x402

x402 is an open payment protocol that revives the HTTP 402 "Payment Required" status code to enable instant stablecoin payments directly over HTTP. It uses a three-actor model — Client (buyer), Resource Server (seller), and Facilitator (settlement) — to let any HTTP endpoint accept payment without API keys, sessions, or accounts. The protocol is built on EIP-3009 transferWithAuthorization for gasless USDC transfers, meaning clients sign an off-chain authorization and the facilitator broadcasts the on-chain settlement.

Package ecosystem:

• Server (TypeScript): @x402/express, @x402/hono, @x402/next
• Client (TypeScript): @x402/fetch, @x402/axios
• Client (Python): pip install x402
• Client (Go): go get github.com/coinbase/x402/go
• Core: @x402/core, @x402/evm, @x402/svm

CDP Facilitator endpoint: https://api.cdp.coinbase.com/platform/v2/x402

What You Probably Got Wrong

LLMs have stale training data. These are the most common mistakes.

• "x402 requires API keys or OAuth" -- There are no API keys. Payment IS authentication. The client signs a USDC transfer authorization, includes it in the X-PAYMENT header, and the server verifies it. If the signature and balance check pass, the request is authorized. No accounts, no sessions, no bearer tokens.

• "x402 only works on Base" -- The protocol supports any EVM chain where USDC implements EIP-3009 (Base, Ethereum, Arbitrum, Optimism, Polygon) and Solana. Network identifiers use CAIP-2 format: eip155:8453 for Base mainnet, eip155:1 for Ethereum, solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp for Solana mainnet.

• "Nonces are sequential counters" -- x402 nonces are random 32-byte values (bytes32), not incrementing integers. Each authorization generates a fresh random nonce. The ERC-3009 contract tracks used nonces on-chain to prevent replay. Using sequential nonces or reusing a nonce will cause settlement to revert.

• "The client pays gas" -- The client never submits an on-chain transaction. The client signs an off-chain EIP-712 authorization. The facilitator broadcasts the transferWithAuthorization call and pays gas. CDP's hosted facilitator absorbs gas costs on Base mainnet for free-tier usage (1,000 tx/month).

• "x402 supports any ERC-20 token" -- The primary path uses USDC via EIP-3009 transferWithAuthorization for truly gasless transfers. A Permit2 fallback exists for arbitrary ERC-20 tokens, but it requires the user to have previously approved the Permit2 contract, which needs native gas. For the simplest integration, use USDC.

• "The old package name is @coinbase/x402" -- The current packages use the @x402/* namespace (e.g., @x402/express, @x402/fetch). Earlier versions used x402-express and x402-fetch without the scope. The @coinbase/x402 name was never published.

• "The payment header is called PAYMENT-SIGNATURE" -- The header name is X-PAYMENT. It contains a base64-encoded JSON payload with the authorization, signature, scheme, and network. The server responds with X-PAYMENT-RESPONSE.

Core Concepts

Three-Actor Model

┌──────────┐      1. Request resource       ┌─────────────────┐
│          │ ─────────────────────────────► │                 │
│          │      2. HTTP 402 + payment     │   Resource      │
│  Client  │ ◄───────────────────────────── │   Server        │
│ (Buyer)  │      3. Request + X-PAYMENT    │   (Seller)      │
│          │ ─────────────────────────────► │                 │
│          │      6. HTTP 200 + resource    │                 │
│          │ ◄───────────────────────────── │                 │
└──────────┘                                └────────┬────────┘
                                                     │
                                              4. Verify │ 5. Settle
                                                     │
                                            ┌────────▼────────┐
                                            │                 │
                                            │  Facilitator    │
                                            │  (Settlement)   │
                                            │                 │
                                            └─────────────────┘

Payment Flow Lifecycle

1. Client sends a standard HTTP request to a protected endpoint
2. Resource Server responds with HTTP 402 and payment requirements in the response body (scheme, network, amount, asset, payTo address)
3. Client constructs an EIP-3009 authorization (from, to, value, validAfter, validBefore, nonce), signs it with EIP-712, and retries the request with the X-PAYMENT header containing the base64-encoded payload
4. Resource Server forwards the payment to the Facilitator for verification (/verify endpoint)
5. Facilitator validates the signature, checks balance, and simulates the transfer
6. Resource Server serves the resource
7. Resource Server requests settlement from the Facilitator (/settle endpoint)
8. Facilitator calls transferWithAuthorization on the USDC contract to move funds on-chain

HTTP 402 Response Format

When a client hits a paid endpoint without a payment header, the server returns:

HTTP/1.1 402 Payment Required
Content-Type: application/json

{
  "x402Version": 2,
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:8453",
      "amount": "10000",
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "payTo": "0xSELLER_ADDRESS",
      "maxTimeoutSeconds": 60,
      "extra": {
        "assetTransferMethod": "eip3009",
        "name": "USDC",
        "version": "2"
      }
    }
  ],
  "error": "X-PAYMENT header is required"
}

The amount is in the token's smallest unit. For USDC (6 decimals), 10000 = $0.01.

