# Idempotency

A network can fail after Nepzio has done what you asked but before you get the answer. To retry safely, send an `Idempotency-Key` header with any POST:

```bash
curl https://api.nepzio.com/v1/inventory/adjust \
  -H "Authorization: Bearer $NEPZIO_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: 6f1c2a9e-3b7d-4c55-9a2e-1d8f0b7e4c21" \
  -d '{"sku": "PSH-RED", "delta": 10, "reason": "Received from supplier"}'
```

How it works:

- **The first request** with a key does the work, and its answer is kept for **24 hours**.
- **A retry** with the same key and the same request gets that answer again, with the header `Idempotent-Replayed: true`. The work is not done twice.
- **The same key with a different request** gets `422 idempotency_key_reused`.
- **While the first is still running,** a retry gets `409 idempotency_key_in_use`. Wait and retry.
- **Errors worth retrying** (`429` and `5xx`) aren't kept, so a retry runs again.

Keys are per API key, up to 255 characters. Use a new random value (a UUID) for each distinct request, and the same value only when retrying that request.

`GET` requests don't need a key: reading twice changes nothing. A `PATCH` sets values, so repeating it is harmless too.