PRODUCTION SCENARIO
You wrap an internal ticketing API as a stdio Model Context Protocol (MCP) server. It prints a 'connecting...' line at start-up, before any request has arrived, and per-request timings during calls, all to standard output; the client reports parse errors and the server's tools vanish from the tool list. The host already captures and displays whatever a server writes to its error stream, and the operator wants to keep reading these lines there. The model should not see the timings.
Where should the diagnostic lines go?
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Show answer and explanation
Answer: Write the lines as UTF-8 to stderr and keep stdout for JSON-RPC messages
On stdio, stdout carries only valid MCP messages delimited by newlines, and a server may write UTF-8 strings to stderr for any logging purpose, which the client is free to capture, forward or ignore, so the lines move one stream over and the parser stops choking. A log notification is a valid message on that channel and is the mechanism for progress a client should render, but it relates to an in-flight request, and the start-up line is emitted before one exists. Attaching detail to a tool result is how you hand the model what it needs to recover, and timings are what should not reach it. A private file is the answer when nothing upstream captures stderr; this host does.