> ## Documentation Index
> Fetch the complete documentation index at: https://402.cat/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Authentication

> x402 micropayment authentication

## x402 Protocol

402.cat uses x402 micropayment protocol instead of API keys. Pay per request with USDC.

## Payment Flow

<Steps>
  <Step title="Request">
    Make request to protected endpoint
  </Step>

  <Step title="402 Response">
    Server returns `HTTP 402 Payment Required` with payment details
  </Step>

  <Step title="Sign">
    Create EIP-3009 authorization signature
  </Step>

  <Step title="Retry">
    Retry request with `PAYMENT-SIGNATURE` header
  </Step>
</Steps>

## Headers

**V2 (Current):**

```http theme={null}
PAYMENT-SIGNATURE: <base64_payment_data>
```

**V1 (Backward Compatible):**

```http theme={null}
X-PAYMENT: <base64_payment_data>
```

## Using httpcat-cli

CLI handles authentication automatically:

```bash theme={null}
httpcat buy MOON 0.10
# Automatically signs and pays
```

## Manual Implementation

See [x402 Protocol](/docs/concepts/x402-protocol) for detailed implementation guide.

## Referrals

Add referrer header to earn 10% of fees:

```http theme={null}
X-402-Referrer: 0xYourAddress...
```

<Card title="x402 Protocol Details" icon="link" href="/docs/concepts/x402-protocol">
  Complete x402 documentation
</Card>