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How an AI agent pays for an API call with x402 — a 3-line quickstart

Updated 2026-10-18. Every request in this article runs against a live production service (Money Reactor) on the Base mainnet. No signup, no API keys, no sandbox.

Traditional monetized APIs assume a human in the loop: someone registers an account, copies an API key into a config file, and tops up a credit card. Autonomous agents break this model completely — an agent that needs one crypto price per decision cannot pass a credit card checkout, and a shared API key is both an operational risk and an accounting nightmare. The x402 protocol solves this by reviving the long-dormant HTTP 402 Payment Required status code: when an unauthenticated request hits a paid resource, the server answers with a machine-readable payment demand, the client signs a USDC transfer on Base (EIP-3009), and the paid retry returns data in the very same round trip. There is nothing to configure in advance — the price list is discovered at call time.

Step 1 — hit the endpoint without a wallet

Any GET on a paid path returns 402 with a JSON challenge. Try it right now with the cheapest probe endpoint:

curl -i https://money-reactor.makar52nn2016.workers.dev/api/uuid

The body describes what to pay, to whom, and on which chain: x402Version: 2, an accepts array with scheme eip155:8453 (Base), maxAmountRequired in USDC units, and the merchant's payTo address. Because the challenge is structured, an agent can parse it without any human help. A free demo of the flagship bounty feed is available if you want to see the data shape before paying anything.

Step 2 — sign and retry with X-PAYMENT

The payment itself is an EIP-3009 transferWithAuthorization of USDC on Base — signed off-chain by the agent's key, verified and submitted on-chain by a facilitator (Coinbase's CDP facilitator handles this for x402 v2). You do not interact with the chain yourself. In Node.js the whole flow collapses to one wrapper call:

import { wrapFetchWithPayment } from "x402-fetch";

const fetchWithPay = wrapFetchWithPayment(fetch, evmClient);
const res = await fetchWithPay(
  "https://money-reactor.makar52nn2016.workers.dev/api/bounties"
);
const feed = await res.json(); // data + settlement receipt in headers

The wrapper receives the 402, builds the signature, retries with the X-PAYMENT header, and hands you the JSON response plus an X-PAYMENT-RESPONSE header containing the settlement proof. The agent's private key is the only credential that ever exists — there is nothing to leak in a repo, nothing to rotate, and every call is individually priced at $0.001–$0.01, so a runaway loop burns cents, not hundreds of dollars.

Step 3 — verify settlement, stay cheap

The settlement receipt names the transaction hash on Base, so payments are auditable per call. Practical tips from running a paid service for months: cache aggressively on your side (our bounty feed refreshes hourly anyway), prefer the $0.001 probes (/api/uuid, /api/time, /api/block-number) for health checks of your own x402 stack, and read openapi.json once at boot so your agent knows the full catalog and every price without another round trip.

Why bother with micropayments at all? Because the alternative for agents is either free-but-rate-limited scraping or $20/month plans designed for humans. A $0.002 call that settles in one round trip fits agent economics: an agent deciding whether a GitHub repo is a honeypot before cloning it will happily pay a fifth of a cent, but will not subscribe to anything. That asymmetry is the entire business case for x402, and it works today on Base with sub-cent fees.