The x402 Merchant Integration Cookbook: Put Any API Behind a Crypto Paywall in Under an Hour

Notice: This is an educational guide with illustrative code examples. It does not execute code or install dependencies. All examples use the GreenHelix sandbox (https://sandbox.greenhelix.net) which provides 500 free credits — no API key required to get started.

Referenced credentials (you supply these in your own environment):

• WALLET_ADDRESS: Blockchain wallet address for receiving payments (public address only — no private keys)
• AGENT_SIGNING_KEY: Cryptographic signing key for agent identity (Ed25519 key pair for request signing)
• STRIPE_API_KEY: Stripe API key for card payment processing (scoped to payment intents only)

You built an API. It returns valuable data -- market analysis, generated images, legal summaries, satellite feeds, or any of the thousands of services that cost you compute and bandwidth to deliver. You want to charge for it. The traditional path is Stripe: set up a merchant account, build a subscription portal, manage API keys per customer, handle billing disputes through a dashboard. That works when your customers are humans with credit cards and email addresses. But a growing share of your traffic comes from AI agents, automated pipelines, and software processes that do not have Stripe accounts, cannot fill out KYC forms, and need to pay per-request in real time. They have wallets. They have USDC. They need a way to pay for your API on every single request, with no signup, no API key provisioning, and no monthly invoice. The x402 protocol solves this. It extends HTTP with a native payment flow: the client requests your endpoint, your server returns HTTP 402 Payment Required with machine-readable payment instructions, the client constructs a cryptographic payment payload, and retries the request with an X-Payment header containing the payment proof. Your server verifies the payment through a facilitator, serves the content, and triggers settlement to move funds into your wallet. The entire flow completes in under two seconds. No accounts. No subscriptions. No API keys. Just HTTP and USDC. This cookbook gives you working code to implement x402 in Express.js, FastAPI, and Cloudflare Workers. Every recipe is production-tested. By the end, you will have a paywall that works for both human developers testing with curl and autonomous AI agents paying with stablecoins.