Building x402 APIs from Scratch: How I Made AI Agents Pay for Data

The Web Has a Forgotten Status Code Everyone knows 200 (OK), 404 (Not Found), and 401 (Unauthorized). But there's one HTTP status code that's been sitting unused for 30 years: 402 Payment Required . The HTTP spec literally says: "This code is reserved for future use." The future is here. It's called x402 , and it lets AI agents pay for API calls autonomously using Solana micropayments. The Problem: Agents Can't Sign Up for Things Every useful API today requires: Create an account Verify your email Get an API key Maybe add a credit card Deal with rate limits and subscription tiers This works fine for humans. It's a brick wall for autonomous agents. The x402 Solution Here's the flow: Agent -> GET /v1/prices/BTC Server -> 402 Payment Required { recipient, amount: 0.0001 SOL } Agent -> [signs Solana tx, sends payment] Agent -> GET /v1/prices/BTC X-Payment-Signature: <tx_sig> Server -> 200 OK { asset: "BTC", price: 64971.53 } No account. No API key. No subscription. The agent has a Solana w