Quick Answer
MCP stdio Error is most commonly caused by server wrote logs to stdout. Check the steps below in order to isolate and fix the issue.
Affected Clients and Transports
Common Causes
- Server wrote logs to stdout
- Process exited early
- Invalid JSON-RPC frame
Step-by-Step Diagnosis
- Check the client logs. Open the client dev tools or log file and look for startup errors or JSON parse failures.
- Verify the server starts manually. Run the server command in a terminal and confirm it prints no errors before waiting for input.
- Use MCP Inspector. Run
npx @modelcontextprotocol/inspectorand connect to the server to inspect capabilities. - Validate credentials. Check that every required environment variable is set and has a valid value.
- Restart the client. Config changes only take effect after a full client restart — not just a reload.
Resolution Checklist
Config file syntax is valid JSON/JSONC
Executable path or npx package name is correct
All required environment variables are set
Credentials have the required permissions/scopes
Client has been fully restarted
Server process has no port conflicts