Authentication

x402 Protocol

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

Request

Make request to protected endpoint

402 Response

Server returns HTTP 402 Payment Required with payment details

Sign

Create EIP-3009 authorization signature

Retry

Retry request with PAYMENT-SIGNATURE header

V2 (Current):

PAYMENT-SIGNATURE: <base64_payment_data>

V1 (Backward Compatible):

X-PAYMENT: <base64_payment_data>

CLI handles authentication automatically:

httpcat buy MOON 0.10
# Automatically signs and pays

See x402 Protocol for detailed implementation guide.

Add referrer header to earn 10% of fees:

X-402-Referrer: 0xYourAddress...