NUT-24: HTTP 402 Payment Required

optional

uses: NUT-12

depends on: NUT-18, NUT-26

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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 creqA or NUT-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 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.