D398 - Guest-visible error codes use the platform's own encoding

Status: decided Date: 2026-08-30

A guest-facing error status is the platform's error encoding - the POSIX error number packed into the low bits of a fixed high word - built only from values this project has confirmed the hardware itself returns, rather than from placeholder codes invented for cases the encoding does not obviously cover.

Why: A single case observed on other tooling was weak evidence, since that tooling could itself have been inferring the same rule; multiple independent failures observed directly on the target hardware confirm the encoding is real rather than assumed. A placeholder code invented to distinguish a case the real encoding already covers is now unnecessary and is retired.

Rejected:

• Keeping placeholder-only codes for cases not directly observed: indistinguishable from a value nothing has ever confirmed, once the real encoding is known.