x402

HTTP 402 ("Payment Required") has been reserved in the web standards since the 1990s but was never widely used. x402 finally puts it to work: when a client calls a paid endpoint, the server responds with 402 and a description of the payment it expects. The client prebuilds and signs a payment transaction; a server, or an optional facilitator, submits that transaction and, once it has settled on-chain, responds with 200 and the actual content requested. The result is metered, pay-per-call access to APIs and data with no signups, subscriptions, or stored card details. Originally contributed by Coinbase, the protocol moved to neutral stewardship under the Linux Foundation in 2026.

Cardano's eUTxO model fits this flow naturally. The buyer prebuilds and signs a transaction that pays an exact amount, and because any change would invalidate the signature, the server or facilitator can only submit it unchanged; the amount that settles is exactly the amount the buyer signed for. This is a different shape from account-based chains, where an x402 payment is commonly made by granting a contract a spending allowance that application logic then draws against. Both models support x402; the eUTxO version simply carries the exact payment in the signed transaction itself rather than in a separate allowance.

x402 matters most for the emerging agent economy, where autonomous AI agents need to pay for the services they consume on the fly, which is also where stablecoins like USDM and Djed fit on Cardano. It is one piece of a wider stack of agentic-commerce standards: AP2 (Agent Payments Protocol) and ACP (Agentic Commerce Protocol) describe how a payment is authorized on the buyer's behalf, MCP (Model Context Protocol) governs how an agent connects to tools and data, and A2A (Agent2Agent) covers how agents talk to one another.