What is HTTP 402 Payment Required?

HTTP 402 is a status code reserved for “Payment Required.” It went unused for decades; x402 gives it a concrete meaning — a 402 response carries a signed challenge with the price and how to pay, so the caller can pay and retry.

In x402, a 402 body is small JSON: it quotes the exact price, the accepted payment details, and a nonce. The client pays the quoted amount and retries the original request with proof attached.

A correct implementation fails closed: if payment can’t be verified, the request returns an error rather than serving paid content for free.