x402 Protocol Documentation

import { createWalletClient, http } from 'viem';
import { base } from 'viem/chains';
import { privateKeyToAccount } from 'viem/accounts';

async function payForResource(url: string) {
  // 1. Make initial request
  const res = await fetch(url);

  if (res.status !== 402) return res;

  // 2. Parse payment requirements
  const { accepts } = await res.json();
  const req = accepts[0];

  // 3. Sign ERC-3009 TransferWithAuthorization
  const account = privateKeyToAccount('0x...');
  const wallet = createWalletClient({
    account, chain: base, transport: http()
  });

  const authorization = {
    from: account.address,
    to: req.payTo,
    value: req.maxAmountRequired,
    validAfter: '0',
    validBefore: String(
      Math.floor(Date.now() / 1000) + req.maxTimeoutSeconds
    ),
    nonce: `0x${crypto.randomBytes(32).toString('hex')}`,
  };

  const signature = await wallet.signTypedData({
    domain: { /* ERC-3009 token domain */ },
    types: { /* TransferWithAuthorization types */ },
    primaryType: 'TransferWithAuthorization',
    message: authorization,
  });

  // 4. Build and encode payment payload
  const payload = btoa(JSON.stringify({
    x402Version: 1,
    scheme: 'exact',
    network: req.network,
    payload: { signature, authorization },
  }));

  // 5. Retry with X-PAYMENT header
  return fetch(url, {
    headers: { 'X-PAYMENT': payload },
  });
}