ERC-3009 transferWithAuthorization

EIP-3009 enables gasless token transfers by separating authorization from execution. The token holder signs an off-chain message; anyone can submit the signed authorization to the contract.

Function Signature

function transferWithAuthorization(
    address from,
    address to,
    uint256 value,
    uint256 validAfter,
    uint256 validBefore,
    bytes32 nonce,
    bytes   signature
) external;

EIP-712 Domain

EIP712Domain({
    name: "USDC",
    version: "2",
    chainId: <chain_id>,
    verifyingContract: <usdc_address>
})

Authorization Struct (for signing)

TransferWithAuthorization({
    from: address,
    to: address,
    value: uint256,
    validAfter: uint256,
    validBefore: uint256,
    nonce: bytes32
})

Key properties:

• nonce is a random bytes32, not a sequential counter. The contract maintains a mapping of (authorizer, nonce) => bool to track used nonces.
• validAfter and validBefore define a time window. Set validAfter to 0 (or current timestamp) and validBefore to current timestamp + maxTimeoutSeconds.
• The facilitator calls this function — the signer never needs native gas.
• USDC on Base, Ethereum, Arbitrum, Optimism, and Polygon all implement EIP-3009.

Server Setup (TypeScript)

Express Middleware

npm install @x402/core @x402/evm @x402/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 facilitatorClient = new HTTPFacilitatorClient({
  url: "https://api.cdp.coinbase.com/platform/v2/x402",
});
 
const resourceServer = new x402ResourceServer(facilitatorClient)
  .register("eip155:8453", new ExactEvmScheme());
 
app.use(
  paymentMiddleware(
    {
      "GET /api/weather": {
        accepts: {
          scheme: "exact",
          network: "eip155:8453",
          price: "$0.01",
          payTo: process.env.PAYMENT_ADDRESS as `0x${string}`,
        },
        description: "Current weather data",
      },
      "POST /api/analyze": {
        accepts: {
          scheme: "exact",
          network: "eip155:8453",
          price: "$0.05",
          payTo: process.env.PAYMENT_ADDRESS as `0x${string}`,
        },
        description: "AI text analysis",
      },
    },
    resourceServer,
  ),
);
 
app.get("/api/weather", (_req, res) => {
  res.json({ temperature: 72, unit: "F", location: "San Francisco" });
});
 
app.post("/api/analyze", (req, res) => {
  res.json({ sentiment: "positive", confidence: 0.92 });
});
 
app.listen(3000, () => {
  console.log("x402 server running on port 3000");
});

Hono and Next.js middleware follow the same pattern — install @x402/hono or @x402/next and use the same paymentMiddleware + x402ResourceServer setup with framework-specific imports.

Client Setup (TypeScript)

Using @x402/fetch (Recommended)

The wrapFetchWithPaymentFromConfig function wraps the native fetch API to automatically handle 402 responses — detect the payment requirement, sign the authorization, and retry with the X-PAYMENT header.

npm install @x402/core @x402/evm @x402/fetch viem

import { wrapFetchWithPaymentFromConfig } from "@x402/fetch";
import { ExactEvmScheme } from "@x402/evm";
import { privateKeyToAccount } from "viem/accounts";
import type { Hex } from "viem";
 
const account = privateKeyToAccount(process.env.PRIVATE_KEY as Hex);
 
const fetchWithPayment = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [
    {
      network: "eip155:8453",
      client: new ExactEvmScheme(account),
    },
  ],
});
 
const response = await fetchWithPayment("https://api.example.com/api/weather");
if (!response.ok) {
  throw new Error(`Request failed: ${response.status}`);
}
 
const data = await response.json();
console.log(data);

