Pay-per-call crypto data for AI agents with x402: a working example

#x402 #ai #web3 #javascript

Most crypto data APIs want an account, an API key and a monthly plan. That's awkward when the caller is an AI agent that needs one price, once.

x402 fixes this by reusing the HTTP 402 Payment Required status: the server answers with a price, the client pays in USDC and retries, and the data comes back. No signup, no key.

We run a small x402 API on Cloudflare Workers. Here's how to call it.

The endpoints

Base URL: https://astra-x402-data.astratc.workers.dev

Endpoint	Price (USDC on Base)	Returns
`GET /`	free	catalog (JSON)
`GET /v1/coin?id=bitcoin`	$0.01	one coin: price, 24h/7d change
`GET /v1/snapshot`	$0.05	top-100 market snapshot: global cap, Fear & Greed, majors, breadth, gainers/losers
`GET /v1/brief`	$0.25	today's Astra Daily Crypto Brief (markdown)

Step 1: see the price (free)

curl -i "https://astra-x402-data.astratc.workers.dev/v1/coin?id=bitcoin"

You get HTTP/2 402 and a body like this:

{
  "x402Version": 1,
  "error": "X-PAYMENT header is required",
  "accepts": [{
    "scheme": "exact",
    "network": "base",
    "maxAmountRequired": "10000",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "payTo": "0x1A7Dc2677391D49FB3001AD5a1aa89A573c9b865",
    "resource": "https://astra-x402-data.astratc.workers.dev/v1/coin?id=bitcoin",
    "maxTimeoutSeconds": 300,
    "extra": { "name": "USD Coin", "version": "2" }
  }]
}

maxAmountRequired is in USDC base units (6 decimals), so 10000 = $0.01. asset is native USDC on Base.

Step 2: pay and fetch (JavaScript)

The x402-fetch package wraps fetch. When it sees a 402, it signs an EIP-3009 USDC authorization for exactly that amount, retries with an X-PAYMENT header, and returns the paid response.

npm i x402-fetch viem

import { wrapFetchWithPayment, decodeXPaymentResponse } from "x402-fetch";
import { privateKeyToAccount } from "viem/accounts";

// Use a dedicated low-balance wallet for your agent, never your main one.
const account = privateKeyToAccount(process.env.AGENT_PRIVATE_KEY);
const fetchWithPay = wrapFetchWithPayment(fetch, account);

const res = await fetchWithPay(
  "https://astra-x402-data.astratc.workers.dev/v1/coin?id=bitcoin"
);
console.log(await res.json());

const receipt = res.headers.get("x-payment-response");
if (receipt) console.log(decodeXPaymentResponse(receipt)); // tx hash, payer

The wallet needs a little USDC on Base. It doesn't need ETH: the facilitator submits the transfer.

Step 3: give it to your agent as a tool

export const cryptoPriceTool = {
  name: "crypto_price",
  description: "Current USD price and 24h/7d change for one coin (CoinGecko id). Costs $0.01.",
  parameters: { type: "object", properties: { id: { type: "string" } }, required: ["id"] },
  run: async ({ id }) =>
    (await fetchWithPay(`https://astra-x402-data.astratc.workers.dev/v1/coin?id=${encodeURIComponent(id)}`)).json(),
};

Put a spending cap in your agent loop, e.g. stop after N paid calls per task. x402 makes paying easy, so you need your own limits.

Why per-call pricing suits agents

No onboarding: the 402 response tells the agent everything it needs.
Cost matches use: one price check costs one cent, not a $29/month plan.
Machine-readable discovery: the catalog at / and our llms.txt describe every endpoint.

Notes

The worker only receives payments. It holds no keys and signs nothing.
Data is information only, not financial advice.
A runnable example is at github.com/astratc/x402-crypto-data-example.

If you'd rather buy a one-off dataset or a written brief through escrow, those are on our services page.

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