What Is x402? The Open Payment Protocol for AI Agents

x402 is an open payment protocol from Coinbase that revives the dormant HTTP 402 "Payment Required" status code. It lets any API charge an AI agent directly over HTTP, settled in stablecoins (USDC). One HTTP request pays for one API call, with no account and no per-merchant API key.

What is x402?

x402 is an open standard for paying over the web. A server answers an HTTP request with a price. The client pays it, and the resource comes back over normal HTTP, with the payment settling on-chain in USDC.

Say you run an API that sells company data. Today a buyer needs an account and a key before they can call it. Put an x402 price on the endpoint and any agent can pay for a single call and get the data, no relationship required.

Here's x402 at a glance:

The dormant HTTP 402 status code

HTTP 402 "Payment Required" was reserved in the HTTP/1.1 spec back in 1997, then sat unused for close to thirty years. Browsers and servers never agreed on what a payment-required response should carry, so it stayed a placeholder with no defined behavior.

x402 finally gives 402 a concrete meaning. The server returns structured payment terms and the client pays against them before the resource comes back. For the raw spec, see the HTTP 402 status code on MDN and the x402 protocol site.

x402 in plain terms

The plainest way to put it: x402 is a card reader for agents instead of humans. A shop turns on card payments so people can buy from it. x402 turns on that same service so agents can buy from it over HTTP, no human in the loop.

How does x402 work?

x402 works as a single round-trip over HTTP. The agent asks for a resource and gets back a 402 with the price. Its wallet pays, and the retry returns the data. The payment is the authorization, so there's no separate login or key exchange.

The exchange runs in four steps:

1. The agent sends a normal HTTP request.
2. The server replies with a 402 naming the price, currency, and destination.
3. The wallet signs a USDC payment and the agent retries with a payment header.
4. The server verifies the payment and returns the resource.

That's the short version. For the full handshake, settlement details and per-call cost, read Payments for AI Agents. Coding agents can also speak x402 through an MCP server that handles the 402 exchange as a paid tool, covered in MCP Payments.

Who created x402, and who governs it now?

Coinbase built x402 and open-sourced it in 2025. In 2026 the Linux Foundation launched the x402 Foundation to steward the standard, so x402 isn't a Coinbase product any more than TCP/IP belongs to one company. Anyone can implement it.

Coinbase shipped x402 to solve a specific problem. Agents call APIs thousands of times per task, and a card charging 30 cents plus a percentage per transaction can't price a two-cent call. A stablecoin payment settled over HTTP can. So Coinbase revived the 402 status code and released the spec as open source.

It's not the only bet. Coinbase built x402 and Stripe built MPP, two payment powerhouses shipping open protocols for agents on different designs. See Coinbase's x402 documentation and the x402 Foundation for the specs and governance.

Why x402 matters: the open, permissionless model

x402 matters because it's open and permissionless. Anyone can put a service behind a 402 and get paid over HTTP, with no gatekeeper approving who's allowed to sell. Picture Amazon in reverse. Instead of clearing an approval process before a marketplace lets you list, anyone in the world publishes a priced endpoint and any agent can pay it. A two-person shop and a Fortune 500 API get the same 402 and the same rail. That's the open-internet model applied to payments, and it's the durable version of this future to bet on.

x402 vs MPP vs the card networks

x402, MPP and the card networks all let agents pay, but they settle very differently. x402 is an open stablecoin protocol that settles each request on-chain. MPP is an open multi-rail protocol that can batch a session and is built to be compatible with x402, not replace it. The card networks (Visa Intelligent Commerce, Mastercard Agent Pay) extend existing card rails to agents inside a closed network.

Each design makes the same bet, that agents will pay for things, and picks a different rail to do it.

How much is x402 actually being used?

x402scan, AgentCash's explorer, shows 242,481 services already accept x402. They span more than 196 million transactions on Base and Solana through dozens of facilitators. That's real breadth for a protocol this young.

Picture an agent that needs one company's funding history. It hits an x402 endpoint and reads a two-cent price. It pays from its wallet and returns the answer. Nobody created an account, and no subscription changed hands.

Now the honest read: it's still small. A percentage jump doesn't mean much at this scale, so we won't wave a 600% increase around as if it settled the debate. What matters is the consistent upward trend, run after run and week after week.

This is the 1997 of the internet. The plumbing is new, most of the web still doesn't speak it, and the pieces are being laid right now. That's the case for paying attention early, not for overclaiming. To go deeper on the parts around it, see how pay-per-call APIs price a single request, and why an agent needs its own wallet to pay at all.

Tradeoffs and what to watch

x402's main limits are coverage and control. It needs server-side support, so not every API answers with a 402 yet, and non-x402 services still need a traditional key. Adoption is early. And because an agent can spend on its own, you set per-call and per-session caps so a runaway loop can't drain the balance.

Settlement finality depends on the chain too, though Base and Solana both confirm in seconds or less. None of these are dealbreakers. They're the normal edges of infrastructure that's still being built out.

If you're still deciding whether x402 is worth building on, the fastest way to feel it is to give an agent a balance and let it pay for one real call. AgentCash gives an agent one wallet to spend across any x402 service. You can get started, or read exactly how the payments flow in Payments for AI Agents.