Unique key per action (e.g. a UUID). Reusing it replays the first response instead of sending again, so a retry can't double-send. Read only by the endpoints that list it. A key is scoped to the endpoint and the account (or chat) it acts on, and kept for 24 hours. Only a successful response is stored and replayed: a 4xx frees the key, while a 5xx keeps it (the action may have gone out; retry with a new key). So does a 400 passing on a LinkedIn, WhatsApp or mail provider failure (code provider-unreachable, provider-408 for the provider's own timeout, or provider- and a 5xx status such as provider-502), unless the request provably never reached the provider (its connection never opened): the action may have gone out. A retry with the same key then gets 409 idempotency_failed. The request body is not compared, so a reused key replays the first response even if the body differs.