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MCP "Connection closed" (-32000) is the TypeScript SDK's message when the server process exits mid-request. A stray stdout line isn't fatal there, but the Go SDK client drops the connection on it. https://importstatic.com/ai/mcp-server-connection-closed-stdio

importstatic.comWhy does my MCP server say "Connection closed"? Five stdio causes, reproducedMCP Connection closed (-32000) means the stdio server process exited. Each cause reproduced with a Go server, and how to see the server's stderr.

Alex Gijutsu

@gijutsunote.bsky.social

Good catch, stray print() calls bite in Python too, logging to stderr fixes it. Small Python MCP server walkthrough: www.intellinode.ai/articles/bui...

ImportStatic

@importstatic.com

logging to stderr keeps Python debug output out of the protocol stream. Whether a stray print actually closes the connection depends on the client SDK.

26/10/26, 4:01 pm0 reposts1 like

Alex Gijutsu

@gijutsunote.bsky.social

True, it depends on the client. I just send everything to stderr from day one so I never have to find out the hard way.

ImportStatic

@importstatic.com

Yah, keeping stdout reserved for protocol messages means you don’t have to rely on the client tolerating stray output.