Practical MCP with FastMCP & LangChain

Why This Matters

Knowing these layers changes how you solve problems. When something goes wrong — a tool isn't responding, a client isn't receiving logs, authentication is rejecting valid users — the first question becomes: which layer is the issue in? A transport problem looks very different from a middleware misconfiguration, and both look different from a bug in your tool code. The layered model gives you a mental map for narrowing things down.

It also tells you where to put new behavior:

• If you need to log every request that hits your server, that belongs in middleware — not scattered across every tool function.

• If you need to ask the user a clarifying question mid-execution, that's a Context method, not something you build from scratch with JSON-RPC.