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, 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/v2/http"
    ginmw "github.com/x402-foundation/x402/go/v2/http/gin"
    evm "github.com/x402-foundation/x402/go/v2/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/v2

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

| Network | CAIP-2 ID | Status | |---------|-----------|--------| | Base Mainnet | eip155:8453 | Mainnet | | Base Sepolia | eip155:84532 | Testnet | | MegaETH Mainnet | eip155:4326 | Mainnet (MegaUSD default, 18 decimals) | | Solana Mainnet | solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp | Mainnet | | Solana Devnet | solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1 | Testnet | | Stellar Mainnet | stellar:pubnet | Mainnet (TypeScript SDK only) | | Stellar Testnet | stellar:testnet | Testnet (TypeScript SDK only) | | Aptos Mainnet | aptos:1 | Mainnet (TypeScript SDK only) | | Aptos Testnet | aptos:2 | Testnet (TypeScript SDK only) | | Monad Mainnet | eip155:143 | Mainnet | | Polygon Mainnet | eip155:137 | Mainnet | | Polygon Amoy | eip155:80002 | Testnet | | Stable Mainnet | eip155:988 | Mainnet | | Stable Testnet | eip155:2201 | Testnet | | Arbitrum One | eip155:42161 | Mainnet | | Arbitrum Sepolia | eip155:421614 | Testnet | | XDC Network Mainnet | eip155:50 | Mainnet (USDC) | | XDC Apothem Testnet | eip155:51 | Testnet (USDC) | | Mezo Mainnet | eip155:31612 | Mainnet (mUSD, 18 decimals, Permit2 + EIP-2612) | | Mezo Testnet | eip155:31611 | Testnet (mUSD, Permit2 + EIP-2612) | | Avalanche | eip155:43114 | Runtime registration only (no default asset; community facilitators) | | Radius Mainnet | eip155:723487 | Mainnet (SBC default) | | Radius Testnet | eip155:72344 | Testnet (SBC default) | | ADI Chain | eip155:36900 | Mainnet (USDC.e default) | | HPP Mainnet | eip155:190415 | Mainnet (Bridged USDC default) | | HPP Sepolia | eip155:181228 | Testnet (Bridged USDC default) | | TON Mainnet | tvm:-239 | Mainnet (jetton transfers; Python + TypeScript SDK) | | TON Testnet | tvm:-3 | Testnet | | Hedera Mainnet | hedera:mainnet | Mainnet (HBAR + HTS tokens) | | Hedera Testnet | hedera:testnet | Testnet | | Algorand Mainnet | algorand:wGHE2Pwdvd7S12BL5FaOP20EGYesN73ktiC1qzkkit8= | Mainnet (USDC ASA) | | Keeta Mainnet | keeta:21378 | Mainnet (TypeScript SDK) | | Keeta Testnet | keeta:1413829460 | Testnet (TypeScript SDK) | | Concordium Mainnet | ccd:9dd9ca4d19e9393877d2c44b70f89acb | Mainnet (native CCD, 6 decimals; TypeScript SDK) | | Concordium Testnet | ccd:4221332d34e1694168c2a0c0b3fd0f27 | Testnet (native CCD; TypeScript SDK) |

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

SDK Packages

TypeScript v2.17.0 (npm, GitHub)

| Package | Purpose | |---------|---------| | @x402/core | Core types, client, server, facilitator | | @x402/evm | EVM exact + upto schemes (EIP-3009, Permit2). Upto via @x402/evm/upto/* subpaths | | @x402/svm | Solana scheme (SPL TransferChecked) | | @x402/stellar | Stellar scheme (SEP-41 Soroban token transfers) | | @x402/aptos | Aptos scheme (Fungible Asset transfers) | | @x402/avm | Algorand (AVM) scheme | | @x402/hedera | Hedera scheme (HBAR + HTS fungible-asset transfers) | | @x402/tvm | TON scheme (jetton transfers) | | @x402/keeta | Keeta scheme (exact) | | @x402/concordium | Concordium scheme (native CCD, exact) | | @x402/express | Express middleware | | @x402/fastify | Fastify middleware | | @x402/hono | Hono edge middleware | | @x402/next | Next.js middleware | | @x402/axios | Axios interceptor | | @x402/fetch | Fetch wrapper | | @x402/paywall | Browser paywall UI | | @x402/mcp | MCP client + server | | @x402/extensions | Bazaar, offer-receipt, payment-identifier, sign-in-with-x, gas sponsoring |

Python v2.14.0 (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 v2.17.0 (GitHub)

The Go module path carries a /v2 suffix - the bare .../x402/go path no longer resolves tagged releases.

go get github.com/x402-foundation/x402/go/v2

Java (Java 17+, GitHub)

A fourth official binding is in the repo (PaymentFilter, FacilitatorClient, X402HttpClient). Not published to a package registry yet - build from source.

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

| File | Content | |------|---------| | references/core-concepts.md | HTTP 402 foundation, client/server/facilitator roles, wallet identity, networks, tokens, custom token config, dynamic registration, self-hosted facilitator, mainnet deployment | | references/protocol-spec.md | v2 protocol types, payment flow, facilitator API, error codes | | references/typescript-sdk.md | TypeScript SDK patterns for server, client, MCP, paywall, facilitator | | references/python-sdk.md | Python SDK patterns for server, client, MCP (server + client), facilitator | | references/go-sdk.md | Go SDK patterns for server, client, MCP, facilitator, signers, custom money parser | | references/evm-scheme.md | EVM exact scheme: EIP-3009, Permit2, default asset resolution, registerMoneyParser, custom tokens | | references/svm-scheme.md | Solana exact scheme: SPL TransferChecked, verification rules, duplicate settlement mitigation | | references/stellar-scheme.md | Stellar exact scheme: SEP-41 Soroban token transfers, ledger-based expiration, fee sponsorship, TypeScript SDK only | | references/upto-scheme.md | Upto (usage-based) scheme: authorize max amount, settle actual usage. EVM via Permit2 only | | references/aptos-scheme.md | Aptos exact scheme: fungible asset transfers, fee payer sponsorship, TypeScript SDK only | | references/transports.md | HTTP, MCP, A2A transport implementations | | references/extensions.md | Bazaar, payment-identifier, sign-in-with-x, gas sponsoring (eip2612 + erc20) extensions | | references/lifecycle-hooks.md | Client/server/facilitator hooks (TypeScript, Python, Go), hook chaining, MCP hooks |

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