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x402-development

Dev Tools

Build internet-native payments with the x402 open protocol - HTTP 402 Payment Required for on-chain micropayments with no accounts or API keys. Use when developing paid APIs, paywalled content, AI agent payment flows, or MCP tools that charge per call. Covers the TypeScript, Python, and Go SDKs across EVM, Solana, Stellar, and Aptos.

Install

openclaw skills install x402-development

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, 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

NetworkCAIP-2 IDStatus
Base Mainneteip155:8453Mainnet
Base Sepoliaeip155:84532Testnet
MegaETH Mainneteip155:4326Mainnet (MegaUSD default, 18 decimals)
Solana Mainnetsolana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdpMainnet
Solana Devnetsolana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1Testnet
Stellar Mainnetstellar:pubnetMainnet (TypeScript SDK only)
Stellar Testnetstellar:testnetTestnet (TypeScript SDK only)
Aptos Mainnetaptos:1Mainnet (TypeScript SDK only)
Aptos Testnetaptos:2Testnet (TypeScript SDK only)
Monad Mainneteip155:143Mainnet
Polygon Mainneteip155:137Mainnet
Polygon Amoyeip155:80002Testnet
Stable Mainneteip155:988Mainnet
Stable Testneteip155:2201Testnet
Arbitrum Oneeip155:42161Mainnet
Arbitrum Sepoliaeip155:421614Testnet
Mezo Testneteip155:31611Testnet (mUSD, Permit2 + EIP-2612)
Avalancheeip155:43114Via community facilitators
Radius Mainneteip155:723487Mainnet (SBC default)
Radius Testneteip155:72344Testnet (SBC default)
TON Mainnettvm:-239Mainnet (jetton transfers; Python SDK)
TON Testnettvm:-3Testnet
Hedera Mainnethedera:mainnetMainnet (HBAR + HTS tokens)
Hedera Testnethedera:testnetTestnet
Algorand Mainnetalgorand:wGHE2Pwdvd7S12BL5FaOP20EGYesN73ktiC1qzkkit8=Mainnet (USDC ASA)

Default facilitator (https://x402.org/facilitator) supports Base Sepolia, Solana Devnet, Stellar Testnet, and Aptos Testnet.

SDK Packages

TypeScript v2.12.0 (npm, GitHub)

PackagePurpose
@x402/coreCore types, client, server, facilitator
@x402/evmEVM exact + upto schemes (EIP-3009, Permit2). Upto via @x402/evm/upto/* subpaths
@x402/svmSolana scheme (SPL TransferChecked)
@x402/stellarStellar scheme (SEP-41 Soroban token transfers)
@x402/aptosAptos scheme (Fungible Asset transfers)
@x402/avmAlgorand (AVM) scheme
@x402/expressExpress middleware
@x402/fastifyFastify middleware
@x402/honoHono edge middleware
@x402/nextNext.js middleware
@x402/axiosAxios interceptor
@x402/fetchFetch wrapper
@x402/paywallBrowser paywall UI
@x402/mcpMCP client + server
@x402/extensionsBazaar, offer-receipt, payment-identifier, sign-in-with-x, gas sponsoring

Python v2.10.0 (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 v2.11.0 (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 (EVM Permit2 only); batch-settlement = commit at request time, settle asynchronously; auth-capture = escrow / authorize-then-capture with void, refund, reclaim. See references/evm-scheme.md, references/svm-scheme.md, references/stellar-scheme.md, references/upto-scheme.md, references/aptos-scheme.md, references/protocol-spec.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, builder-code attribution, http-message-signatures, auth-hints). 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

FileContent
references/core-concepts.mdHTTP 402 foundation, client/server/facilitator roles, wallet identity, networks, tokens, custom token config, dynamic registration, self-hosted facilitator, mainnet deployment
references/protocol-spec.mdv2 protocol types, payment flow, facilitator API, error codes
references/typescript-sdk.mdTypeScript SDK patterns for server, client, MCP, paywall, facilitator
references/python-sdk.mdPython SDK patterns for server, client, MCP (server + client), facilitator
references/go-sdk.mdGo SDK patterns for server, client, MCP, facilitator, signers, custom money parser
references/evm-scheme.mdEVM exact scheme: EIP-3009, Permit2, default asset resolution, registerMoneyParser, custom tokens
references/svm-scheme.mdSolana exact scheme: SPL TransferChecked, verification rules, duplicate settlement mitigation
references/stellar-scheme.mdStellar exact scheme: SEP-41 Soroban token transfers, ledger-based expiration, fee sponsorship, TypeScript SDK only
references/upto-scheme.mdUpto (usage-based) scheme: authorize max amount, settle actual usage. EVM via Permit2 only
references/aptos-scheme.mdAptos exact scheme: fungible asset transfers, fee payer sponsorship, TypeScript SDK only
references/transports.mdHTTP, MCP, A2A transport implementations
references/extensions.mdBazaar, payment-identifier, sign-in-with-x, gas sponsoring (eip2612 + erc20) extensions
references/lifecycle-hooks.mdClient/server/facilitator hooks (TypeScript, Python, Go), hook chaining, MCP hooks

Official Resources

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