idempotency_key_reused

HTTP 409 · The same Idempotency-Key was sent with a different body.

What causes it

A key is bound to the exact request that first used it. A second request with the same key and different content is ambiguous, so nothing is created.

What to do

This is a bug on your side. Either you are reusing a key across genuinely different operations, or you changed the payload while retrying. Derive the key from your own record id, one key per logical operation.

Retry?

No — the same request produces the same answer.

The response

{
  "type": "https://developers.vaki.co/errors/idempotency_key_reused",
  "title": "…",
  "status": 409,
  "code": "idempotency_key_reused",
  "detail": "…",
  "instance": "/v1/checkout_links"
}

Branch on code, never on title or detail. The full list of codes, and the shape of the problem document, is on Errors.