# x402

## What is x402?

**x402** is an open payment protocol over HTTP from **Coinbase** that brings the long-reserved **HTTP 402 Payment Required** status code to life. The server responds with 402 and a `PAYMENT-REQUIRED` header, the agent client sends a `PAYMENT-SIGNATURE`, a facilitator verifies and settles the transaction — all without human involvement.

```http
GET /premium-report HTTP/1.1

HTTP/1.1 402 Payment Required
PAYMENT-REQUIRED: scheme="exact", amount="0.01", currency="USDC",
  network="eip155:8453", recipient="0x..."

# Agent pays and retries the request
GET /premium-report HTTP/1.1
PAYMENT-SIGNATURE: <signed_payment_payload>

HTTP/1.1 200 OK
```

## Why do you need x402?

In a world of autonomous AI agents, a standard for micropayments without human involvement is essential. x402 solves three problems:

- **Pay-per-use API** — an agent pays $0.001 per data unit or tool call.
- **AI traffic monetization** — the server earns on every agent request without subscriptions.
- **Agent-to-agent billing** — one agent pays another for compute resources.

The protocol uses stablecoins (USDC) on Base, Polygon, Arbitrum, and Solana networks. The network identifier uses CAIP-2 format (`eip155:8453` = Base). As of April 2026, the protocol had processed around 165 million transactions with roughly 69,000 agents connected.

## How do you implement x402?

Three roles in the protocol:

| Role | Responsibility |
|------|----------------|
| **Resource Server** | Responds with 402 + `PAYMENT-REQUIRED`, accepts the retry with `PAYMENT-SIGNATURE` |
| **Client (AI agent)** | Parses the 402, forms the payment, adds `PAYMENT-SIGNATURE` |
| **Facilitator** | Verifies the signature and processes the on-chain transaction |

Minimal declaration of support in [`ai-agent.json`](/glossary/ai-agent-json):

```json
{
  "payments": {
    "x402": "https://api.example.com/x402"
  }
}
```

Or via a Link header:

```http
Link: <https://api.example.com/x402>; rel="x402-endpoint"
```

## How do we check x402?

The x402 check in the scanner is **informational** (does not affect the main score). The scanner checks for protocol support declaration in two ways:

1. GET `/.well-known/ai-agent.json` → `payments.x402` field
2. HEAD `/` → Link header with `rel="x402-endpoint"`

The actual payment flow (sending `PAYMENT-SIGNATURE`, settlement via facilitator) is not tested. If a declaration is found → `pass`, if absent → `fail`.

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