x402 Protocol brings HTTP micro payments to Sei, enabling you to monetize APIs, premium content, and digital services with instant, low-cost payments. Whether you’re building AI APIs, data feeds, or premium content platforms, x402 makes it simple to add payment gates to any HTTP endpoint. Works with Sei’s advantages: Sei’s fast finality, low gas fees, and EVM compatibility make it perfect for micro payments. x402 leverages these features to enable seamless payment flows that complete in milliseconds.

Why x402 on Sei?

Fast & Cheap Payments: Sei’s 400ms finality and low gas fees make micropayments practical. Perfect for pay-per-request APIs and streaming content.
EVM Compatible: Use familiar tools like Viem, Ethers.js, and Hardhat. All existing Ethereum tooling works seamlessly on Sei.
Built-in Wallet Support: Integrates with Sei wallets, MetaMask, and any EIP-6963 compatible wallet for smooth user experiences.

Use Cases on Sei

The x402 protocol enables a wide range of monetization strategies for web services and APIs:

AI & Machine Learning Services: Per-inference pricing for LLM APIs, image generation, and data processing.
Premium Content & Media: Pay-per-view articles, videos, and subscription gates.
Real-Time Data & APIs: Market data feeds, weather and IoT data monetization.
Infrastructure & CDN Services: Bandwidth metering and storage payments.

sei-js Integration

The sei-js library provides a suite of packages to simplify working with x402 on Sei. You can find more details in the sei-js x402 documentation.

Core Concepts

Protocol Overview: Learn about the architecture of x402.
Quickstart Guide: Build your first paid API.
Facilitators: Understanding payment facilitators.
Client Integration: How to integrate x402 in your frontend.

Available Packages

x402: The core protocol implementation.
x402-fetch: A fetch wrapper for making x402-compliant requests.
x402-axios: Axios interceptors for x402 payments.
x402-express: Express middleware for serving paid content.
x402-hono: Middleware for Hono applications.
x402-next: Components and utilities for Next.js applications.

These packages help streamline both the client-side (paying) and server-side (charging) aspects of the protocol.

Axiom Kit Integration

While you can build x402 integrations using standard tools and the libraries mentioned above, you can also optionally use Axiom Kit for a more agent-centric approach. This guide demonstrates an end-to-end x402 (HTTP 402 Payment Required) micropayment flow on the Sei testnet using AxiomKit.

What is x402?

x402 is an open standard protocol for internet-native payments that enables users to send and receive payments globally in a simple, secure, and interoperable manner. The protocol leverages the HTTP 402 status code (“Payment Required”) to facilitate blockchain-based micropayments directly through HTTP requests.

Key Features of x402:

HTTP-Native: Uses standard HTTP status codes and headers
Blockchain Integration: Supports multiple blockchain networks
Real-time Settlement: Enables instant payment verification
Interoperable: Works across different payment schemes and networks
Micropayment Support: Designed for small, frequent transactions

Protocol Overview

The x402 protocol follows a specific flow:

Initial Request: Client makes a request to a protected resource
402 Response: Server responds with HTTP 402 and payment requirements
Payment Execution: Client executes blockchain payment
Payment Proof: Client includes payment proof in subsequent request
Resource Access: Server verifies payment and grants access

Axiom Integration

Axiom is a blockchain interaction framework that provides tools and libraries for building decentralized applications. In this implementation, Axiom integrates with x402 to enable seamless blockchain payments within Sei network applications.

Axiom Components Used:

@axiomkit/core: Core framework for building blockchain agents
@axiomkit/sei: Sei blockchain integration
AxiomSeiWallet: Wallet management for Sei transactions
Context and Actions: Framework for building interactive blockchain agents

Sei Blockchain Implementation

This implementation uses the Sei testnet for x402 payments with the following configuration:

Network Configuration

export const X402_CONFIG = {
  network: 'sei-testnet',
  chainId: 1328,
  asset: 'USDC',
  assetAddress: '0x4fCF1784B31630811181f670Aea7A7bEF803eaED', // Sei Testnet USDC
  assetDecimals: 6,
  recipient: '0x9dC2aA0038830c052253161B1EE49B9dD449bD66',
  rpcUrl: 'https://evm-rpc-testnet.sei-apis.com'
};

Technical Architecture

The x402 implementation consists of several key components:

1. Payment Challenge Generation

function generatePaymentChallenge() {
  const reference = `sei-${Date.now()}-${Math.random().toString(36).substring(7)}`;
  const amountInUnits = parseUnits('0.001', X402_CONFIG.assetDecimals);

  return {
    x402Version: 1,
    accepts: [
      {
        scheme: 'exact',
        network: X402_CONFIG.network,
        maxAmountRequired: amountInUnits.toString(),
        resource: '/api/weather',
        description: 'Get current weather data',
        mimeType: 'application/json',
        payTo: X402_CONFIG.recipient,
        maxTimeoutSeconds: 300,
        asset: X402_CONFIG.assetAddress,
        extra: {
          name: X402_CONFIG.asset,
          version: '2',
          reference: reference
        }
      }
    ]
  };
}

