and VS Code, each for Windows and macOS. Variants: `cmd /c npx` on Windows, plain `npx` on macOS, full path, `uvx`, and an `env` block.

- `check_mcp_config.py`. Written for Python 3.8 or newer (tested on 3.9), standard library only, runs offline, no AI. It reads one config file and prints OK, WARN or ERROR lines per server: invalid JSON with line and column, wrong top-level key for the client, missing command, wrong types for args and env, warnings for bare npx on Windows and relative paths, and more.

- FILE-LOCATIONS.md: where each client keeps its config file on Windows and macOS, and where its logs are.

- TROUBLESHOOTING.md: one page, from exact error text ("spawn npx ENOENT", "spawn EINVAL", "Client closed", config ignored, JSON parse errors) to the usual cause and fix.

- SOURCES.md: the official doc pages each format was checked against, and a list of what we could not confirm.

- Unit tests for the checker.

What is not inside:

- No guarantee your server will start. The checker finds known mistakes. It cannot prove a config works.

- Not a server installer. You install Node.js or uv yourself.

- No Linux file location for Claude Desktop, because the docs we read do not give one.

- Only the four clients named above.

- No support promise.

How it was tested: offline unit tests (the checker on good and bad sample files and on all 40 templates) and a review against the official docs. The templates were not tried inside the live Claude Desktop, Claude Code, Cursor or VS Code apps.

Who it is for: people setting up MCP servers by hand who want correct starting files and a quick check, especially on Windows.

Client config formats change. If a client's docs differ from this kit, the docs win.

License: one person, for use in their own personal or commercial work. No sharing or resale of the kit itself.

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