Idempotent replay
A stored answer returned again for a repeated idempotency key.
What it means
A replay is the server handing back the response it already produced for a request it has seen, instead of running the request again.
How Onekey handles it
A replay is marked in two places: metadata.idempotent_replay: true and the header X-Idempotent-Replay: true. It is free, carries the original fetched_at and age_s, and omits X-Credits-Remaining because nothing was charged. Responses over 64 KB are not stored, so their replay answers 409 IDEMPOTENCY_REPLAY_UNAVAILABLE rather than billing twice.