idempotency_key_in_flight
A request with this key is still running (or died between claiming the key and recording its result), so this one must not perform the side effect a second time. Retry in a moment.
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_in_flight",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}