x402

x402 is an open payment standard that lets a server ask for payment, and a client pay, inside an ordinary request-response exchange, built around the HTTP 402 Payment Required status code.

Background. RFC 9110 defines HTTP status 402 Payment Required but marks it as reserved for future use. x402 gives it a concrete flow. The protocol started at Coinbase. In July 2026 the Linux Foundation announced the operational launch of the x402 Foundation, which now stewards it under open governance.

How it works (v2, HTTP transport).

1. The client requests a resource.
2. The server answers 402 with a base64-encoded PaymentRequired object in the PAYMENT-REQUIRED header. It lists the accepted options, each with a scheme, network, amount, asset and recipient.
3. The client picks an option, signs a PaymentPayload, and retries with it in the PAYMENT-SIGNATURE header.
4. The server verifies the payload, either locally or through a facilitator’s /verify endpoint, then does the work.
5. The server settles, directly or through the facilitator’s /settle endpoint, and returns 200 with a PAYMENT-RESPONSE header that holds the settlement result.

Networks are named in CAIP-2 format, such as eip155:8453. Schemes such as exact define how a payment is built and checked.

In A2A. The x402 v2 specification includes an A2A transport. The server agent moves the task to input-required and puts the payment requirements in message metadata under x402.payment.required. The client replies on the same task with x402.payment.payload, and the server reports the outcome in x402.payment.receipts. Agents declare support in the Agent Card:

{
  "capabilities": {
    "extensions": [
      {
        "uri": "https://github.com/google-a2a/a2a-x402/v0.1",
        "description": "Supports payments using the x402 protocol for on-chain settlement.",
        "required": true
      }
    ]
  }
}

The transport’s message examples use A2A v0.3 shapes, such as kind fields, message/send and lowercase states like input-required. The v1.0 names differ, for example SendMessage and TASK_STATE_INPUT_REQUIRED.

Neighbouring terms. AP2 covers what the user authorized. x402 covers the payment request and the settlement.