HTTP 402 Payment Required

An HTTP status code reserved for future use since 1997, now used by x402 to signal that a request requires payment before processing.

HTTP 402 Payment Required is a standard HTTP response status code that was included in the HTTP/1.1 specification (RFC 2616) but marked as 'reserved for future use.' For decades, it had no standard implementation. The x402 protocol gives it concrete semantics: when a server returns 402, the response headers specify the price, accepted payment token (typically USDC), blockchain network, and payment address. This allows automated clients — particularly AI agents — to handle payment negotiation programmatically.