What Is x402? The HTTP Protocol for AI Agent Payments

HTTP status code 402 has existed since 1997. It was reserved for 'future use' — a placeholder for the day the web had a native payment layer. That day arrived when stablecoins made programmable money practical and AI agents made autonomous spending necessary.

x402 is FurlPay's implementation of that vision. It is an open protocol that lets any HTTP server monetise an endpoint, and any HTTP client — human or AI agent — pay for access, all within the normal request-response cycle.

How it works

The flow has four steps. No redirect, no popup, no checkout page.

• 1. The client sends a normal HTTP request to a protected endpoint.
• 2. The server responds with 402 Payment Required and an x-payment header containing the price, accepted assets, and a payment address.
• 3. The client reads the header, constructs an EIP-3009 authorisation (gasless USDC transfer), signs it, and retries the request with the signed payment in the x-payment-response header.
• 4. The server verifies the signature, submits the transfer on-chain, and returns the 200 response with the requested content.

Why this matters for AI agents

When an AI agent needs to call a paid API — a premium data source, a compute service, a SaaS tool — it cannot click a checkout button. It needs a protocol: a machine-readable price, a programmable payment method, and a way to prove it is authorised to spend.

x402 provides all three. The agent reads the price from the 402 header, checks its spending mandate (set by its human principal via FurlPay's control plane), and signs the payment if the mandate allows it. No human in the loop for routine spending. Immediate human escalation for anything outside policy.

The control plane

An AI agent with a credit card is a liability. An AI agent with a signed spending mandate, per-transaction limits, daily budgets, merchant allowlists, and real-time human approvals for exceptions — that is a controllable economic actor.

FurlPay's agent trust framework sits between the agent and the money. The human sets the rules. The agent operates within them. Every transaction is logged, auditable, and reversible. This is the infrastructure that x402 needs to be safe, and it is the moat that makes FurlPay's x402 implementation different from a raw 402 header.

Try it

The FurlPay Chrome extension includes an x402 DevTools inspector that shows 402 headers on any page. The extension can also auto-pay x402 endpoints using your FurlPay wallet, with spending limits you control.

For developers: the x402 server middleware is available in the @furlpay/js SDK, and agent-side x402 clients are available for LangChain, Vercel AI, OpenAI Agents, CrewAI, LlamaIndex, Mastra, and PydanticAI. See docs.furlpay.com for integration guides.