Definition
x402 is an open protocol, introduced by Coinbase in 2025, that turns HTTP 402 Payment Required into a working payment rail for machines. A client requests a resource, receives a 402 response describing the price and how to pay, submits payment, and retries the request. No account creation, API key provisioning, or human checkout step is involved, which is what makes it usable by autonomous AI agents.
The first reference implementations settle in stablecoins on-chain, and adoption grew quickly through 2025 and 2026 as agent traffic increased. A later revision added wallet credentials that prove both identity and payment authorization in the same exchange, which gives sites something user-agent strings never could: evidence of which specific user delegated authority to an agent.
x402 is deliberately narrow. It does not model carts, refunds, or the commerce lifecycle—that is the job of protocols like the Agentic Commerce Protocol for checkout and AP2 for authorization mandates. x402 just embeds payment into HTTP. In practice these layers stack: AP2 says the agent is authorized, x402 moves the money, and a licensing layer such as RSL says what the buyer may do with what they bought.
For publishers, x402 is the machine-to-machine plumbing behind pay per crawl and metered APIs. For GEO teams, it is worth understanding as part of the emerging economics of agentic search: as agents gain the ability to pay for access, the question shifts from whether a bot can reach your content to whether paying for it is worth more to you than being cited for free across AI search.
Examples of x402
- An AI agent requests a metered API endpoint, receives HTTP 402 with payment details, settles the payment, and retries the request successfully.
- A documentation site charges agents a fraction of a cent per fetch through x402 while keeping the same pages free for human visitors.
- A crawler presents wallet credentials that prove both which operator it belongs to and that a specific user authorized the purchase.
- A platform team stacks x402 for settlement, AP2 for authorization, and RSL for licensing terms rather than trying to solve all three with one protocol.
