# Idempotency

### How it works

Include an `Idempotency-Key` header with a unique value on any POST request:

```bash
curl -X POST https://zynta-lastmile-api.up.railway.app/api/v1/off-ramp \
  -H "x-api-key: sk_live_..." \
  -H "Idempotency-Key: txn-abc-123-unique" \
  -H "Content-Type: application/json" \
  -d '{"quoteId": "...", "recipientId": "..."}'
```

If you send the same request with the same idempotency key:

* **Same body** — returns the original response without creating a duplicate
* **Different body** — returns a `409 Conflict` error

### Key rules

* Keys must be unique per request — use UUIDs or a combination of your internal reference and timestamp
* Keys expire after **24 hours**
* Keys are scoped to your API key — different API keys can use the same idempotency key without conflict

{% hint style="danger" %}
**Always** use idempotency keys when creating transfers. A duplicate transfer means a duplicate payment.
{% endhint %}

### Generating good keys

```javascript
// UUID
const key = crypto.randomUUID();

// Deterministic (based on your internal data)
const key = `offramp-${invoiceId}-${Date.now()}`;
```


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