2. Payment Verification

async function verifyPayment(paymentHeader: string) {
  const paymentData = JSON.parse(Buffer.from(paymentHeader, 'base64').toString());
  const { x402Version, scheme, network, payload } = paymentData;

  // Validate payment format
  if (x402Version !== 1 || scheme !== 'exact' || network !== X402_CONFIG.network) {
    return { isValid: false, reason: 'Invalid payment format or network' };
  }

  // Verify transaction on-chain
  const receipt = await publicClient.getTransactionReceipt({
    hash: payload.txHash as `0x${string}`
  });

  return { isValid: receipt?.status === 'success', txHash: payload.txHash };
}

3. Axiom Agent Integration

The Axiom agent handles the complete x402 flow:

action({
  name: 'getWeather',
  description: 'Get current weather data. This requires an x402 payment of $0.001 USDC.',
  async handler({ location }, { memory }) {
    // Step 1: Request weather data (returns 402 Payment Required)
    const weatherResponse = await fetch(`${baseUrl}/api/weather`);

    if (weatherResponse.status !== 402) {
      throw new Error(`Expected 402 Payment Required, got ${weatherResponse.status}`);
    }

    const paymentChallenge = await weatherResponse.json();

    // Step 2: Make x402 payment
    const txHash = await makeX402Payment(amount, recipient, reference);

    // Step 3: Retry request with payment proof
    const paymentProof = {
      x402Version: 1,
      scheme: 'exact',
      network: X402_CONFIG.network,
      payload: { txHash, amount, from: seiWallet.walletAddress }
    };

    const weatherDataResponse = await fetch(`${baseUrl}/api/weather`, {
      headers: { 'X-Payment': Buffer.from(JSON.stringify(paymentProof)).toString('base64') }
    });

    return weatherDataResponse.json();
  }
});

Payment Flow

1. Initial Request

Client → GET /api/weather
Server → 402 Payment Required + Payment Challenge

2. Payment Challenge Response

{
  "x402Version": 1,
  "accepts": [
    {
      "scheme": "exact",
      "network": "sei-testnet",
      "maxAmountRequired": "1000",
      "resource": "/api/weather",
      "description": "Get current weather data",
      "mimeType": "application/json",
      "payTo": "0x9dC2aA0038830c052253161B1EE49B9dD449bD66",
      "maxTimeoutSeconds": 300,
      "asset": "0x4fCF1784B31630811181f670Aea7A7bEF803eaED",
      "extra": {
        "name": "USDC",
        "version": "2",
        "reference": "sei-1234567890-abc123"
      }
    }
  ]
}

3. Payment Execution

The Axiom agent executes a USDC transfer on Sei testnet:

const transferData = encodeFunctionData({
  abi: [
    {
      name: 'transfer',
      type: 'function',
      stateMutability: 'nonpayable',
      inputs: [
        { name: 'to', type: 'address' },
        { name: 'amount', type: 'uint256' }
      ],
      outputs: [{ name: '', type: 'bool' }]
    }
  ],
  functionName: 'transfer',
  args: [recipient, amountInUnits]
});

const hash = await seiWallet.walletClient.sendTransaction({
  to: X402_CONFIG.assetAddress,
  data: transferData
});

4. Payment Proof Submission

Client → GET /api/weather + X-Payment Header (base64 encoded payment proof)
Server → Verifies payment + Returns weather data

Code Examples

Complete Weather API Implementation

export async function GET(req: NextRequest) {
  const paymentHeader = req.headers.get('x-payment');

  if (!paymentHeader) {
    // No payment provided, return 402 with payment requirements
    return NextResponse.json(generatePaymentChallenge(), { status: 402 });
  }

  // Verify the payment
  const verification = await verifyPayment(paymentHeader);

  if (!verification.isValid) {
    // Invalid payment, return 402 with error
    const challenge = generatePaymentChallenge();
    challenge.error = verification.reason || 'Payment verification failed';
    return NextResponse.json(challenge, { status: 402 });
  }

  // Payment verified, return weather data
  const weatherData = {
    location: 'Sei Network',
    temperature: '99°F',
    conditions: 'Sunny',
    humidity: '45%',
    windSpeed: '8 mph',
    timestamp: new Date().toISOString(),
    payment: verification
  };

  return NextResponse.json(weatherData);
}

Axiom Agent Weather Action

action({
  name: 'getWeather',
  description: 'Get current weather data. This requires an x402 payment of $0.001 USDC.',
  schema: {
    location: z.string().optional().describe('Optional location for weather data.')
  },
  async handler({ location }, { memory }) {
    try {
      // Step 1: Request weather data (this will return 402 Payment Required)
      const weatherResponse = await fetch(`${baseUrl}/api/weather`);

      if (weatherResponse.status !== 402) {
        throw new Error(`Expected 402 Payment Required, got ${weatherResponse.status}`);
      }

      const paymentChallenge = await weatherResponse.json();

      // Step 2: Make x402 payment
      const reference = paymentChallenge.accepts[0].extra.reference;
      const amount = '0.001'; // $0.001 USDC
      const recipient = paymentChallenge.accepts[0].payTo;

      const txHash = await makeX402Payment(amount, recipient, reference);

