Append a campaign destination suppression

Stable key per logical intent, 1–255 printable characters. Persist it before sending. Retry the same key/body after an uncertain result for 24 hours. A changed body conflicts. Beyond retention, reconcile the resource before retrying; this is not permanent deduplication. Current authorization is rechecked on replay. Evaluation order: authenticate and check current authorization; look up intent; replay its stored receipt before checking the current revision; only a new intent evaluates If-Match and executes. Fingerprint includes method, path, body and original If-Match. A changed request returns idempotency_key_reused. The server stores only a keyed (HMAC) fingerprint, and no error or response ever returns a fingerprint or hash of a request. In-flight intent returns 409 idempotency_in_progress with Retry-After; retry the same intent after reconciliation. Receipts describe that attempt, not current resource state. A replay returns the original status code and body, with the response header Idempotent-Replayed: true.