MCP server errors and fixes

Fixes for the most common MCP server errors in Claude Desktop, Claude Code, Cursor, VS Code, Windsurf and ChatGPT. Each page explains why the error happens and the fix most likely to work first.

spawn npx ENOENT: MCP server fails to start

The error means your AI app could not find the npx command.

uvx: command not found (spawn uvx ENOENT)

Python MCP servers usually start with uvx, which comes with the uv package manager.

MCP server not showing up in Claude Desktop

Claude Desktop only reads claude_desktop_config.json when it starts, and it skips the whole file if the JSON is invalid.

Cursor MCP server shows a red dot or "client closed"

A red dot in Cursor's MCP settings means the server process failed to start or crashed.

Claude Code MCP server "failed to connect"

Claude Code marks a server as failed when its start command exits or does not answer in time.

ChatGPT custom connector cannot connect to an MCP server

ChatGPT only connects to remote MCP servers over a public HTTPS URL.

MCP server returns 401 Unauthorized or "missing API key"

Most MCP servers that connect to a service need an API key or token, passed as an environment variable in the config.

MCP config file has invalid JSON

One stray character makes the whole MCP config unreadable, so every server in it disappears.