Monetize Express.js APIs for AI Agents in 3 Lines of Code with x402

#javascript #web3 #express #ai

AI agents are increasingly making programmatic requests, but traditional billing (credit cards, OAuth, complex API keys) creates friction for automated microtransactions.

With x402-express, you can monetize any Express.js endpoint using HTTP 402 payment requirements on Base in 3 lines of code.

1. Installation

bash
npm install x402-express

2. Implementation

Add the middleware to any route you want to protect:
javascript
const express = require('express');
const { x402Middleware } = require('x402-express');

const app = express();

app.get('/api/agent-data', x402Middleware({ price: '0.01' }), (req, res) => {
res.json({ message: "Payment verified via x402!" });
});

app.listen(3000, () => console.log('Server running on port 3000'));

3. Verification & Registry

GitHub Repository: github.com/IkeHentrel/x402-express
MCP Registry: io.github.ihen404/x402-express

When an unauthenticated agent hits your endpoint, x402-express interceptively returns an HTTP 402 Payment Required payload containing payment parameters, verifying settlements autonomously.

🌐 Paid API Specification

Endpoint: POST https://your-deployed-app.up.railway.app/api/scrape

Protocol: x402 (HTTP 402 Payment Required)

Price: $0.02 USDC per request

Network: Base (chainId: 8453)

Asset Contract: 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913 (USDC)

Payout Address: YOUR_BASE_WALLET_ADDRESS

1. Unauthenticated Request (HTTP 402 Challenge)

Send a standard POST request without payment headers:


bash
curl -i -X POST [https://your-deployed-app.up.railway.app/api/scrape](https://your-deployed-app.up.railway.app/api/scrape) \
  -H "Content-Type: application/json" \
  -d '{"url": "[https://news.ycombinator.com](https://news.ycombinator.com)"}'
Expected Response Header:
HTTP/1.1 402 Payment Required
X-Payment-Required: eyJzY2hlbWUiOiJleGFjdCIsInBheVRvIjoiMHhZT1V...
Paid Request (Machine-to-Machine via x402 Client)
AI agents using @ihentrel/x402-express or x402-fetch automatically parse the challenge header, sign a $0.02 USDC payment payload on Base, and retry:
import { fetchWithX402 } from 'x402-client';

const response = await fetchWithX402('[https://your-deployed-app.up.railway.app/api/scrape](https://your-deployed-app.up.railway.app/api/scrape)', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ url: '[https://news.ycombinator.com](https://news.ycombinator.com)' })
});

const data = await response.json();
console.log(data.markdown); // Returns clean Markdown ready for LLM context

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