Building Python MCP Servers

MCP servers expose tools to an LLM client (Claude Desktop, Claude Code, etc.). The LLM is the caller, so the failure modes differ from a normal library: errors must be machine-readable, every input is untrusted, and a green test suite often proves nothing about whether the tools actually work. This skill encodes the patterns that recur when these go wrong.

Quick Start (FastMCP)

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("my-server")