fastmcp

FastMCP is the fastest way to build MCP servers in Python. Decorate a function and it becomes a tool your AI assistant can call, with schema generation, validation, auth, and transport handled for you. Apache 2.0 and free. Version 4 turned it from a server library into a full framework with three pillars: servers that wrap your functions, clients that connect to any server, and Apps that render interactive UIs directly in the conversation.

Setup is one uv add and a few lines of code. Written a Flask route? You can write an MCP server. FastMCP 1.0 was folded into the official MCP Python SDK back in 2024, and the standalone project has kept moving well past it, so read the upgrade guides before assuming existing code carries forward. There is an official TypeScript counterpart from the same team if Python is not where you live.

Free at every size, and the library is complete rather than a demo that funnels you toward a paid tier. Prefect sells Prefect Horizon separately: an enterprise MCP gateway that deploys servers from GitHub with branch previews, keeps a private registry of every MCP in the company, and adds SSO, tool-level RBAC, and audit logs. Free for personal projects, enterprise pricing on request. Solo builders and small teams will not need it. Anyone running many servers across many teams is buying governance, not the framework.

The catch is churn. Version 4 is in beta while 3.4.x is still shipping patches, and the docs carry separate upgrade guides from FastMCP 3, FastMCP 2, and two different MCP SDK generations. That is the cost of being the default in a protocol that is still moving. Pin your version and read the release notes before you bump.