x402 — HTTP Payment Protocol

Overview

x402 is an open payment protocol from Coinbase that revives the HTTP 402 (Payment Required) status code for instant, automatic stablecoin micropayments. It enables APIs and services to be monetized natively over HTTP — no accounts, sessions, or traditional payment processors required. Both human clients and AI agents can pay programmatically.

How It Works

Client                          Server                      Facilitator
  │                               │                             │
  │── GET /weather ──────────────►│                             │
  │◄── 402 Payment Required ─────│                             │
  │    (price, payTo, network)    │                             │
  │                               │                             │
  │── sign payment ──────────────►│                             │
  │   (PAYMENT-SIGNATURE header)  │── verify + settle ────────►│
  │                               │◄── confirmation ────────────│
  │◄── 200 OK (content) ─────────│                             │

Server Middleware

Express (TypeScript)

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

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

app.use(paymentMiddleware(
  {
    "GET /weather": {
      accepts: {
        scheme: "exact",
        price: "$0.001",
        network: "eip155:84532",
        payTo: "0xYourAddress",
      },
    },
  },
  resourceServer,
));

Flask (Python)

from flask import Flask
from x402.flask.middleware import PaymentMiddleware

app = Flask(__name__)
payment_middleware = PaymentMiddleware(app)

payment_middleware.add(
    path="/weather",
    price="$0.001",
    pay_to_address="0xYourAddress",
    network="base-sepolia",
)

@app.route("/weather")
def get_weather():
    return {"report": {"weather": "sunny", "temperature": 70}}

Client Integration

TypeScript

import { x402Client } from "@x402/core/client";
import { ExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client()
  .register("eip155:*", new ExactEvmScheme(privateKeyToAccount(privateKey)));

let response = await fetch(url);
if (response.status === 402) {
  const paymentRequired = decodePaymentRequiredHeader(
    response.headers.get("PAYMENT-REQUIRED")
  );
  const payload = await client.createPaymentPayload(paymentRequired);
  response = await fetch(url, {
    headers: { "PAYMENT-SIGNATURE": encodePaymentSignatureHeader(payload) },
  });
}

Python (httpx)

from eth_account import Account
from x402.clients.httpx import x402HttpxClient

account = Account.from_key(os.getenv("PRIVATE_KEY"))

async with x402HttpxClient(account=account, base_url="https://api.example.com") as client:
    response = await client.get("/protected-endpoint")

HTTP Headers

Header	Direction	Description
PAYMENT-REQUIRED	Response (402)	Payment terms (price, payTo, network, scheme)
PAYMENT-SIGNATURE	Request (retry)	Signed payment payload

Supported Networks

x402 works with fast, low-fee blockchains:

• Base (Coinbase L2) — primary network
• Base Sepolia — testnet
• Solana — supported via Solana scheme
• Ethereum mainnet — supported but higher fees

Best Practices

• Use testnet (Base Sepolia) during development; switch to mainnet for production.
• Set prices in USD strings ("$0.001") — the protocol handles conversion.
• Use the public facilitator (https://x402.org/facilitator) for verification and settlement.
• For AI agents, wrap the x402 client in the agent's HTTP layer so payments are automatic.
• Keep per-request prices low (fractions of a cent) for API monetization — x402 is designed for micropayments.
• Add x402 middleware only to routes that need monetization, not globally.