# Idempotency

> How to retry without paying twice.

Source: https://docs.nocamclub.com/api/idempotency

Networks fail. If a call times out, you can't know whether the generation started. An `Idempotency-Key` makes retrying safe.

```http
POST /v1/generations
Idempotency-Key: order-1234-hero-photo
```

| You send | You get |
|---|---|
| A new key | A new generation |
| The same key and the same body | The first generation, with `Idempotent-Replayed: true`. Nothing new is charged |
| The same key and another body | [`idempotency_conflict`](https://docs.nocamclub.com/errors/idempotency_conflict.md) |

- Up to 200 characters. Keys belong to your account.
- A key is remembered for as long as its generation exists, which is for good: deleting a generation removes its files, not the record. So never reuse a key for new work.
- Use one key for each thing you want made: an order id, a UUID, a hash of what you're asking for. Keys in these docs' examples are examples: make your own.
- **A refused call doesn't use up its key.** If the answer is a 4xx, nothing was made: fix the request and send it again with the same key.
- **To retry a failed generation, use a new key.** The old key returns the same failure, because it's the same order.
- After [`internal`](https://docs.nocamclub.com/errors/internal.md) or a dropped connection, retry with the **same** key.

## Through MCP

The `generate` tool has no key field: it does this for you. A call with exactly the same arguments within the same 10 minutes returns the same generation, unless that one failed or was deleted. Pass `run_again: true` to make another on purpose.