Stdout Purity Check

Assert that stdout carries only protocol messages, because one stray print breaks every client

Use when: the server logs, warns or prints anything during normal operation

Avoid when: the server is not a stdio transport and has no dedicated protocol channel

Procedure

1. Run the server and capture stdout separately from stderr
2. Parse every stdout line as JSON
3. Fail the test when any line is not a JSON-RPC message
4. Trigger a warning path and confirm it appears on stderr only
5. Check the process exit path for a trailing print
6. Add the check to CI so a new log statement cannot sneak in

Signals

• every stdout line parses as a JSON object with a jsonrpc field
• warnings appear on stderr
• the exit path prints nothing to stdout
• a deliberately injected print makes the test fail

Patterns

1. Split streams

stdout=protocol, stderr=diagnostics

2. Line parse

json.loads(line) for every non-empty stdout line

3. Injected failure

temporarily print('debug') and expect a failing test

Pitfalls

• Using the default logging configuration, which writes to stdout
• Printing a startup banner for humans, which the client tries to parse
• Capturing both streams together in the test and missing the leak

Done when

• every stdout line parses as JSON during a full session
• a diagnostic on stderr is proven not to reach stdout

References

• MCP - transports
• Python - logging