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The x402 protocol enables pay-per-call USDC micropayments for AI agents, LLM APIs, and any HTTP service. Buyers pay in USDC per request; sellers validate payments server-side with zero configuration overhead.

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How It Works

  1. Buyer calls pay(url, amount) with the seller’s URL and the expected USDC amount
  2. The SDK negotiates a payment session with the x402 facilitator, submits an ERC-20 Transfer transaction on-chain, and attaches the authorization header to the request
  3. Seller middleware (@x402/express) validates the header, extracts the payment details, and allows the request through
  4. Both sides receive the on-chain transaction hash as proof of payment

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Buyer SDK (@xpaylabs/x402)

Install: 
npm install @xpaylabs/x402

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One-shot Payment

import { pay } from '@xpaylabs/x402';

const response = await pay('https://api.example.com/weather', {
  amount: '0.001',       // USDC amount
  network: 'base-sepolia',
  privateKey: '0x...',   // buyer's EVM private key
});

console.log(response.data);     // response body
console.log(response.paymentId); // on-chain tx hash

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Session-based Client

import { XPayClient } from '@xpaylabs/x402';

const client = new XPayClient({
  privateKey: '0x...',
  network: 'base-sepolia',
  facilitatorUrl: 'https://x402-facilitator-testnet.vercel.app/api',
});

const res1 = await client.request('https://api.example.com/weather', {
  amount: '0.001',
});
const res2 = await client.request('https://api.example.com/weather', {
  amount: '0.001',
});
Each call pays independently. The client tracks the last PAYMENT-RESPONSE header for audit.

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Payment ID Extraction

import { extractPaymentId } from '@xpaylabs/x402';

const paymentId = extractPaymentId(response.headers);
console.log(paymentId); // "0xabc...123"

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Seller Middleware (@x402/express)

npm install @x402/express

import express from 'express';
import { x402Middleware } from '@x402/express';

const app = express();

app.use(x402Middleware({
  network: 'base-sepolia',
  facilitatorUrl: 'https://x402-facilitator-testnet.vercel.app/api',
  evmAddress: '0x...',       // seller's USDC receiving address
}));

// Paid route — client must include valid x402 payment
app.get('/api/generate', (req, res) => {
  // req.x402 contains decoded payment info
  res.json({ result: 'llm response', paymentId: req.x402.paymentId });
});

// Public route — no payment required
app.get('/health', (req, res) => {
  res.json({ status: 'ok' });
});

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AI Agent Payment Flow

x402 is designed for autonomous agent-to-API payments: 
AI Agent (buyer)                    AI API (seller)
      │                                   │
      │  1. POST /api/generate            │
      │     (no payment yet)              │
      │                                   │
      │  2. 402 Payment Required          │
      │     { amount: "0.001" }           │
      │                                   │
      │  3. Submit USDC Transfer (on-chain)│
      │     via x402 facilitator          │
      │                                   │
      │  4. retry with x402 auth header   │
      │                                   │
      │  5. 200 OK + response             │
      │     (paymentId in headers)        │
No API keys, no subscriptions, no billing dashboard — just pay-per-call with USDC.
Last modified on June 5, 2026