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Call a paid AI API over x402 — in 5 minutes

No API key, no signup. You hit an endpoint, the server answers 402 with a price, your client signs a one-off USDC authorization on Base and retries. Below is the whole thing as one runnable file.

What you need

• Node.js 18+ and one dependency: npm i viem
• A wallet private key whose address holds a little USDC on Base mainnet (a few cents is enough to test).
• Nothing else — no account, no key provisioning, no dashboard.

The flow

Copy-paste client

Save as pay-and-call.mjs, then run X402_PRIVATE_KEY=0x... node pay-and-call.mjs:

// pay-and-call.mjs — pay Clawy in USDC over x402 and read the answer.
// Setup:  npm i viem
import { privateKeyToAccount } from "viem/accounts";
import { randomBytes } from "crypto";

const GATEWAY  = "https://clawy.uk";
const ENDPOINT = "/v1/chat/auto";  // cheapest tier: $0.04 USDC/call
const MESSAGE  = "Reply with exactly: HELLO";
const USDC     = "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"; // USDC on Base
const CHAIN_ID = 8453;                                          // Base mainnet

const account = privateKeyToAccount(process.env.X402_PRIVATE_KEY);
const body = JSON.stringify({ messages: [{ role: "user", content: MESSAGE }] });
const headers = { "Content-Type": "application/json" };

// 1) Unpaid request -> 402 with the price in the PAYMENT-REQUIRED header
const r1 = await fetch(GATEWAY + ENDPOINT, { method: "POST", headers, body });
if (r1.status !== 402) throw new Error("expected 402, got " + r1.status);
const reqs = JSON.parse(Buffer.from(r1.headers.get("payment-required"), "base64").toString());
const a = reqs.accepts[0];   // { scheme:"exact", network, amount, payTo, ... }
console.log("price:", a.amount, "(6-dp USDC) ->", a.payTo);

// 2) Sign an EIP-3009 transferWithAuthorization for the exact amount
const nonce = "0x" + randomBytes(32).toString("hex");
const validBefore = BigInt(Math.floor(Date.now() / 1000) + 300); // valid 5 min
const signature = await account.signTypedData({
  domain: { name: "USD Coin", version: "2", chainId: CHAIN_ID, verifyingContract: USDC },
  types: { TransferWithAuthorization: [
    { name: "from", type: "address" }, { name: "to", type: "address" },
    { name: "value", type: "uint256" }, { name: "validAfter", type: "uint256" },
    { name: "validBefore", type: "uint256" }, { name: "nonce", type: "bytes32" },
  ] },
  primaryType: "TransferWithAuthorization",
  message: { from: account.address, to: a.payTo, value: BigInt(a.amount),
             validAfter: 0n, validBefore, nonce },
});

// 3) Build the X-PAYMENT header and retry -> 200 + OpenAI-shape response
const xPayment = Buffer.from(JSON.stringify({
  x402Version: 2, scheme: "exact", network: a.network,
  payload: { signature, authorization: {
    from: account.address, to: a.payTo, value: a.amount,
    validAfter: "0", validBefore: validBefore.toString(), nonce } },
})).toString("base64");

const r2 = await fetch(GATEWAY + ENDPOINT, {
  method: "POST", headers: { ...headers, "X-PAYMENT": xPayment }, body,
});
const out = await r2.json();
console.log(out.choices[0].message.content);  // -> HELLO
console.log("served by:", out.x_clawy?.provider);

This is the minimal happy path. For production add balance and error handling — a fuller reference (testnet support, balance checks, dry-run) lives in the repo as scripts/x402-client.mjs.

For AI agents (no human in the loop)

Discovery files let an autonomous client find, price-check, and call Clawy on its own:

• /llms.txt — one-page summary for LLM clients
• /openapi.json — OpenAPI 3.1 spec with payment info per endpoint
• /.well-known/x402.json — x402 Bazaar manifest (input/output schemas)
• /.well-known/agent-services.json — agent-services catalog
• /health — live status + current pricing