validation_failed
HTTP 422§9.10 Agreements
PATCH /v1/onboarding named a step and supplied no answers object of that name. Distinct from invalid_request (§9.2) because the body is individually well-formed - it is the cross-field agreement between step and its payload that failed, which no field-level schema can express.
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/validation_failed",
"title": "…",
"status": 422,
"detail": "…",
"instance": "/v1/…"
}