Invalid request (malformed body, missing/invalid parameter).

Missing or invalid API key.

API key lacks a required scope (named in error.param) or test-only endpoint called with a live key.

No such resource (or it belongs to another mode/subaccount).

Invalid state transition, in-flight registration conflict, or idempotency key reuse with a different body.

If-Match ETag no longer matches the resource.

Rate limit exceeded; see Retry-After. RateLimit-* headers appear on every response, not only 429s.