Always consider the destination and lifecycle of output streams to prevent protocol interference, data loss, and unexpected behavior. Choose stdout for application output and stderr for debugging/logging. Avoid accidentally closing streams and be mindful of output pollution in protocol-sensitive contexts. Key practices: - Use stderr for debug/log output to avoid interfering with application protocols - When piping streams, use `{ end: false }` option to prevent closing the target stream - In protocol-sensitive contexts (like JSON-RPC), redirect console output to stderr to prevent stdout pollution - Prefer `process.stderr.write` over `console.warn` in tests to avoid mock interference - Use `console.error.bind(console)` instead of custom implementations to preserve formatting Example: ```typescript // Good: Prevent stdout closure when piping fileStream.pipe(process.stdout, { end: false }) // Good: Redirect console in protocol contexts if (process.argv.includes('mcp')) { console.log = console.error.bind(console) } // Good: Use stderr for debug output console.warn(`${namespace} ${format}`, ...rest) ```