Multi-Network Client

import { x402Client, wrapFetchWithPayment } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { registerExactSvmScheme } from "@x402/svm/exact/client";
import { privateKeyToAccount } from "viem/accounts";
import { Keypair } from "@solana/web3.js";
import type { Hex } from "viem";
 
const evmAccount = privateKeyToAccount(process.env.EVM_PRIVATE_KEY as Hex);
const solanaKeypair = Keypair.fromSecretKey(
  Buffer.from(process.env.SOLANA_PRIVATE_KEY as string, "base64")
);
 
const client = new x402Client();
registerExactEvmScheme(client, { signer: evmAccount });
registerExactSvmScheme(client, { signer: solanaKeypair });
 
const fetchWithPayment = wrapFetchWithPayment(fetch, client);
 
// Automatically handles both EVM and Solana payment requirements
const evmResponse = await fetchWithPayment("https://api.example.com/evm-data");
const solResponse = await fetchWithPayment("https://api.example.com/sol-data");

For manual header construction without the wrapper, see the agent-client and python-client examples in this skill's examples/ directory.

Client Setup (Python)

pip install "x402[evm,requests]"

Using the x402 Client

from x402 import x402ClientSync
from x402.evm import ExactEvmScheme
from eth_account import Account
 
wallet = Account.from_key("0xYOUR_PRIVATE_KEY")
 
client = x402ClientSync()
client.register("eip155:*", ExactEvmScheme(signer=wallet))
 
# Make a paid request — the client handles the 402 flow automatically
import requests
 
session = requests.Session()
response = session.get("https://api.example.com/api/weather")
 
if response.status_code == 402:
    payment_required = response.json()
    payload = client.create_payment_payload(payment_required)
    response = session.get(
        "https://api.example.com/api/weather",
        headers={"X-PAYMENT": payload.to_header()},
    )
 
print(response.json())

For manual EIP-712 signing without the x402 package, see the python-client example in this skill's examples/ directory.

Facilitator Patterns

CDP Hosted Facilitator (Default)

Coinbase Developer Platform provides a hosted facilitator with two endpoints:

Pricing:

• Free tier: 1,000 transactions/month
• Standard: $0.001 per transaction thereafter
• Base mainnet: Zero gas fees (gas absorbed by facilitator)

import { HTTPFacilitatorClient } from "@x402/core/server";
 
const facilitator = new HTTPFacilitatorClient({
  url: "https://api.cdp.coinbase.com/platform/v2/x402",
});

Custom Facilitator

Run your own facilitator for full control over settlement:

import { createFacilitatorServer } from "@x402/core/facilitator";
import { ExactEvmScheme } from "@x402/evm/exact/server";
import { createWalletClient, http } from "viem";
import { privateKeyToAccount } from "viem/accounts";
import { base } from "viem/chains";
import type { Hex } from "viem";
 
// Facilitator wallet pays gas for settlement
const facilitatorAccount = privateKeyToAccount(
  process.env.FACILITATOR_PRIVATE_KEY as Hex
);
 
const walletClient = createWalletClient({
  account: facilitatorAccount,
  chain: base,
  transport: http(),
});
 
const server = createFacilitatorServer({
  schemes: {
    "eip155:8453": new ExactEvmScheme({ walletClient }),
  },
});
 
server.listen(4000, () => {
  console.log("Custom facilitator on port 4000");
});

Settlement Flow

1. Resource server sends the payment payload to facilitator /verify
2. Facilitator checks: signature validity, signer balance, authorization parameters, transfer simulation
3. If valid, resource server serves the response and calls facilitator /settle
4. Facilitator submits transferWithAuthorization(from, to, value, validAfter, validBefore, nonce, signature) to the USDC contract
5. Facilitator returns the transaction hash and receipt

The facilitator cannot modify the amount or destination. It serves only as the transaction broadcaster.

Agent-to-Agent Payments

x402 enables autonomous agent commerce where AI agents discover services, negotiate prices, and pay without human intervention.

