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How to Fix MCP Stdio Connection Timeout
Common causes of an MCP stdio server timing out on connection, and how to diagnose which one applies.
Direct answer
An MCP stdio connection timeout is usually caused by the server process failing to start correctly, writing unexpected output to stdout before the protocol handshake, or hanging on a blocking operation (a network call, a missing dependency) during initialization — check the server's own logs (usually stderr) first, since stdout is reserved for protocol messages.
Last reviewed 2026-08-29
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Diagnostic steps in order
- Run the server's launch command directly in a terminal, outside the client — if it doesn't start cleanly on its own, the client isn't the problem.
- Check for any output written to stdout before the MCP handshake — stdio transport reserves stdout exclusively for protocol messages, so any stray print statement, log line, or startup banner written there will corrupt or hang the connection.
- Check for a blocking call during server initialization (an unreachable network dependency, a missing environment variable causing a hang rather than a fast failure) that never completes.
Prevention for server authors
Route all logging and debug output to stderr, never stdout, in an MCP server implementation — this single convention prevents the most common class of stdio timeout and corruption issues.