From println!() Disasters to Production. Building MCP Servers in Rust

I shipped my first MCP server on a Friday. It worked perfectly in my tests. Then Claude Code started hallucinating gibberish responses, and I spent the weekend figuring out why. The culprit? A single println!() . That debugging session taught me more about building MCP servers than any documentation ever could. So let me save you the weekend. This tutorial walks through three patterns I wish someone had shown me before I started building MCP servers in Rust : the stdio trap, typed tool schemas, and the error-as-UI pattern that changed how I think about tool design entirely. We are building a real thing here. A code-stats MCP server with three tools that analyse your codebase. No weather APIs. No toy examples. What MCP Actually Is (in 30 Seconds) MCP is a JSON-RPC 2.0 protocol. It is now a Linux Foundation project with over 97 million SDK downloads. AI clients (like Claude Code) spawn your server as a child process and send JSON-RPC messages over stdin. Your server responds on stdout. T