x402 (HTTP 402 payment protocol)
A Coinbase-originated protocol built on the long-dormant HTTP 402 "Payment Required" status code, letting a server request a stablecoin micropayment before an API call completes.
A server returns 402; the calling agent signs a stablecoin transfer (typically USDC on Base); a facilitator settles it on-chain; the original request then completes. It's designed for exactly the kind of sub-cent, high-frequency payments an autonomous agent might make calling many APIs — a pricing shape that doesn't fit traditional invoicing. It's one of several protocols (alongside Google's AP2 and Stripe/OpenAI's Agentic Commerce Protocol) emerging for agent-initiated payments.
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