      // Step 3: Wait for transaction confirmation
      await new Promise((resolve) => setTimeout(resolve, 3000));

      // Step 4: Retry weather request with payment proof
      const paymentProof = {
        x402Version: 1,
        scheme: 'exact',
        network: X402_CONFIG.network,
        payload: {
          txHash: txHash,
          amount: parseUnits(amount, X402_CONFIG.assetDecimals).toString(),
          from: seiWallet.walletAddress
        }
      };

      const paymentHeader = Buffer.from(JSON.stringify(paymentProof)).toString('base64');

      const weatherDataResponse = await fetch(`${baseUrl}/api/weather`, {
        headers: {
          'Content-Type': 'application/json',
          'X-Payment': paymentHeader
        }
      });

      const weatherData = await weatherDataResponse.json();

      // Update memory
      memory.transactions.unshift(txHash);
      memory.lastTransaction = txHash;

      return actionResponse(`🌤️ **Weather Data Retrieved**

**Location:** ${weatherData.location}
**Temperature:** ${weatherData.temperature}
**Conditions:** ${weatherData.conditions}
**Humidity:** ${weatherData.humidity}
**Wind Speed:** ${weatherData.windSpeed}

✅ **Payment Successful!**

**Transaction Hash:** ${txHash}
**Amount:** $${amount} USDC
**Status:** Confirmed on Sei Testnet

🔗 **View Transaction:** [Seiscan](https://testnet.seiscan.io/tx/${txHash})`);
    } catch (error) {
      return actionResponse(`❌ **Weather Request Failed**

${error.message}

Please try again or check your wallet balance for USDC tokens needed for the payment.`);
    }
  }
});

Security Considerations

Payment Verification

On-chain Verification: All payments are verified against the Sei blockchain.
Transaction Receipt Validation: Ensures transaction success and proper recipient.
Payment Caching: Prevents double-spending by caching verified payments.
Reference Validation: Unique payment references prevent replay attacks.

Network Security

HTTPS Required: All API communications use secure connections.
Base64 Encoding: Payment proofs are base64 encoded for safe transmission.
Timeout Handling: Payment challenges include timeout mechanisms.
Error Handling: Comprehensive error handling prevents information leakage.

Wallet Security

Private Key Management: Private keys are stored securely in environment variables.
Transaction Signing: All transactions are properly signed before submission.
Balance Validation: Sufficient balance checks before payment execution.

Tutorials & Resources

Sei-js X402 Documentation: Comprehensive guide on using the x402 protocol with the sei-js library, including package details and examples.
AxiomKit X402 Demo Repository: The complete source code for the Axiom integration demo used in this guide, including installation and configuration instructions.

Overview

Essentials

seid CLI

Frontend Development

Smart Contracts

sei-js Library

Videos

Ecosystem Tutorials

Reference

Hardware Wallets

x402 Protocol on Sei

Use Cases on Sei

sei-js Integration

Core Concepts

Available Packages

Axiom Kit Integration

What is x402?

Key Features of x402:

Protocol Overview

Axiom Integration

Axiom Components Used:

Sei Blockchain Implementation

Network Configuration

Technical Architecture

1. Payment Challenge Generation

2. Payment Verification

3. Axiom Agent Integration

Payment Flow

1. Initial Request

2. Payment Challenge Response

3. Payment Execution

4. Payment Proof Submission

Code Examples

Complete Weather API Implementation

Axiom Agent Weather Action

Security Considerations

Payment Verification

Network Security

Wallet Security

Tutorials & Resources

Overview

Essentials

seid CLI

Frontend Development

Smart Contracts

sei-js Library

Videos

Ecosystem Tutorials

Reference

Hardware Wallets

Documentation Index

​Use Cases on Sei

​sei-js Integration

​Core Concepts

​Available Packages

​Axiom Kit Integration

​What is x402?

​Key Features of x402:

​Protocol Overview

​Axiom Integration

​Axiom Components Used:

​Sei Blockchain Implementation

​Network Configuration

​Technical Architecture

​1. Payment Challenge Generation

​2. Payment Verification

​3. Axiom Agent Integration

​Payment Flow

​1. Initial Request

​2. Payment Challenge Response

​3. Payment Execution

​4. Payment Proof Submission

​Code Examples

​Complete Weather API Implementation

​Axiom Agent Weather Action

​Security Considerations

​Payment Verification

​Network Security

​Wallet Security

​Tutorials & Resources

Use Cases on Sei

sei-js Integration

Core Concepts

Available Packages

Axiom Kit Integration

What is x402?

Key Features of x402:

Protocol Overview

Axiom Integration

Axiom Components Used:

Sei Blockchain Implementation

Network Configuration

Technical Architecture

1. Payment Challenge Generation

2. Payment Verification

3. Axiom Agent Integration

Payment Flow

1. Initial Request

2. Payment Challenge Response

3. Payment Execution

4. Payment Proof Submission

Code Examples

Complete Weather API Implementation

Axiom Agent Weather Action

Security Considerations

Payment Verification

Network Security

Wallet Security

Tutorials & Resources