x402 Monetization Basics

x402 lets you charge per request for anything served over HTTP — an API, a dataset, a model endpoint — and lets AI agents pay for it without an account, an API key, or a subscription.

The idea in one paragraph

HTTP has always had a status code 402 Payment Required; x402 finally gives it a protocol. Your endpoint answers an unpaid request with a 402 carrying a price quote. The caller — typically an AI agent — signs a USDC payment authorization and retries with it attached. A facilitator settles the payment on-chain, and your endpoint releases the response. No signup, no invoicing, no key management: paying is the authentication.

Two protocol dialects, handled for you

x402 v1 carries payment data in X-PAYMENT headers and the 402 body; v2 (since December 2025) moved to PAYMENT-SIGNATURE / PAYMENT-REQUIRED headers with different shapes. FurlPay's tooling speaks both at once and answers in whichever dialect the caller used — you never configure a version.

Monetize an endpoint in three commands

npm install @furlpay/gateway
npx @furlpay/gateway secret        # generates your quote secret
npx @furlpay/gateway expose http://localhost:8000 \
  --price 0.01 --pay-to 0xYourWallet --network base

That reverse proxy paywalls an API you cannot even modify. For code-level integration (Next.js routes, Express middleware), see the package docs on npm.

What the gateway protects you from

• Replay: one payment unlocks one response, exactly once — even under concurrent retries.
• Substitution: a payment quoted for a cheap endpoint cannot unlock an expensive one, or a different request body at the same price.
• Underpayment, expired quotes, wrong-network payments, and facilitator outages all fail closed.

Full protocol documentation: Agentic Payments (x402).