X_IDEMPOTENCY_KEY_INVALID
Means
an Idempotency-Key header was sent that cannot identify one request
Typical cause
a blank value — Headers.get() answers '' for Idempotency-Key: and never null, so a blank header used to become a live key every blank sender shared — or one past the 255 characters the OpenAPI operation publishes. A 400, raised before the handler, so nothing was written
Fix
set the header to a fresh crypto.randomUUID() on the client, one per request — or omit it entirely to run this call without idempotency