idempotency_in_progress

HTTP 409. A request carrying this Idempotency-Key is still being processed. The API holds the key while the first request is in flight so a concurrent retry cannot run the operation twice.

What to do: Wait briefly and retry with the same key - once the first request settles you will get its stored result.

Every error is an RFC 9457 problem document - match on code (stable forever), show detail to humans, and quote instance when reporting an occurrence to support.