x402 Agent Skill

Automatically detect and pay x402 crypto paywalls when your AI agent encounters HTTP 402 Payment Required responses.

When to Activate

Activate this skill when an HTTP response returns status 402 Payment Required and the response body contains an x402Version field with an accepts array describing payment options.

Detection pattern:

• HTTP status code: 402
• Response body: JSON with x402Version (number) and accepts (array of payment requirements)
• Response header: PAYMENT-REQUIRED (base64-encoded payment requirements)

How to Use

Step 1: Install

npm install @x402-kit/agent @x402-kit/shared

Step 2: Set Up the Agent Fetch Wrapper

import { createAgentFetch } from "@x402-kit/agent";

const agentFetch = createAgentFetch({
  walletPrivateKey: process.env.X402_WALLET_PRIVATE_KEY as `0x${string}`,
  network: "eip155:8453",        // Base mainnet
  policy: {
    maxPerRequest: "1.00",       // Max 1 USDC per request
    maxDailySpend: "10.00",      // Max 10 USDC per day
    allowedNetworks: ["eip155:8453"],
    allowedAssets: ["0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"], // USDC on Base
    requireHumanApproval: false,
  },
  logFilePath: "./x402-payments.jsonl",
});

Step 3: Use as a Drop-in Fetch Replacement

// Use agentFetch exactly like regular fetch
const response = await agentFetch("https://api.example.com/premium/data");
const data = await response.json();

What Happens Internally

1. The initial request is made normally
2. If the server returns HTTP 402 with x402 payment requirements, the skill:
   • Parses the payment requirements from the response headers and body
   • Finds a matching payment option for the configured network
   • Converts the raw token amount to human-readable format
   • Checks the spending policy (per-request limit, daily limit, network/asset/domain rules)
   • If approved: signs an EIP-3009 payment authorization and retries with the X-PAYMENT header
   • Records the spend and logs the payment event
3. If the policy denies the payment, the original 402 response is returned without spending

Configuration

Environment Variables

Policy Configuration

The policy object controls what the agent is allowed to spend:

interface X402Policy {
  maxPerRequest: string;        // Max USDC per single request (e.g., "1.00")
  maxDailySpend: string;        // Max USDC per day (e.g., "10.00")
  allowedNetworks: string[];    // CAIP-2 networks (e.g., ["eip155:8453"])
  allowedAssets: string[];      // Token contract addresses
  domainAllowlist?: string[];   // Only pay these domains (optional)
  domainDenylist?: string[];    // Never pay these domains (optional)
  requireHumanApproval: boolean;
}

Supported Networks

Full Config Reference

interface AgentFetchConfig {
  walletPrivateKey: Hex;        // From env: X402_WALLET_PRIVATE_KEY
  policy: X402Policy;           // Spending rules
  network: Network;             // CAIP-2 format (e.g., "eip155:8453")
  spendFilePath?: string;       // Daily spend persistence (default: ".x402/daily-spend.json")
  logFilePath?: string;         // Payment log file path
  rpcUrl?: string;              // Custom JSON-RPC URL
  tokenDecimals?: number;       // Token decimals (default: 6 for USDC)
}

Examples

Basic: Pay for Premium API Data

Agent calls api.example.com/premium, gets 402, pays 0.10 USDC, gets data:

const agentFetch = createAgentFetch({
  walletPrivateKey: process.env.X402_WALLET_PRIVATE_KEY as `0x${string}`,
  network: "eip155:8453",
  policy: {
    maxPerRequest: "0.50",
    maxDailySpend: "5.00",
    allowedNetworks: ["eip155:8453"],
    allowedAssets: ["0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"],
    requireHumanApproval: false,
  },
});

const response = await agentFetch("https://api.example.com/premium/weather");
// First request returns 402 -> agent auto-pays 0.10 USDC -> retries -> gets 200 with data

Domain-Restricted: Only Pay Trusted APIs

const agentFetch = createAgentFetch({
  walletPrivateKey: process.env.X402_WALLET_PRIVATE_KEY as `0x${string}`,
  network: "eip155:8453",
  policy: {
    maxPerRequest: "1.00",
    maxDailySpend: "20.00",
    allowedNetworks: ["eip155:8453"],
    allowedAssets: ["0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"],
    domainAllowlist: ["api.trusted-service.com", "data.partner.io"],
    requireHumanApproval: false,
  },
});

Testnet Development

const agentFetch = createAgentFetch({
  walletPrivateKey: process.env.X402_WALLET_PRIVATE_KEY as `0x${string}`,
  network: "eip155:84532",  // Base Sepolia testnet
  policy: {
    maxPerRequest: "10.00",
    maxDailySpend: "100.00",
    allowedNetworks: ["eip155:84532"],
    allowedAssets: ["0x036CbD53842c5426634e7929541eC2318f3dCF7e"], // USDC on Base Sepolia
    requireHumanApproval: false,
  },
});

Safety Rules

1. Never expose the private key. Always load X402_WALLET_PRIVATE_KEY from environment variables. Never hardcode it in source files or logs.
2. Respect spending limits. The policy engine enforces per-request and daily limits. Configure conservative limits for production.
3. Log all payments. Set logFilePath to maintain an audit trail. The log file records every payment attempt (approved, denied, or failed) in JSONL format.
4. Use domain filtering. Set domainAllowlist or domainDenylist to restrict which services the agent can pay.
5. Start with testnet. Use Base Sepolia (eip155:84532) during development. Only switch to mainnet (eip155:8453) for production.
6. Daily spend resets at UTC midnight. The spend tracker persists to disk and resets automatically each day.

Payment Log Format

Each payment event is logged as a JSON line in the log file:

{
  "timestamp": "2026-02-27T12:00:00.000Z",
  "url": "https://api.example.com/premium/data",
  "amount": "0.1",
  "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
  "network": "eip155:8453",
  "facilitator": "",
  "success": true,
  "policyDecision": "approved"
}