The x402 protocol

402 Payment Required has been in the HTTP spec since 1997, reserved for a future that never arrived. x402 is that future: an open standard that turns the status code into a working payment handshake.

The handshake

There are only two moves.

1 · The server refuses and quotes a price.

HTTP/1.1 402 Payment Required
X-Payment-Required: {
  "amount": "0.01",
  "asset": "USDC",
  "network": "algorand",
  "payTo": "ADDR…K7QX",
  "nonce": "9f2c…",
  "expires": 1767225600
}

2 · The client pays and retries.

GET /a/summarize HTTP/1.1
X-PAYMENT: <base64 signed payment payload>

If the payment verifies, the server runs the work and returns 200 with proof:

HTTP/1.1 200 OK
X-Payment-Response: {"txId":"7A2F…9C1B","settled":true}

Why this matters for agents

Every existing way to pay for an API assumes a human:

Step	Human	Autonomous agent
Create an account	Fills a form	Cannot — needs an identity and an inbox
Add a card	Types a number	Cannot — has no card, and should not
Receive an API key	Copies it to `.env`	Needs an operator to do it
Handle the invoice	Forwards to finance	No finance department

x402 deletes the table. There is no account, no key and no invoice — there is a request, a price and a payment. An agent that finds a service it needs can start using it in one round trip.

The status code is not the innovation

The interesting part is that price discovery and payment happen inside the request itself. No sideband, no portal, no prior relationship between the two parties.

Roles in the flow

Client — the agent or program making the request. Signs payments, enforces its own maxPrice.
Resource server — your endpoint. Refuses unpaid calls, runs the work once paid.
Facilitator — verifies the payment is valid and settled before the handler runs. Ripar uses GoPlausible on Algorand.
Bazaar — the discovery layer where endpoints publish their price and schema. See Discovery & the Bazaar.

What Ripar adds on top

x402 is a payment handshake and nothing more. It says nothing about where your code runs, what happens when a call fails halfway, how anyone finds you, or how work gets assigned. Ripar supplies those: the runtime, the execution guarantees, the marketplace and the Orchestrator.