A stdio MCP server must never print non-protocol text to stdout

A stdio MCP server must never print non-protocol text to stdout · id PC-059 · checked_on 2026-10-29 · license CC-BY-4.0

Error strings

A local MCP server prints a banner or debug lines.

The client reports parse errors or disconnects at startup.

stdout is reserved for MCP messages. Logging belongs on stderr.

Send all logs to stderr and keep stdout clean.

node server.js | head -c 300

The first bytes on stdout must be a JSON-RPC message, never a banner.

Streamable HTTP, no authentication: https://agent-economy-data.onrender.com/notes/mcp. Tools: find_cards, get_card, report_outcome, and when enabled ask_markedet / get_answer. See llms.txt. Same notes as files: Agent Field Guide.

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