Same Key New Body

A buggy client sends POST /charges with idempotency key abc123 for $50, then retries with the same key but a $500 amount after the user edited the form. What should a well-designed idempotent API do?

Overwrite the stored record with the $500 amount and return the amended result, so the most recent client intent wins and the customer is charged what the form showed

Reject the second request with an error, because the key is bound to the parameters of the original request and reusing it with a different body is a client bug rather than a retry

Process the second request as a brand new charge under a freshly minted key, since the differing amount proves the user intended a distinct operation rather than a retry

Replay the stored $50 response without raising an error, since a matching key always identifies an operation the server has already completed and the request body is only advisory on a retry