X402 API Server
x402 protocol API server β pay-per-call DeFi data endpoints using USDC micropayments on Base
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x402 API Server
Pay-per-call crypto/DeFi data API using HTTP 402 Payment Required.
No API keys. No subscriptions. AI agents pay USDC on Base, per request.
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π Live at: https://x402-api.fly.dev
π€ Agent Identity: #18763 on Base (ERC-8004)
π Blog Post: I Built a DeFi Data API Where AI Agents Pay Per Call
What Is This?
This is a DeFi/crypto data API server where payment is part of the HTTP protocol itself.
Every endpoint costs a small USDC micropayment β between 0.001 and 0.008 USDC (fractions of a cent). When a client hits a protected endpoint, the server responds with HTTP 402 Payment Required and precise payment instructions. The client pays, retries with proof, and gets data.
No accounts. No OAuth. No billing dashboard. Just: "this costs 0.003 USDC β pay here."
This model is designed for AI agents: autonomous software that needs to call APIs without a human reaching for a credit card. The agent holds a wallet, pays exactly what's needed, and moves on.
The stack:
- x402 Protocol β HTTP 402-based micropayment standard by Coinbase
- USDC on Base β L2 mainnet, sub-cent fees, instant finality
- ERC-8004 β on-chain identity registration for AI agents
- Express.js backend β straightforward, auditable, no framework magic
Endpoints
| Endpoint | Price | Description |
|---|---|---|
GET /api/price-feed | 0.001 USDC | BTC/ETH/SOL prices + top 24h movers (live CoinGecko) |
GET /api/gas-tracker | 0.001 USDC | Multi-chain gas prices (ETH, Base, Polygon, Arbitrum) with speed tiers |
GET /api/dex-quotes | 0.002 USDC | Compare swap quotes across Uniswap, SushiSwap, 1inch |
GET /api/token-scanner | 0.003 USDC | Token security & risk analysis β rug-pull detection flags |
GET /api/whale-tracker | 0.005 USDC | Token holder concentration, Gini coefficient, whale alerts |
GET /api/yield-scanner | 0.005 USDC | Top DeFi yields across Aave, Compound, Morpho, Lido, Pendle + more |
GET /api/funding-rates | 0.008 USDC | Perp funding rates across 6 venues + arb ranking |
GET /api/wallet-profiler | 0.008 USDC | Wallet portfolio analysis, holdings, activity, risk profile |
GET /api/endpoints | Free | Machine-readable endpoint catalog |
GET /health | Free | Health check |
Query Parameters
| Endpoint | Parameter | Example |
|---|---|---|
/api/dex-quotes | from, to, amount | ?from=ETH&to=USDC&amount=1.5 |
/api/token-scanner | token | ?token=PEPE or ?token=0x... |
/api/whale-tracker | token | ?token=ETH |
/api/yield-scanner | chain, min_tvl | ?chain=base&min_tvl=1000000 |
/api/funding-rates | asset | ?asset=BTC |
/api/wallet-profiler | address | ?address=0x... |
How x402 Works
The x402 protocol extends the long-forgotten HTTP 402 Payment Required status code into a machine-readable micropayment standard.
The Payment Flow
Step 1 β Initial request (no payment)
GET /api/price-feed HTTP/1.1
Host: x402-api.fly.dev
Step 2 β Server returns 402
HTTP/1.1 402 Payment Required
Content-Type: application/json
{
"x402Version": 1,
"error": "Payment Required",
"accepts": [{
"scheme": "exact",
"network": "base",
"asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
"payTo": "0x60264c480b67adb557efEd22Cf0e7ceA792DefB7",
"maxAmountRequired": "1000",
"resource": "https://x402-api.fly.dev/api/price-feed",
"description": "Live crypto price feed",
"extra": {
"name": "USD Coin",
"chainId": 8453,
"supportedProofs": ["txHash"],
"experimentalProofs": ["eip3009_transferWithAuthorization"]
}
}]
}
Step 3 β Client pays and retries
GET /api/price-feed HTTP/1.1
Host: x402-api.fly.dev
X-Payment: <base64-encoded payment proof>
Step 4 β Server verifies and responds
HTTP/1.1 200 OK
X-Payment-Response: {"success":true,"txHash":"0x..."}
Content-Type: application/json
{ "btc": 95420.12, "eth": 3241.88, ... }
Payment Verification
In the current repo, the production-ready payment proof path is:
- Transaction hash β Confirms a submitted Base transaction contains a USDC transfer to the receiving address.
The repo also supports EIP-3009 transferWithAuthorization settlement when you configure one of these:
- Direct settlement via
X402_SETTLEMENT_PRIVATE_KEYβ the server submitstransferWithAuthorizationon-chain and pays gas. - Custom facilitator settlement via
X402_FACILITATOR_URLβ the server forwards the signed authorization to your facilitator.
Without either of those configured, the deployment stays on the txHash payment proof path only.
Receiving wallet: 0x60264c480b67adb557efEd22Cf0e7ceA792DefB7
Network: Base mainnet (chain ID 8453)
Asset: USDC (0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913)
Using with x402-fetch
x402-fetch is the official client library from Coinbase that handles the payment flow automatically. Drop it in as a fetch replacement.
