400
idempotency_key_requiredplatformThis write needs an Idempotency-Key header
Returned by the implementation; no specification rule declares it yet.
Every partner-surface write requires one. It is not optional and it is not defaulted, because a default would be a key shared between two different requests.
Why the API refuses
Idempotency-Key header is required on this surface
What to do
Send Idempotency-Key: <uuid> — a fresh one per logical operation, and the same one on every retry of that operation. Generate it before the first attempt and store it with the work you are doing; generating it inside a retry loop defeats the entire mechanism.