D170 - Guest standard output goes to host stderr

Status: decided Date: 2026-09-26

A guest's writes to descriptors 1 and 2, and its printf-family output, land on the host's error stream, never its output stream.

Why: a conformance probe that cannot write to standard output cannot report at all, and a guest's own diagnostics name what failed. The worker's standard output carries the protocol as newline-delimited JSON, and guest bytes interleaved into it would break the reader.

Rejected:

• Refusing descriptors 1 and 2: a guest that cannot talk.
• Host standard output: corrupts the worker protocol.