Build an AI Agent That Can Pay

Agents that can only talk are demos. Agents that can pay are businesses. This guide wires a real payment rail into your agent — a USDC wallet on Base, the x402 payment flow, a spend cap — and ends with a free live check you can run on this page, no wallet connection, no money moved.

Step 1 — Give the agent its own wallet

Generate a dedicated keypair for the agent and fund it with USDC on Base L2. Keep the agent's money separate from yours so a prompt injection can never reach your main funds. The wallet needs no ETH: x402 payments settle through transferWithAuthorization (EIP-3009), and the payer signs a message rather than a gas-bearing transaction.

# example: fund the agent wallet with USDC on Base
# any USDC transfer works — the agent just needs a balance
agent_wallet = "0xYourAgentWallet"
network = "eip155:8453"   # Base L2

Step 2 — Make the call, read the 402

x402 is HTTP-native pricing. Your agent calls an endpoint the normal way; if it costs money, the server answers 402 Payment Required with everything needed to pay: price, payee, chain, and a facilitator URL that verifies and settles.

GET https://agentpaystore.com/forge/api/chat

HTTP/402 Payment Required
{
  "x402Version": 1,
  "maxAmountRequired": "100000",          // 0.10 USDC, 6 decimals
  "payTo": "0x367F1b3D8Ca90D1e087481a9A40d585Bf3451a03",
  "asset": "0x833589fCD5eDb6e08F4c7C32D4F71b54bdA02913",  // USDC on Base
  "network": "base",
  "description": "FORGE agent chat"
}

Step 3 — Sign, attach, retry

The agent signs an EIP-712 transferWithAuthorization for that exact price and payee, base64-encodes it into the X-Payment header, and retries the call. A facilitator verifies the signature against the chain and settles in the background — the response itself returns instantly. Most agent frameworks wrap this in one function call.

import { fetchWithPayment } from "x402-fetch";

const agent = privateKeyToAccount(process.env.AGENT_WALLET_KEY);

const res = await fetchWithPayment(url, {
  method: "POST",
  body: JSON.stringify({ query: "Summarize today's crypto news" }),
  x402Version: 1,
  accountAddress: agent.address,
  signMessage: (m) => agent.signMessage({ message: m }),
});

// res is the real answer — paid, verified, settled on Base

Retry on 402 rather than prepaying: the authorization is bound to one nonce, so a failed call never burns funds. Coinbase's x402 reference client and our live facilitator both follow this exact flow on Base.

Step 4 — Bound the spend, then test before you spend

Two safety rails before production:

• Spend authority. Publish a signed grant that caps the wallet: a budget per rolling hour/day/month, a no-confirm threshold under which the agent pays freely, and an expiry. A facilitator that enforces authorities refuses anything outside the envelope before it can settle. See the authority routes on our facilitator.
• Readiness check. Before the agent ever signs a real payment, confirm the whole buyer side works: terms signable, facilitator answering, wallet funded. Ours is free — try it right now.

What the check proves

The handshake answers one question a buyer actually has: could this payment be constructed and funded right now? It names the endpoint's price, confirms the USDC balance covers it (with the exact shortfall if not), and flags defects that would bite later — a facilitator that does not answer, or terms too incomplete to sign. Gas never blocks the verdict, because settling through a facilitator costs the payer no gas.

Common questions