I Built an API That AI Agents Pay in USDC — Full x402 Walkthrough (27 Endpoints, Real Transactions)

I built an Express API that AI agents (or humans, or anything with fetch ) can pay per call, in USDC, with no signup and no API key. It's live on Base mainnet with 27 paid endpoints, and I've run real settled transactions against it. This is the technical walkthrough — the code, the protocol, and the things that actually broke — not an "agentic economy" pitch. What x402 is, in 5 lines x402 resurrects the dormant HTTP 402 Payment Required status code as a real payment handshake. A client calls a paid route → the server replies 402 with payment requirements (amount, asset, network) instead of the resource → the client signs a USDC transfer on Base and replays the request with a PAYMENT header → a facilitator (a third party, or Coinbase's CDP service in production) verifies and settles the transfer on-chain → the server serves the response. No account creation, no API key issuance, no OAuth dance — the wallet address is the identity, and payment is the auth. The seller side The server is plain Express. Each endpoint is a file in endpoints/ exporting { path, method, price, handler } ; server.js loads them all, builds the x402 route table, and mounts one middleware: import { paymentMiddleware , x402ResourceServer } from " @x402/express " ; import { ExactEvmScheme } from " @x402/evm/exact/server " ; import { HTTPFacilitatorClient } from " @x402/core/server " ; import { createFacilitatorConfig } from " @coinbase/x402 " ; const facilitatorConfig = config . isMainnet ? createFacilitatorConfig ( config . cdpApiKeyId , config . cdpApiKeySecret ) : { url : config . testnetFacilitatorUrl }; // https://x402.org/facilitator, no key const facilitatorClient = new HTTPFacilitatorClient ( facilitatorConfig ); const resourceServer = new x402ResourceServer ( facilitatorClient ). register ( config . caip2Network , // "eip155:8453" on mainnet new ExactEvmScheme () ); const paidRoutes = {}; for ( const ep of endpoints ) { if ( ep . price == null ) continue ; paidRoutes [ ` ${ ep . method }