Idempotency-Key
A unique key that makes retries of a non-idempotent request safe.
Request header (sent by clients) other
How it works
The server stores the first result for the key and replays it for retries, so a network failure cannot create a duplicate payment or order. Widely used by payment APIs.
Example
Idempotency-Key: 4f9c2c1e-8a0b-4d5e-9b6a-2f3e1d0c9b8a
See it live: send any request in Endpoint Client and open the Headers tab, where every response header is explained. Specification: IETF draft.