How to Build a Pay-Per-Call MCP Server with x402 and USDC

If you've built MCP servers for Claude Code, you know the distribution problem: anyone who installs your server gets unlimited free access. There's no clean way to charge per call without rolling your own auth + billing. x402 solves this at the HTTP layer. Here's how to build a pay-per-call MCP server where every tool call costs USDC — settled instantly on Base. What is x402? x402 is an HTTP payment protocol that revives the abandoned 402 Payment Required status code. The flow: Client requests a