x402
An open payment protocol from Coinbase that repurposes the long-dormant HTTP 402 status code. A server responds with a price, the client pays in stablecoins on-chain, and the request is fulfilled. No accounts, no API keys, no card details.
Also known as: HTTP 402, x402 protocol
x402 is an HTTP-native payment protocol that lets a server quote a price for a resource and a client (typically an AI agent) settle that price on-chain in stablecoins. The server returns an HTTP 402 response with a payment requirements payload. The client signs a transfer authorisation, sends it back as a request header, and the server delivers the resource once settlement clears. The whole exchange takes one extra round trip and adds no per-user account state.
Settlement on Base, the most common path, uses EIP-3009 on USDC. EIP-3009’s transferWithAuthorization lets a third party (a “facilitator”) submit and pay gas for a USDC transfer that the payer has signed off-chain. This means the agent making the request never needs ETH for gas, only the USDC for the payment itself. The settlement on a block explorer looks like an ordinary USDC Transfer event accompanied by an AuthorizationUsed log, which is the only on-chain marker that distinguishes EIP-3009 from a regular ERC-20 transfer.
The editorial implication of that indistinguishability matters. Independent classifiers (Allium, Artemis) have to identify x402 traffic by tracking known facilitator wallets and request signatures rather than by the on-chain logs alone. That makes “how much volume is x402 actually doing” a non-trivial question to answer with on-chain data, and it explains why headline figures from the Coinbase facilitator’s own dashboard run higher than the on-chain reconstructions: the dashboard counts everything the facilitator processed, the classifiers count only what they have learned to label, and neither can authoritatively measure x402 traffic that flows through facilitators they do not know about.
x402 sits in the broader agent commerce stack alongside Coinbase AgentKit (the wallet SDK that signs the authorisations), Privy (a security-first agent wallet), and the Google A2P protocol (which embeds x402 as one of its payment options). The OYM “When AI Agents Hold Wallets” article walks through what is shipping versus what is narrative, and embeds a live tracker that classifies x402 settlement on Base via the public Coinbase facilitator. The number on that tracker is a floor on x402 volume, not a measure of organic demand: gas fees are subsidised, wash-trade filtering is not applied, and only the public facilitator path is covered.