x402

x402 is a payment protocol that uses the HTTP 402 Payment Required status to request and complete machine-to-machine payments. A server returns 402 with payment instructions; a client (often an AI agent) settles the required amount—commonly on a crypto rail—and retries the request with proof of payment so the resource is released.

Why it Matters

Classic checkout assumes a browser session and a human card form. Agents calling APIs need a pay-per-request pattern that fits HTTP: challenge, pay, continue. x402 makes that loop explicit, so metering, micropayments, and paid tool access can sit next to ordinary REST or RPC traffic.

From a ledger perspective, each 402 settlement is a discrete economic event that must be recorded once, attributed to the paying agent or wallet, and reconciled against the rail that moved the funds. Token balances, prepaid commits, and drawdowns map cleanly onto that pattern when every paid request posts to a double-entry system of record rather than living only in gateway logs.