Show HN: X402-express. Pay-per-call HTTP 402 middleware for AI agents on Base L2

M2MCent released X402-express, an open-source middleware that enables AI agents to pay per API call using USDC on Base L2. The npm package implements the x402 payment protocol, allowing developers to add a paywall to any Express endpoint with three lines of code, processing atomic on-chain settlements through an escrow contract. The tool is designed for AI-to-AI commerce, with over 100 production Model Context Protocol servers already using the system for real USDC micro-payments.

Ultra-lightweight x402 payment interceptor for AI Agents and MCP Servers. Monetize any API in 3 lines of code. npm install m2mcent-sdk js import { X402Handler } from 'm2mcent-sdk'; const x402 = new X402Handler { rpcUrl: 'https://mainnet.base.org', privateKey: process.env.RELAYER PRIVATE KEY , recipient: process.env.TREASURY ADDRESS } ; // Protect any Express endpoint with a USDC paywall app.post '/api/analyze', x402.requirePayment "100000" , req, res = { res.json { result: "Premium analysis complete", receipt: req.paymentTx } ; } ; M2MCent implements the x402 Payment Protocol — a machine-native payment standard inspired by HTTP 402: Agent requests a protected endpoint Server responds with 402 Payment Required + payment metadata Base64 encoded Agent signs an EIP-712 typed data authorization gasless for the payer Server settles atomically on-chain via the M2MCent Escrow contract Agent receives the premium response + transaction receipt Agent ──► API Server ──► 402 + metadata Agent ◄── signs EIP-712 authorization Agent ──► API Server + Payment-Signature header └──► Escrow.settle on Base L2 Agent ◄── Premium Response + tx hash | Parameter | Description | Required | |---|---|---| rpcUrl | Base Mainnet RPC endpoint | ✅ | privateKey | Relayer wallet private key for settlement | ✅ | recipient | Treasury address to receive payments | ✅ | Non-custodial : Funds flow directly from payer → treasury via on-chain escrow Gasless for payers : Uses EIP-3009 transferWithAuthorization USDC native Zero-Leak : No agent data is retained after settlement Atomic : Payment and service delivery happen in a single request cycle | Parameter | Value | |---|---| Network | Base Mainnet Chain ID: 8453 | USDC Contract | 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913 | Escrow Contract | 0xf3c3416A843d13C944554A54Ac274BB7fF264BcC | Settlement | Atomic, sub-second finality | Perfect for Model Context Protocol https://modelcontextprotocol.io servers: python import express from 'express'; import { X402Handler } from 'm2mcent-sdk'; const app = express ; const x402 = new X402Handler { rpcUrl: process.env.BASE RPC URL , privateKey: process.env.RELAYER PRIVATE KEY , recipient: process.env.TREASURY ADDRESS } ; // Any MCP tool endpoint becomes monetizable app.post '/api/tools/analyze', x402.requirePayment "50000" , async req, res = { const result = await runMCPTool req.body ; res.json { ...result, paymentTx: req.paymentTx } ; } ; M2MCent powers 100+ production MCP servers on Base Mainnet, processing real USDC micro-payments for AI-to-AI commerce. MIT © M2MCent https://github.com/Evozim/m2mcent-sdk