x402 Payments Are Live: The Stablecoin Rail for AI Agents and APIs

The payment problem nobody built for machines

Every payment system we use was designed for a human: a person typing a card number, clicking "subscribe," approving a checkout. That model breaks the moment the buyer is software. An AI agent can't open an account, pass a KYC form, or click a confirmation button — but it increasingly needs to buy things: an API call, a dataset, a compute cycle, a tool invocation.

For 35 years the HTTP spec has held a reserved status code for exactly this moment: 402 Payment Required. Defined in 1991, it was never standardized — a placeholder waiting for a payment system that didn't exist yet.

That system now exists. It's called x402, and today Crypto APIs is launching a managed facilitator for it.

What x402 actually does

x402 is an open protocol for paying for HTTP APIs with stablecoin, using the standard 402 response. The flow is simple enough to describe in one sentence:

An agent requests a paid endpoint → the server replies with 402 Payment Required and the price → the agent signs a USDC payment locally → it retries the request with the signed payload attached → the server verifies and settles onchain, then returns the data.

The entire cycle takes seconds, requires no login, and settles onchain with zero protocol fees. It's already moved serious volume — over 119 million transactions on Base and roughly $600M in annualized volume across the ecosystem — and it's growing precisely because the agent economy needs a rail built for machines.

Why both sides matter

Here's the part that's rarely discussed: x402 only works if both halves of the transaction are solved. You need the seller side — an API that can issue a 402 challenge, verify a signed payment, and settle it — and the buyer side — an agent that can read the challenge, sign, and retry. Building either from scratch means running gas wallets, verifying EIP-712 signatures, broadcasting settlements, enforcing spending limits, and staying non-custodial through all of it.

Crypto APIs handles both.

Monetize any API (the seller side)

Add a per-request paywall to your endpoint in a few lines. Our zero-dependency, non-custodial merchant SDK (@cryptoapis-io/x402-merchant-sdk) ships as drop-in middleware for Express, Hono and Next.js. Under the hood it calls a managed facilitator:

• POST /verify — validate a buyer's signed payment without settling
• POST /settle — verify, then broadcast the settlement onchain using our gas wallet
• GET /supported — list live assets, networks and signers

Every response carries settlement-quota headers (X-X402-Settlement-Limit, -Remaining, -Reset) so you can watch your billing cycle and self-throttle before you hit a limit.

Let your agents pay (the buyer side)

On the other side, your apps and agents can pay 402-gated endpoints automatically using our buyer SDK, an x402_pay MCP tool, and an agent plugin. Payments are signed locally with EIP-712 — your keys never leave your side, and there is no /sign endpoint. You keep full custody. And because runaway spend is the first thing anyone worries about with autonomous payments, we enforce per-transaction, daily and monthly spending limits out of the box.

Non-custodial, by design

This is the point we won't compromise on. The facilitator verifies signatures and settles on-chain, but it can never move funds beyond what the buyer explicitly authorized. The client signs every payment locally. Settlement happens in USDC across supported EVM networks like Base.

Get started

x402 is live now on Crypto APIs, and you can start on the Free tier — no credit card required. Scale up through Starter, Scale, Pro and Enterprise as your volume grows.

• 🔗 Product overview
• 📖 Merchant & Facilitator API docs
• 💳 Pricing

The agent economy is being built right now. x402 is how it pays. Let's build it together.