Idempotency-Key header. This includes call creation and call-control mutations, number creation, and billing checkout. The checkout operation requires a key; on other supported routes, the key is optional.
Idempotency-Replayed: true. Reusing the same key for different request details returns a conflict; an in-progress request may return 409 with Retry-After: 1.
Idempotency middleware captures at most 1 MiB of request body. It is not automatically active on every mutation; see the endpoint reference and do not assume unsupported operations are safely replayable.