x402 and HTTP 402 for AI Agents

HTTP 402 Payment Required is a long-reserved status code that lets a server ask a client to pay before it serves a response; x402 is the modern convention that puts it to work for machine payments. For agents it matters because it makes payment a native part of an HTTP request rather than a separate checkout flow — an agent calls an endpoint, receives a 402 with payment terms, pays programmatically, and retries. Pay-per-call becomes a protocol, not an integration.

HTTP 402

Payment Required — long reserved, now used

x402

A convention for machine payments over HTTP

Enables

Pay-per-call APIs, micropayments

Pairs with

Stablecoins, agent wallets

Why HTTP payments matter again

For agents, the payment and the request want to be one flow. A 402 with machine-readable terms lets an agent handle payment the way it handles any other response code — no browser, no form, no human at a terminal. That is what makes pay-per-call and micropayments practical between machines.

x402 is one of several approaches; AP2 and others address adjacent parts of the agent-payment problem. A neutral comparison of them matters more than betting the stack on one, which is why the estate treats payment protocols as composable rather than as a single choice.

Related standards

x402 / HTTP 402AP2

Questions

Is x402 a blockchain?

No. It is a way to use HTTP’s payment-required status for machine payments. It can settle in stablecoins or other rails, but the convention itself is HTTP-native.

How is x402 different from a normal payment API?

A payment API is a separate service an integration calls; x402 makes payment part of the resource request itself, so an agent needs no bespoke checkout flow.