idempotency_key_conflict
The mobile Idempotency-Key was already used for a different task action. Generate a new key before submitting a different update. Distinct from the /v1 idempotency guard's codes (§9.2).
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/idempotency_key_conflict",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}