Declare a contact property: its key, its type (string or number) and an optional fallback_value that a broadcast uses for a contact with no value of its own.

Makes this call safe to retry. Send the same key with the same body and the original response is replayed instead of the work happening twice. Keys are 1-256 characters and are remembered for 7 days. While the first attempt is still running, a second call with that key returns 409 idempotency_in_flight (Resend calls this concurrent_idempotent_requests); the same key with a different body returns 409 idempotency_payload_mismatch (Resend: invalid_idempotent_request). Resend-Idempotency-Key is accepted as the same header.

Alias of Idempotency-Key. Idempotency-Key wins if both are sent.