MCP Server stdout: One console.log Kills the Connection

Description and use

Printing to stdout in an MCP server over stdio breaks the JSON‑RPC protocol. Redirect all console.log and print to stderr to keep the stream clean.

Problem

A console.log statement written to stdout in an MCP server over stdio corrupts the JSON-RPC protocol stream, causing the client to crash.

Outcome

Redirecting all console output to stderr (or configuring logging to stderr) prevents protocol corruption, restoring stable client-server communication.

Workflow

• 1. Launch MCP server via stdio transport. 2. Server writes JSON-RPC messages to stdout. 3. Developer adds console.log("fetching", url) inside a tool handler. 4. The log line is written to stdout, interleaving with JSON messages. 5. Client reads the second line, attempts JSON parse, fails. 6. Client tears down the connection and reports server failure.
• 7. Fix: redirect console.log and other console methods to stderr at entry point (Node) or configure logging to stderr (Python).
• 8. Ensure no other stdout writes (dependencies, child processes, shell scripts, spinners).
• 9. Test by piping server stdout through jq to validate each line is valid JSON.
• 10. Restart server and verify stable operation.

Evidence

Any console.log, print(), banner, spinner, or echo that lands on stdout injects a non‑JSON line and the client's parser blows up.

Source

Fix in Node: alias console.log to console.error at the top of your entry file.

Source