x402

x402 Protocol Development

x402 is an open standard (Apache-2.0) that activates the HTTP 402 Payment Required status code for programmatic, on-chain payments. Originally created by Coinbase, now maintained by the x402 Foundation. No accounts, sessions, or API keys required - clients pay with signed crypto transactions directly over HTTP.

When to Use

• Building a paid API that accepts crypto micropayments
• Adding paywall to web content or endpoints
• Enabling AI agents to autonomously pay for resources
• Integrating MCP tools that require payment
• Building agent-to-agent (A2A) payment flows
• Working with EVM (Base, Ethereum, MegaETH, Monad, Polygon, Stable, Arbitrum), Solana, Stellar, Aptos, NEAR, or XRPL payment settlement
• Implementing usage-based billing with the upto scheme (LLM tokens, bandwidth, compute)
• Running an in-process facilitator (self-facilitation) without external facilitator dependency

Core Architecture

Three roles in every x402 payment:

1. Resource Server - protects endpoints, returns 402 with payment requirements
2. Client - signs payment authorization, retries request with payment header
3. Facilitator - verifies signatures, settles transactions on-chain

Payment flow (HTTP transport):

Client -> GET /resource -> Server returns 402 + PAYMENT-REQUIRED header
Client -> signs payment -> retries with PAYMENT-SIGNATURE header
Server -> POST /verify to Facilitator -> POST /settle to Facilitator
Server -> returns 200 + PAYMENT-RESPONSE header + resource data

Quick Start: Seller (TypeScript + Express)

import express from "express";
import { paymentMiddleware, x402ResourceServer } from "@x402/express";
import { ExactEvmScheme } from "@x402/evm/exact/server";
import { HTTPFacilitatorClient } from "@x402/core/server";

const app = express();
const payTo = "0xYourWalletAddress";

const facilitator = new HTTPFacilitatorClient({ url: "https://x402.org/facilitator" });
const server = new x402ResourceServer(facilitator)
  .register("eip155:84532", new ExactEvmScheme());

app.use(
  paymentMiddleware(
    {
      "GET /weather": {
        accepts: [
          { scheme: "exact", price: "$0.001", network: "eip155:84532", payTo },
        ],
        description: "Weather data",
        mimeType: "application/json",
      },
    },
    server,
  ),
);

app.get("/weather", (req, res) => {
  res.json({ weather: "sunny", temperature: 70 });
});

app.listen(4021);