HTTP 422 · type invalid_request_error · retryable false

defaultMessage Idempotency-Key was reused with a different request body.

idempotency_key_conflict

The same Idempotency-Key was reused with a different request body.

What to do

1. Reuse an idempotency key only for exact retries of the same JSON body.
2. For a new send (different recipient, template, or variables), generate a new key.
3. Do not mutate body fields while retrying a 5xx or network timeout with the same key.
4. If you need the original message id, look up the first successful accept response or poll by other criteria — do not force a conflicting body through.