What is x402?
x402 is an HTTP-native micropayment protocol that uses the HTTP 402 "Payment Required" status code to settle paid API calls on-chain in stablecoin in a single round-trip, with no API keys.
x402 reuses the long-reserved HTTP 402 status code to turn a regular HTTP request into a paid one. When a resource requires payment, the server responds with HTTP 402 plus a machine-readable payment-required header describing the chain, asset (typically USDC), amount, and recipient. The client (an autonomous agent or its wallet) signs an on-chain payment proof and re-sends the original request with a payment-signature header attached.
The server passes the proof to the x402 facilitator, which verifies and settles the payment on-chain, then returns the actual resource, all in the same round-trip the client expected. No API keys, no subscription, no out-of-band billing.
Sella runs x402 across 10+ chains, including Stellar (sub-cent fees), Base, Ethereum, and Solana. Sub-cent unit pricing for per-call data and inference is the design point.