FastMCP Turns a Python Function Into an MCP Tool

Building an MCP server from scratch means writing transport, auth, schemas, validation, protocol lifecycle, and error handling before you touch your own logic. FastMCP does that plumbing for you.

Decorate a Python function with @mcp.tool and it becomes an MCP-compatible tool, with schema generation, validation, and documentation handled automatically. One decorator is the whole idea: Python function, decorator, AI-ready tool.

What Happened

FastMCP is a framework for building MCP servers and clients in Python, with a TypeScript version as well. Beyond single tools, it covers:

• Server — expose tools, resources, and prompts to LLMs
• Client — connect to local or remote MCP servers
• Apps — build interactive UIs that render inside conversations
• Proxy and composition — combine and filter multiple MCP servers
• Auth — OAuth 2.0, API keys, and custom flows
• Tasks — run long-running async tools with streamed progress

FastMCP 1.0 became the foundation of the official MCP Python SDK, and the project says it now powers a huge share of MCP servers.

Why It Matters

MCP is quickly becoming the connective tissue between AI agents and real software, and the boring parts — schemas, validation, auth, lifecycle — are where most of the time goes. FastMCP moves those out of your way, so the function you already wrote is the function the model can call.