Discovery Pattern

// Agent discovers paid endpoints via standard HTTP
const discoveryResponse = await fetch("https://agent-service.com/.well-known/x402");
const services = await discoveryResponse.json();
 
// services contains available endpoints with pricing
// {
//   "endpoints": [
//     { "path": "/api/summarize", "price": "$0.02", "network": "eip155:8453" },
//     { "path": "/api/translate", "price": "$0.01", "network": "eip155:8453" }
//   ]
// }

Autonomous Payment Execution

import { wrapFetchWithPaymentFromConfig } from "@x402/fetch";
import { ExactEvmScheme } from "@x402/evm";
import { privateKeyToAccount } from "viem/accounts";
import type { Hex } from "viem";
 
const agentAccount = privateKeyToAccount(process.env.AGENT_PRIVATE_KEY as Hex);
 
const agentFetch = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [
    { network: "eip155:8453", client: new ExactEvmScheme(agentAccount) },
  ],
});
 
// Agent autonomously pays for another agent's service
async function callPaidService(url: string, body: Record<string, unknown>): Promise<unknown> {
  const response = await agentFetch(url, {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify(body),
  });
 
  if (!response.ok) {
    throw new Error(`Service call failed: ${response.status}`);
  }
 
  return response.json();
}
 
// Multi-step agent workflow: summarize -> translate -> store
const summary = await callPaidService("https://summarizer.agent/api/summarize", {
  text: "Long document content...",
});
 
const translation = await callPaidService("https://translator.agent/api/translate", {
  text: summary,
  targetLanguage: "es",
});

Pricing Strategies

Per-Request (Fixed Price)

Charge a flat fee per API call. Simplest model.

"GET /api/weather": {
  accepts: {
    scheme: "exact",
    network: "eip155:8453",
    price: "$0.01",
    payTo: process.env.PAYMENT_ADDRESS as `0x${string}`,
  },
  description: "Weather data — $0.01 per request",
},

Tiered Pricing

Different prices per endpoint based on compute cost.

const routes = {
  "GET /api/basic-query": {
    accepts: {
      scheme: "exact",
      network: "eip155:8453",
      price: "$0.001",
      payTo: paymentAddress,
    },
    description: "Basic database query",
  },
  "POST /api/ai-analysis": {
    accepts: {
      scheme: "exact",
      network: "eip155:8453",
      price: "$0.05",
      payTo: paymentAddress,
    },
    description: "AI-powered analysis — higher compute cost",
  },
  "POST /api/image-generation": {
    accepts: {
      scheme: "exact",
      network: "eip155:8453",
      price: "$0.10",
      payTo: paymentAddress,
    },
    description: "Image generation — GPU cost",
  },
};

Dynamic Pricing

Adjust prices based on demand, time of day, or resource utilization.

function getDynamicPrice(basePrice: number, currentLoad: number): string {
  // Surge pricing: 1x at 0% load, up to 3x at 100% load
  const multiplier = 1 + (currentLoad / 100) * 2;
  const price = basePrice * multiplier;
  return `$${price.toFixed(4)}`;
}

Replay Protection

x402 uses two mechanisms to prevent payment replay:

Random Nonces

Every authorization includes a random 32-byte nonce. The USDC contract's authorizationState mapping tracks whether a (from, nonce) pair has been used. Attempting to settle with a used nonce reverts with authorization is used or canceled.

import { keccak256, encodePacked, toHex } from "viem";
 
// Generate a unique nonce per authorization
function generateNonce(): Hex {
  const randomBytes = crypto.getRandomValues(new Uint8Array(32));
  return toHex(randomBytes);
}

Expiration Timestamps

The validBefore field ensures authorizations cannot be settled after their time window. Best practice: set validBefore to now + maxTimeoutSeconds (typically 30-60 seconds).

const now = Math.floor(Date.now() / 1000);
const authorization = {
  validAfter: BigInt(0),
  validBefore: BigInt(now + 60), // 60-second window
  nonce: generateNonce(),
  // ... other fields
};

If a facilitator does not settle within the validBefore window, the authorization expires and the client's funds are never moved. The client can safely retry with a new authorization.

Permit2 Fallback

For ERC-20 tokens that do not implement EIP-3009, x402 supports a Permit2-based fallback.

How It Differs

The Permit2 path requires the user to have approved the Permit2 contract (0x000000000022D473030F116dDEE9F6B43aC78BA3). If the allowance is missing, the server returns HTTP 412 with error code PERMIT2_ALLOWANCE_REQUIRED.

Supported Networks

Last verified: February 2026

References

• Spec: https://github.com/coinbase/x402/tree/main/specs
• TypeScript SDK: https://github.com/coinbase/x402/tree/main/typescript
• Python SDK: https://pypi.org/project/x402/
• Go SDK: https://github.com/coinbase/x402/tree/main/go
• CDP Docs: https://docs.cdp.coinbase.com/x402/welcome
• EIP-3009: https://eips.ethereum.org/EIPS/eip-3009
• x402.org: https://www.x402.org
• Cloudflare x402 Foundation: https://blog.cloudflare.com/x402/