Definition
x402 Protocol
An open payment protocol that uses HTTP 402 Payment Required to enable pay-per-call API transactions for AI agents.
x402 is an open protocol that builds on the HTTP 402 Payment Required status code — a code reserved since HTTP/1.1 in 1997 but never widely adopted. When an API server returns a 402 response, x402-compatible clients automatically read the payment requirements from response headers (price, token, chain, payment address), sign a USDC transaction on-chain, and retry the request with a payment receipt. This turns any API into a pay-per-call service without requiring API keys, subscriptions, or pre-registration. The entire flow — request, payment, retry — typically completes in under 2 seconds.
See also
Related terms
HTTP 402 Payment Required
An HTTP status code reserved for future use since 1997, now used by x402 to signal that a request requires payment before processing.
USDC
A US dollar-pegged stablecoin issued by Circle, used as the payment currency for all Mithril transactions.
Pay-Per-Call
A pricing model where each individual API call is billed as a separate micro-transaction, rather than through subscriptions or rate-limited tiers.
Agent Wallet
A blockchain wallet assigned to an AI agent, managed by Mithril, with configurable spending limits and controls.
On-Chain Settlement
The process of recording and finalizing a payment transaction on a blockchain, providing immutable proof of payment.