I spent two weeks building, breaking, and rebuilding a pay-per-call crypto signal API. Here's what actually matters if you're thinking about x402 — not the hype, the implementation. What x402 Actually Is HTTP 402 Payment Required. Coinbase open-sourced a protocol that lets APIs demand payment per-request using stablecoins. No API keys. No subscriptions. No Stripe integration. The client (which can be an AI agent) pays in USDC on Base, gets a receipt onchain, and the server delivers the response. The full spec lives at x402.org . What matters practically: three headers, one smart contract interaction, and a facilitator that handles the plumbing. The Architecture That Worked My service has three layers: 1. Discovery — GET /.well-known/x402 returns a manifest: endpoints, prices, network, contact. This is how other agents find you. Without it, you're invisible. 2. Payment Gate — GET /signal/BTC returns 402 Payment Required with X-Payment-Required headers.…