# Idempotency (/concepts/idempotency)



<!-- generated from docs/wallet-api/QUICKSTART.md §5 — edit THAT file -->

Send `Idempotency-Key: <unique string>` on any POST. &#x2A;*It is required on
`POST /v1/payouts`** — a retry without one can send funds twice, so the API
rejects the request with `400 idempotency_key_required` rather than let you find
out expensively.

| Situation                                 | Result                                                      |
| ----------------------------------------- | ----------------------------------------------------------- |
| First use                                 | request executes, response stored                           |
| Same key, same body                       | the stored response replays, with `Idempotent-Replay: true` |
| Same key, **different** body              | `409 idempotency_key_reuse`                                 |
| Same key while the first is still running | `409 idempotency_in_progress` + `Retry-After`               |

Keys expire after 24h. Use a UUID, or something derived from your own record id.

> A 5xx or a network timeout does **not** mean the request failed — only that you
> did not see the answer. Retry with the same key.