HTTP 402 Payment Required has been in the spec since 1997, “reserved for future use.” Turns out the future is AI agents that need to pay for API calls without signing up for anything.

I built a DeFi/crypto data API that uses the x402 protocol (by Coinbase) to handle payment natively in the HTTP layer. No API keys, no subscriptions, no accounts. An agent sends a request, gets back a 402 with payment details, pays in USDC on Base, and retries — all in one flow.

How it works:

1. Client calls an endpoint 2. Server returns 402 with payment instructions (x402) 3. Client signs a USDC payment (0.001–0.008 USDC per call) and retries with a payment proof header 4. Server verifies payment and returns the data

The x402 facilitator (middleware) handles verification, so the server just defines prices and serves data.

8 endpoints currently live: token prices, gas tracker, DEX swap quotes, token security scanner, whale tracker, DeFi yield scanner, funding rates, and wallet profiler. Each call costs fractions of a cent (1,000 lookups/day is roughly $1–5).

Built with Express.js. The server is registered as ERC-8004 Agent #18763 on Base mainnet. The whole thing is ~500 lines of code — most of the complexity lives in the x402 facilitator library.

Live: https://x402-api.fly.dev (https://x402-api.fly.dev/) Code: https://github.com/fernsugi/x402-api-server Writeup: https://dev.to/fernsugi/i-built-a-defi-data-api-where-ai-age...