How to build a paid MCP tool that AI agents pay to use

MCP is everywhere right now. Everyone is building MCP servers — for search, for databases, for file systems. But there's one thing nobody is talking about: how do you get paid when an AI agent uses your MCP tool? Today we'll build a complete MCP server with a paid tool. An AI agent calls your tool, pays per use, and gets the result. No API keys, no subscriptions, no signup. This isn't theoretical. We'll build it, run it, and test it. What we're building A code review MCP server. An AI agent sends a code snippet, pays $0.05, and gets a structured review back — issues found, suggestions, severity ratings. Why code review? Because it's expensive to run (it calls an LLM under the hood), so charging for it makes sense. And it's a tool that AI agents actually want — coding agents like Cursor, Windsurf, and Claude Code could use a second opinion on the code they write. How payment works in MCP Quick primer if you haven't seen this before. MCP tools normally work like this: agent calls tool →