x402
An open protocol that lets an AI agent pay for a web resource automatically using the old, unused HTTP "402 Payment Required" status code.
Definition
x402 is an open payment protocol that revives the long-unused HTTP "402 Payment Required" status code: when an agent or app requests a resource that costs money, the server replies with a 402 and payment details, the client signs a small stablecoin payment, and the request is retried and fulfilled once payment settles. It needs no account sign-up or stored card, just a crypto wallet that can sign a transaction, which makes it well suited to AI agents paying per-request for APIs, data, or services on their own. It has become a common building block underneath higher-level agent checkout standards like the Agentic Commerce Protocol and Agent Payments Protocol.