Installation
npm install x402-fetch viem
Basic Usage
import { wrapFetchWithPayment } from 'x402-fetch';
import { createWalletClient, http } from 'viem';
import { privateKeyToAccount } from 'viem/accounts';
import { base } from 'viem/chains';
const account = privateKeyToAccount(process.env.PRIVATE_KEY as `0x${string}`);
const walletClient = createWalletClient({
account,
chain: base,
transport: http(),
});
// Wrap fetch β payments handled automatically
const fetchWithPayment = wrapFetchWithPayment(fetch, walletClient);
// Just call the API β 402 is handled transparently
const response = await fetchWithPayment('https://x402-api.fly.dev/api/price-feed');
const data = await response.json();
console.log(data);
What wrapFetchWithPayment Does
- Makes the initial request
- If 402 β parses payment requirements
- Signs the payment authorization (EIP-3009
transferWithAuthorization) - Retries the request with
X-Paymentheader - Returns the successful response
Note: this is the standard x402 client flow. The current server repo also supports a simpler proof format where a client pays on-chain first, then sends a Base64 JSON X-Payment header like:
{ "txHash": "0x...", "payer": "0x..." }
Your wallet needs USDC on Base mainnet. You can bridge USDC from Ethereum to Base using the Base Bridge or buy directly on Coinbase.
ERC-8004 Identity
This API is registered as an AI agent on Base via ERC-8004 β the Ethereum standard for on-chain agent identity.
Agent ID: #18763
Registry contract: 0x8004A169FB4a3325136EB29fA0ceB6D2e539a432
Network: Base mainnet
The registration includes:
- Agent name and description
- Service endpoints (web + x402)
- x402 capability flag
- Reputation-based trust signals
This lets discovery services, AI marketplaces, and other agents find and verify this API on-chain β no centralized registry required.
See the registration data: agent-registration.json
Integrations
MCP Server (Claude Desktop / Claude API)
The MCP (Model Context Protocol) server now lives in its own standalone repo:
- GitHub: fernsugi/x402-api-mcp-server
- npm:
@x402-api/mcp-server
Quick setup:
# Run without payment (inspect mode)
npx @x402-api/mcp-server
# Run with auto-pay
X402_WALLET_PRIVATE_KEY=0x... npx @x402-api/mcp-server
Add to Claude Desktop (claude_desktop_config.json):
{
"mcpServers": {
"x402-api": {
"command": "npx",
"args": ["@x402-api/mcp-server"],
"env": {
"X402_WALLET_PRIVATE_KEY": "0x<your_key>"
}
}
}
}
β See the standalone MCP repo for full docs: x402-api-mcp-server README
ElizaOS Plugin
The elizaos-plugin/ directory contains an ElizaOS plugin that gives autonomous agents natural-language access to all 8 endpoints.
Install:
npm install @x402-api/elizaos-plugin
Register in your ElizaOS agent:
import { x402ApiPlugin } from '@x402-api/elizaos-plugin';
export const agent: Character = {
name: 'DeFi Agent',
plugins: [
x402ApiPlugin({
walletPrivateKey: process.env.X402_WALLET_PRIVATE_KEY,
}),
],
};
The plugin adds 8 actions: GET_CRYPTO_PRICES, GET_GAS_PRICES, GET_DEX_QUOTES, SCAN_TOKEN, TRACK_WHALES, SCAN_YIELDS, GET_FUNDING_RATES, PROFILE_WALLET.
β See elizaos-plugin/README.md for full docs.
Local Development
git clone https://github.com/sugi/x402-api-server
cd x402-api-server
cp .env.example .env # defaults to development mode
npm install
npm run dev # auto-reload on changes
In development mode (NODE_ENV=development), any non-empty X-PAYMENT header is accepted β no real payments needed for local testing.
# Get 402 response with payment instructions
curl -i http://localhost:4020/api/price-feed
# Test with mock payment (dev only)
curl http://localhost:4020/api/price-feed -H "X-Payment: test"
Architecture
src/
βββ index.js # Express server + graceful shutdown
βββ payment-config.js # Settlement mode + proof support detection
βββ middleware/
β βββ x402.js # x402 payment gate middleware
βββ routes/
β βββ priceFeed.js # /api/price-feed (live CoinGecko)
β βββ gasTracker.js # /api/gas-tracker (real RPC + mock)
β βββ dexQuotes.js # /api/dex-quotes
β βββ tokenScanner.js # /api/token-scanner
β βββ whaleTracker.js # /api/whale-tracker
β βββ yieldScanner.js # /api/yield-scanner
β βββ fundingRates.js # /api/funding-rates
β βββ walletProfiler.js # /api/wallet-profiler
βββ services/
β βββ verifier.js # txHash verifier + EIP-3009 settlement
βββ views/
βββ index.html # Landing page
elizaos-plugin/ # @x402-api/elizaos-plugin
Standalone sibling repo:
x402-api-mcp-server/ # @x402-api/mcp-server
Deployment
Fly.io (recommended)
curl -L https://fly.io/install.sh | sh
fly auth login
./deploy.sh
Docker
docker compose up -d
Environment Variables
| Variable | Default | Description |
|---|---|---|
PAY_TO_ADDRESS | 0x60264c...DefB7 | USDC receiving wallet on Base |
PORT | 4020 | Server port |
NODE_ENV | development | Set production for real payment verification |
BASE_RPC_URL | https://mainnet.base.org | Base RPC URL |
X402_SETTLEMENT_MODE | auto | auto, direct, facilitator, or disabled |
X402_SETTLEMENT_PRIVATE_KEY | none | Sponsor key for direct transferWithAuthorization settlement |
X402_FACILITATOR_URL | none | Custom facilitator URL for EIP-3009 settlement |
X402_FACILITATOR_API_KEY | none | Optional bearer token for your facilitator |
Links
- π Landing Page
- π x402 Protocol
- βοΈ Base Chain
- π Agent #18763 on BaseScan
- π ERC-8004 Spec
- π€ MCP Server
- π€ ElizaOS Plugin
License
MIT
