402Briefbar

Protocol

HTTP already had a place for money. Nobody could put money there.

x402 is not a new website and it is not a new coin. It is a way to use the long-reserved 402 Payment Required status so a server can charge for a single HTTP request, in stablecoins, without creating an account.

The original sin of the commercial web

The web is good at moving documents and terrible at moving small amounts of money. Credit cards have a floor around thirty cents plus a percent. Bank transfers take days. Chargebacks exist because the payer can change their mind. So publishers sold attention to advertisers, and APIs sold monthly keys to developers. Both patterns assume a human who can be identified, billed, and shown a form.

Agents fail that assumption. They arrive without cookies, they do not watch pre-rolls, and they may make ten thousand GETs in an hour, each worth a fraction of a cent. If you cannot charge them, they will scrape you. If you block them, you also block a buyer class that is about to be larger than the human one.

HTTP 402 was reserved in the 1990s for exactly this. The missing piece was a rail that could settle a thousandth of a dollar in about the time it takes to fetch JSON. USDC on cheap L2s (Base is the usual example) is that rail. x402 is the handshake that hangs the rail on an HTTP response.

The loop

Coinbase shipped the first implementation; the protocol is Apache-2.0 and now sits with the x402 Foundation. You do not need Coinbase to speak it. You need a client that can sign, a server that can quote, and a facilitator that can settle.

  1. GET01

    Ask for the resource

    An agent requests a brief the same way it requests any JSON API. No account, no API key, no OAuth dance.

  2. 40202

    Read the invoice

    If PAYMENT-SIGNATURE is missing, the desk answers Payment Required. The PAYMENT-REQUIRED header is a base64 JSON quote: price, asset, network, payTo.

  3. PAY03

    Sign and retry

    The client (wallet SDK, agent runtime, or this playground) signs a payload and retries the same URL with PAYMENT-SIGNATURE.

  4. 20004

    Settle and read

    The desk verifies, fetches the brief, then settles. PAYMENT-RESPONSE comes back with the body. Failed briefs are not charged.

Three headers

Version 2 of the spec keeps the bodies out of the URL. Everything payment-shaped is base64 JSON in a header so ordinary caches and browsers do not choke on it.

PAYMENT-REQUIRED
Server → client, on a 402. Lists accepted schemes, CAIP-2 networks (for example eip155:84532 for Base Sepolia), atomic token amounts, the asset contract, and the payTo address.
PAYMENT-SIGNATURE
Client → server, on the retry. Proves the buyer authorized a transfer that matches one of the accepted quotes. On EVM this is usually an EIP-3009 transferWithAuthorization signature over USDC.
PAYMENT-RESPONSE
Server → client, after verify/settle. Success or failure, plus a transaction id the buyer can check.

Facilitators, schemes, discovery

A resource server should not have to run a chain indexer. A facilitator verifies the payload and, for the exact scheme, settles it. Public testnet facilitators live at x402.org. Production desks choose a facilitator they trust or run their own.

exact means “pay this amount, then get the bytes.” upto is for metered work (tokens generated, seconds of compute). Batch settlement exists for high-frequency vouchers. Briefbar uses exact because a brief has a known cost before we fetch it.

Discovery is unsolved in the way payments used to be. The Foundation has a Bazaar for listing services. This desk also publishes a free catalog, /llms.txt, and /.well-known/x402 so an agent that has already found us does not need a UI.

What this instance actually settles

The 402, the headers, the retry, and the “do not charge on failure” rule are real. The money is not. A local demo facilitator issues HMAC-signed payloads so you can learn the protocol without a wallet or testnet faucet. Swap that signer for a wallet SDK and HTTPFacilitatorClient when you want USDC to move.

That split is the point of Briefbar as a first slice: understand the product (priced packets of information) and the rail (x402) before you couple them to mainnet.

Glossary