What is x402? HTTP 402 payments for APIs and AI agents

x402 is an open payment protocol that lets any HTTP API charge for a request, in stablecoins, with no account or API key. It revives the long-unused HTTP status code 402 Payment Required: the server answers an unpaid request with a price, the client pays, and the server returns the response. It was created by Coinbase and is now governed by the x402 Foundation under the Linux Foundation.

Because the whole exchange happens over plain HTTP, an AI agent can discover a paid API, pay a fraction of a cent for one call and use the result, without a human signing up, entering a card or managing keys.

How an x402 payment works

1. The client (a script, app or AI agent) calls an endpoint, e.g. GET /report.
2. The server replies 402 Payment Required with its payment requirements: price, network (for example Base or Solana), token (usually USDC) and the address to pay. In x402 v2 these travel in the PAYMENT-REQUIRED header.
3. The client signs a payment for exactly that amount (on EVM chains, a USDC transfer authorization) and retries the request with the signed payment attached (PAYMENT-SIGNATURE in v2).
4. The server, usually through a facilitator, verifies the payment and settles it on-chain, then returns the data with a PAYMENT-RESPONSE header containing the settlement result.

A facilitator is a service that checks and settles payments so the API owner doesn’t have to run blockchain infrastructure. Coinbase runs one (the CDP Facilitator; per its docs the first 1,000 on-chain transactions each month are free, then $0.001 each), and there are independent facilitators too.

Which networks and tokens does x402 use?

Most x402 APIs charge USDC. In our directory, 239 listings use x402: 231 accept payment on Base and 94 on Solana, with smaller numbers on Polygon, Arbitrum and other chains. Coinbase’s facilitator docs list Base, Polygon, Arbitrum, Monad, World and Solana among supported mainnets.

Why it matters for AI agents

• No sign-up. An agent doesn’t need an account or stored API key for every service it uses.
• Pay per call. Prices of $0.001 to $0.01 per request are common in our listings, which makes one-off use practical. See pay-per-call AI APIs.
• Discoverable. Services settled through Coinbase’s facilitator can be indexed in the x402 Bazaar, a discovery API agents can query for paid endpoints.

Some low-cost x402 services

Five listings with the lowest published price per call. Full comparison: best x402 APIs.

x402 vs other agent payment protocols

x402 is a payment rail for single HTTP requests. Other efforts such as Google’s AP2, OpenAI and Stripe’s ACP, Stripe and Tempo’s MPP, and Lightning’s L402 cover overlapping ground. We compare them in AI agent payment protocols compared.

Want to charge for your own API?

Read how to accept USDC for your API, then submit it to YourAITools for free.

Sources

• x402 specification and SDKs (x402 Foundation on GitHub)
• Coinbase CDP: Quickstart for sellers
• Coinbase CDP: Facilitator (networks, pricing)