HTTP already knows how to request a page, return a response, and report an error. Paying for that response usually happens somewhere else, through a checkout, a subscription, or a separate account. x402 brings payment negotiation into the HTTP request itself using the 402 Payment Required status code.
That becomes especially useful when the client is an AI agent. An agent calling an API needs a way to discover the price, authorize a payment, and receive the result without a person stepping through a checkout each time.
The guide follows that exchange from the first unpaid request to payment verification and resource delivery. It also covers the facilitator's role, the security model, and how developers can connect the pieces in a working integration.
