Errors

Stable codes, and what each one means.

Every failure carries a machine-readable code and an explicit HTTP status. Client-facing codes are append-only — one never changes meaning.

Code	Status	Meaning
`invalid_api_key`	401	The key is unknown, revoked, or expired
`insufficient_credit`	402	Not enough available balance to start
`spend_cap_exceeded`	402	The hold ran out mid-stream
`unknown_model`	404	No alias by that name in this bundle
`usage_unmeasurable`	500	The provider reported no usage — nothing was billed

usage_unmeasurable is deliberately a 500 that pages. We would rather take an incident than bill a guess.