Payer Guide

import { privateKeyToAccount } from "viem/accounts";
import { signEip3009Authorization } from "@xentfi/x402-facilitator";
import type { Eip3009TokenDomain, X402Challenge } from "@xentfi/x402-facilitator";

const account = privateKeyToAccount(process.env.PAYER_PRIVATE_KEY! as `0x${string}`);

const domain: Eip3009TokenDomain = {
  chainId: 8453,
  tokenAddress: "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
  tokenName: "USD Coin",
  tokenVersion: "2",
  tokenDecimals: 6,
};

async function main() {
  const url = "https://merchant.example.com/premium-report";

  // 1. First request — expect a 402 challenge.
  const firstResponse = await fetch(url);
  if (firstResponse.status !== 402) {
    console.log("Already accessible without payment:", await firstResponse.json());
    return;
  }

  const challenge = (await firstResponse.json()) as X402Challenge;
  const requirement = challenge.accepts.find((r) => r.scheme === "eip3009");
  if (!requirement) throw new Error("Server doesn't support the 'eip3009' scheme");

  console.log("Got 402 challenge, paying:", requirement.maxAmountRequired, "to", requirement.payTo);

  // 2. Sign a real EIP-3009 authorization for the required amount.
  const paymentPayload = await signEip3009Authorization({
    account,
    to: requirement.payTo,
    amount: requirement.maxAmountRequired,
    domain,
  });

  // 3. Retry with the signed payment attached.
  const header = Buffer.from(JSON.stringify(paymentPayload)).toString("base64");
  const secondResponse = await fetch(url, { headers: { "X-PAYMENT": header } });

  console.log("Result:", secondResponse.status, await secondResponse.json());
}

main();