What is x402?
An open payment standard, introduced by Coinbase in 2025, that uses HTTP 402 so an API can charge per request and a client or AI agent can pay instantly, typically in stablecoins.
The flow: the client calls a paid endpoint; the server replies 402 with its payment requirements (price, asset, network, address); the client signs a payment and retries the request with it attached; the server, often with a facilitator service, verifies and settles the payment and returns the data. No account, API key or card form is needed.
It suits AI agents that need one call from a service they have never used before, where signing up for a monthly plan would not make sense.
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Related terms
- HTTP 402 Payment RequiredAn HTTP status code reserved for payment that HTTP itself never defined in detail, now used by agent payment schemes such as x402.
- Tool call (tools/list, tools/call)How an AI model uses an MCP tool: the client lists the server's tools with tools/list, the model picks one, and the client runs it with tools/call and returns the result to the model.