An email address or phone number — auto-detected, no separate field per channel.

Optional. Makes a retry safe: the same key with the same body within 24 hours returns the original response (header X-Idempotent-Replayed: true) instead of repeating the action. The same key with a different body returns 422; a request still in flight returns 409. Only 2xx responses are remembered. Scoped to your client, venue and route.