# NUT-24: HTTP 402 Payment Required

`optional`

`uses: NUT-12`

`depends on: NUT-18, NUT-26`

---

This NUT describes how Cashu tokens can be used with HTTP 402 Payment Required responses to enable payments for HTTP resources.

## Server response

HTTP servers may respond with the HTTP status code 402 and an `X-Cashu` header containing a payment request, encoded in either [NUT-18][18] `creqA` or [NUT-26][26] `creqB` formats. Wallets **MUST** support both formats.

The decoded payment request has the following fields:

```
{
    "a": int,
    "u": str,
    "m": Array[str],
    "nut10": NUT10Option <optional>
}
```

- `a`: The amount required in the specified unit
- `u`: The currency unit (e.g., "sat", "usd", "api")
- `m`: Array of mint URLs that the server accepts tokens from
- `nut10`: The required [NUT-10][10] locking condition

Note: there is no transport field since we expect an in-band payment.

## Client payment

After receiving a 402 response with an `X-Cashu` header, the client may retry the request with a `cashuB` token in the `X-Cashu` header as payment.

The token MUST be from one of the mints listed in the `m` array, and MUST be of the same `u` unit and greater than or equal to the `a` amount.

## Payment response

If the server receives tokens from a mint that is not in the `m` array, an incorrect `u` unit, an insufficient amount of tokens, or with insufficient locking conditions, it should respond with an HTTP `400` status code.

[10]: 10.md
[12]: 12.md
[18]: 18.md
[26]: 26.md