API errors

Idempotency key reused

This X-Idempotency-Key was already used with a different request body within the last 48 hours.

422application/problem+json

What it means

An idempotency key pins one exact request for 48 hours: a replay with the same key must carry an equivalent body to receive the original cached response. This key arrived with a body that fingerprints differently from the one it was first used with, so the API refused rather than guess which request you meant.

422 · application/problem+json

{
  "type": "https://developers.firsty.app/errors/idempotency-key-reused",
  "title": "Idempotency key reused",
  "status": 422,
  "detail": "This idempotency key was already used with a different request body."
}

How to handle it

Generate a fresh key (a UUID works well, up to 255 characters) for every logical operation. Reuse a key only to replay the identical request.
Never derive keys from something that repeats, like a user id or an order number alone. Two different requests will collide on the same key.
If you intended a replay, diff the body against the original. Bodies are compared by canonicalized JSON, so key order does not matter but values do.

Idempotency, in the rate limits guide Errors reference

What it means

How to handle it

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