x402 Protocol Development

x402 is an open standard (Apache-2.0) that activates the HTTP

402 Payment Required

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, or Aptos 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);

Install:

npm install @x402/express @x402/core @x402/evm

Quick Start: Buyer (TypeScript + Axios)

import { x402Client, wrapAxiosWithPayment } from "@x402/axios";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";
import axios from "axios";

const signer = privateKeyToAccount(process.env.EVM_PRIVATE_KEY as `0x${string}`);
const client = new x402Client();
registerExactEvmScheme(client, { signer });

const api = wrapAxiosWithPayment(axios.create(), client);
const response = await api.get("http://localhost:4021/weather");
// Payment handled automatically on 402 response

Install:

npm install @x402/axios @x402/evm viem

Quick Start: Seller (Python + FastAPI)

from fastapi import FastAPI
from x402.http import FacilitatorConfig, HTTPFacilitatorClient, PaymentOption
from x402.http.middleware.fastapi import PaymentMiddlewareASGI
from x402.http.types import RouteConfig
from x402.mechanisms.evm.exact import ExactEvmServerScheme
from x402.server import x402ResourceServer

app = FastAPI()

facilitator = HTTPFacilitatorClient(FacilitatorConfig(url="https://x402.org/facilitator"))
server = x402ResourceServer(facilitator)
server.register("eip155:84532", ExactEvmServerScheme())

routes = {
    "GET /weather": RouteConfig(
        accepts=[PaymentOption(scheme="exact", pay_to="0xYourAddress", price="$0.001", network="eip155:84532")],
        mime_type="application/json",
        description="Weather data",
    ),
}
app.add_middleware(PaymentMiddlewareASGI, routes=routes, server=server)

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

Install:

pip install "x402[fastapi,evm]"

Quick Start: Seller (Go + Gin)

import (
    x402http "github.com/x402-foundation/x402/go/http"
    ginmw "github.com/x402-foundation/x402/go/http/gin"
    evm "github.com/x402-foundation/x402/go/mechanisms/evm/exact/server"
)

facilitator := x402http.NewHTTPFacilitatorClient(&x402http.FacilitatorConfig{URL: facilitatorURL})

routes := x402http.RoutesConfig{
    "GET /weather": {
        Accepts: x402http.PaymentOptions{
            {Scheme: "exact", Price: "$0.001", Network: "eip155:84532", PayTo: evmAddress},
        },
        Description: "Weather data",
        MimeType:    "application/json",
    },
}

r.Use(ginmw.X402Payment(ginmw.Config{
    Routes:      routes,
    Facilitator: facilitator,
    Schemes:     []ginmw.SchemeConfig{{Network: "eip155:84532", Server: evm.NewExactEvmScheme()}},
}))

Install:

go get github.com/x402-foundation/x402/go

Multi-Network Support (EVM + Solana)

Servers can accept payment on multiple networks simultaneously:

import { ExactEvmScheme } from "@x402/evm/exact/server";
import { ExactSvmScheme } from "@x402/svm/exact/server";

const server = new x402ResourceServer(facilitator)
  .register("eip155:84532", new ExactEvmScheme())
  .register("solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1", new ExactSvmScheme());

// Route config with both networks
"GET /weather": {
  accepts: [
    { scheme: "exact", price: "$0.001", network: "eip155:84532", payTo: evmAddress },
    { scheme: "exact", price: "$0.001", network: "solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1", payTo: svmAddress },
  ],
}

Clients register both schemes and auto-select based on server requirements:

const client = new x402Client();
registerExactEvmScheme(client, { signer: evmSigner });
registerExactSvmScheme(client, { signer: svmSigner });

Supported Networks

eip155:8453

eip155:84532

eip155:4326

solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp

solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1

stellar:pubnet

stellar:testnet

aptos:1

aptos:2

eip155:143

eip155:137

eip155:80002

eip155:988

eip155:2201

eip155:42161

eip155:421614

eip155:31611

eip155:43114

Default facilitator (

https://x402.org/facilitator

SDK Packages

TypeScript v2.9.0 (npm, GitHub)

@x402/core

@x402/evm

@x402/evm/upto/*

@x402/svm

@x402/stellar

@x402/aptos

@x402/express

@x402/fastify

@x402/hono

@x402/next

@x402/axios

@x402/fetch

@x402/paywall

@x402/mcp

@x402/extensions

Python v2.6.0 (PyPI, GitHub)

pip install "x402[httpx]"      # Async HTTP client
pip install "x402[requests]"   # Sync HTTP client
pip install "x402[fastapi]"    # FastAPI server
pip install "x402[flask]"      # Flask server
pip install "x402[svm]"        # Solana support
pip install "x402[mcp]"        # MCP integration
pip install "x402[extensions]" # Extensions (bazaar, gas sponsoring, etc.)
pip install "x402[all]"        # Everything

Go v2.7.0 (GitHub)

go get github.com/x402-foundation/x402/go

Key Concepts

• Client/Server/Facilitator: The three roles in every payment. Client signs, server enforces, facilitator settles on-chain. See

  references/core-concepts.md

• Wallet: Both payment mechanism and identity for buyers/sellers. See

  references/core-concepts.md

• Networks & Tokens: CAIP-2 identifiers, EIP-3009 tokens on EVM, SPL on Solana, custom token config. See

  references/core-concepts.md

• Scheme: Payment method.

  exact

  = transfer exact amount;

  upto

  = authorize max, settle actual usage (TS + Go, EVM Permit2 only). See

  references/evm-scheme.md

  ,

  references/svm-scheme.md

  ,

  references/stellar-scheme.md

  ,

  references/upto-scheme.md

  ,

  references/aptos-scheme.md

• Self-facilitation: Run an in-process facilitator instead of calling an external URL. See

  references/typescript-sdk.md

  ,

  references/go-sdk.md

• Transport: How payment data is transmitted (HTTP headers, MCP

  _meta

  , A2A metadata). See

  references/transports.md

• Extensions: Optional features (bazaar discovery, offer-receipt attestations, payment-identifier idempotency, sign-in-with-x auth, gas sponsoring). See

  references/extensions.md

• Hooks: Lifecycle callbacks on client/server/facilitator (TS, Python, Go). See

  references/lifecycle-hooks.md

• Protocol types:

  PaymentRequired

  ,

  PaymentPayload

  ,

  SettlementResponse

  . See

  references/protocol-spec.md

• Custom tokens: Use

  registerMoneyParser

  for non-USDC tokens, Permit2 for non-EIP-3009 tokens. See

  references/evm-scheme.md

• Mainnet deployment: Switch facilitator URL, network IDs, and wallet addresses. See

  references/core-concepts.md

References

references/core-concepts.md

references/protocol-spec.md

references/typescript-sdk.md

references/python-sdk.md

references/go-sdk.md

references/evm-scheme.md

references/svm-scheme.md

references/stellar-scheme.md

references/upto-scheme.md

references/aptos-scheme.md

references/transports.md

references/extensions.md

references/lifecycle-hooks.md

Official Resources

• GitHub: https://github.com/x402-foundation/x402
• Spec: https://github.com/x402-foundation/x402/tree/main/specs
• Docs: https://docs.x402.org
• Website: https://x402.org
• Ecosystem: https://x402.org/ecosystem
• Foundation Charter: https://github.com/x402-foundation/x402/tree/main/foundation