The internet had a payment status code for 33 years. Nobody used it. Until now.

HTTP 402 Payment Required has been in the spec since 1991. It was reserved. Servers couldn't actually charge for a request because the client half of the protocol was missing. No browser knew what to do with a 402. No client library could sign and replay. The status code sat in RFC 7231 with the word "experimental" next to it.

What x402 actually does

x402 is the missing client half. The protocol shipped in 2025. Here is what a paid request looks like end to end:

1. Client calls a paid endpoint with no auth.
2. Server returns 402 with a JSON body that includes accepts[], the array of acceptable payment options. Each option carries scheme (exact, upto), network (CAIP-2, e.g. eip155:8453 for Base), amount (atomic units), asset (USDC contract on Base), payTo (recipient wallet), maxTimeoutSeconds, and extra (the EIP-712 domain name and version for the asset).
3. Client picks an option. The wallet signs an EIP-3009 TransferWithAuthorization over those exact terms.
4. Client base64-encodes the signed payload and replays the request with an X-PAYMENT header.
5. Server verifies the signature, settles the transfer on-chain via a facilitator, and serves the response.

The whole flow is HTTP. No new infrastructure. Your existing API gets paid endpoints by emitting a 402. Your client library learns to sign and replay. That is it.

Why now

Agents need a payment primitive that does not require accounts. Stripe and the rest of the SaaS billing stack assume a human is at the door. Wallets do not. A wallet signing typed data is a clean, programmable, account-less primitive that any agent can use.

For the supply side, x402 turns a single endpoint into a paid endpoint with one HTTP middleware. No new vendor relationship. No new contract. No new auth surface. The agentic.market directory indexes the providers so agents can discover them.

I built a paid memory API on top of this. The Coinbase CDP facilitator handles on-chain settlement on Base. The whole round-trip is three seconds.

Watch it move real money: bmdpat.com/memory/demo

What I had to learn the hard way

CDP's V2 facilitator enforces an undocumented minimum payment threshold somewhere between 100 and 1000 atomic USDC. Below the floor, V2 verify rejects with a generic invalid_payload and no message. The V1 endpoint with the same body said "amount is too low." Took three PRs of payload-shape fixes to figure out the body was always fine and the number was the bug. Bumped my prices to a uniform $0.001 per call and the rejections went away.

If you are building on x402 and getting invalid_payload from CDP, the first thing to try is bumping the amount.

What's next

The next problem isn't whether agents can pay. It's giving them a budget.

A typical 4-tool agent loop hits five priced endpoints per turn. A long-running task does this thousands of times a day. A single rogue loop drains a wallet in minutes. Per-tool caps, per-agent budgets, kill switches, spend visibility — that's the next layer.

AgentGuard.