Pay from an agent

import { randomBytes } from 'node:crypto';
import { privateKeyToAccount } from 'viem/accounts';

const BUY_URL = process.argv[2];
const MAX_USDC = 5; // your own spending limit for this purchase
const account = privateKeyToAccount(process.env.AGENT_PRIVATE_KEY);

// 1. Ask for the price.
const first = await fetch(BUY_URL, { headers: { Accept: 'application/json' } });
if (first.status !== 402) throw new Error(`Expected 402, got ${first.status}: ${await first.text()}`);
const challenge = await first.json();
const req = challenge.accepts.find((a) => a.scheme === 'exact' && a.network === 'eip155:8453');
if (!req) throw new Error('No USDC-on-Base offer');
const amount = BigInt(req.amount ?? req.maxAmountRequired);
if (amount > BigInt(MAX_USDC * 1e6)) throw new Error('Over budget');

// 2. Sign an EIP-3009 transferWithAuthorization for exactly that amount.
const now = Math.floor(Date.now() / 1000);
const authorization = {
  from: account.address,
  to: req.payTo,
  value: amount.toString(),
  validAfter: String(now - 60),
  validBefore: String(now + (req.maxTimeoutSeconds ?? 300)),
  nonce: `0x${randomBytes(32).toString('hex')}`,
};
const signature = await account.signTypedData({
  domain: {
    name: req.extra?.name ?? 'USD Coin',
    version: req.extra?.version ?? '2',
    chainId: 8453,
    verifyingContract: req.asset,
  },
  types: {
    TransferWithAuthorization: [
      { name: 'from', type: 'address' },
      { name: 'to', type: 'address' },
      { name: 'value', type: 'uint256' },
      { name: 'validAfter', type: 'uint256' },
      { name: 'validBefore', type: 'uint256' },
      { name: 'nonce', type: 'bytes32' },
    ],
  },
  primaryType: 'TransferWithAuthorization',
  message: {
    ...authorization,
    value: BigInt(authorization.value),
    validAfter: BigInt(authorization.validAfter),
    validBefore: BigInt(authorization.validBefore),
  },
});

// 3. Retry with the payment attached.
const payment = {
  x402Version: challenge.x402Version,
  scheme: req.scheme,
  network: req.network,
  payload: { signature, authorization },
};
const paid = await fetch(BUY_URL, {
  headers: {
    Accept: 'application/json',
    'PAYMENT-SIGNATURE': Buffer.from(JSON.stringify(payment)).toString('base64'),
  },
});

// 4. Keep the receipt.
const body = await paid.json();
if (paid.status !== 200) throw new Error(`Payment failed (${paid.status}): ${body.error}`);
console.log('Order', body.data.orderId, 'tx', body.data.receipt?.transaction);

AGENT_PRIVATE_KEY=0x… node buy.mjs "https://apps.apiswop.co/api/v5/x402/store/travis.swop.id/buy/6a7f63002e22feb6ad48dd7c"