A request with this idempotency key is still in progress

request_in_progress HTTP 409

A request with this Idempotency-Key is still running. Wait and retry with the same key — you will get the original result once it completes, not a second execution.

How to react to this in code

Switch on the code member — request_in_progress — which is stable. Do not parse the type URI: it points here, and where "here" is may change.