x402 PAYMENT PROTOCOL - AI AGENT QUICK REFERENCE
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When you receive HTTP 402, follow these steps:

1. PARSE THE 402 RESPONSE
   Extract: accepts[0].{scheme, network, asset, amount, payTo, maxTimeoutSeconds, extra}

2. SIGN EIP-3009 TRANSFER AUTHORIZATION
   Domain: {name: extra.name, version: extra.version, chainId: <from network>, verifyingContract: asset}
   Types: TransferWithAuthorization(from, to, value, validAfter, validBefore, nonce)
   Message: {from: YOUR_WALLET, to: payTo, value: amount, validAfter: 0, validBefore: now+120, nonce: random32bytes}

3. BUILD X-PAYMENT HEADER (base64 JSON)
   {
     "x402Version": 2,
     "scheme": "exact",
     "network": "<from 402>",
     "payload": {
       "x402Version": 2,
       "payload": {"authorization": <message>, "signature": <sig>},
       "accepted": {<copy accepts[0]>, "extra": {<copy extra>, "assetTransferMethod": "eip3009"}}
     }
   }

4. RESEND REQUEST WITH HEADER
   X-PAYMENT: <base64 of above JSON>

5. SAVE JWT FROM RESPONSE
   Header: X-Payment-Response: {"token": "<jwt>", "transactionId": "<txhash>"}

6. USE JWT FOR SUBSEQUENT REQUESTS
   Authorization: Bearer <jwt>

NETWORKS
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Testnet: eip155:84532 (Base Sepolia), USDC: 0x036CbD53842c5426634e7929541eC2318f3dCF7e
Mainnet: eip155:8453 (Base), USDC: 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913

MINIMAL NODE.JS EXAMPLE
-----------------------
import {ethers} from "ethers";
const wallet = new ethers.Wallet(PRIVATE_KEY);

// After getting 402 with accepts[0] as `req`:
const nonce = ethers.hexlify(ethers.randomBytes(32));
const validBefore = Math.floor(Date.now()/1000) + 120;
const sig = await wallet.signTypedData(
  {name: req.extra.name, version: req.extra.version, chainId: 84532, verifyingContract: req.asset},
  {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"}
  ]},
  {from: wallet.address, to: req.payTo, value: req.amount, validAfter: 0, validBefore, nonce}
);
const header = btoa(JSON.stringify({x402Version:2, scheme:"exact", network:req.network, payload:{
  x402Version:2, payload:{authorization:{from:wallet.address,to:req.payTo,value:req.amount,validAfter:"0",
  validBefore:String(validBefore),nonce},signature:sig},
  accepted:{...req,extra:{...req.extra,assetTransferMethod:"eip3009"}}}}));
const resp = await fetch(URL, {headers:{"X-PAYMENT":header}});
const jwt = JSON.parse(resp.headers.get("X-Payment-Response")).token;

ERROR CODES
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402 = Need payment (parse accepts array)
401 = JWT expired (pay again)
400 = Bad X-PAYMENT header format
403 = Insufficient USDC or invalid signature