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FastMCP for building Python tool servers
Turn Python functions and related content into MCP servers with concise definitions.
Key Takeaways
- FastMCP derives tool schemas from Python definitions and offers server composition and client facilities.
- Use type annotations and informative descriptions on the first tool.
- Register a read-only function with one required parameter and test it through a local client.
- Automatically generated schemas still need useful field names and documentation.
How it works
FastMCP derives tool schemas from Python definitions and offers server composition and client facilities. A small service can start with direct function registration and grow into a composed server.
What you need
Use type annotations and informative descriptions on the first tool. Inspect them as a caller would, since the generated tool schema is part of the API.
What to try first
Register a read-only function with one required parameter and test it through a local client. Inspect the generated description and schema to see what callers are told.
Compose two tiny servers with deliberately overlapping tool names. Review the naming and generated schemas before adding several independent service owners.
Where to look for failures
Automatically generated schemas still need useful field names and documentation. Review error output and authentication behavior before exposing a service remotely.
Related Franklin pages
Check tool discovery, argument validation and error behavior before introducing a model. The same fixture can serve as a contract test across server implementations and clients.
- Inspect MCP connections and exercise tools, resources and prompts during development. MCP Inspector for testing server behavior
- Implement Model Context Protocol capabilities in Python. MCP Python SDK for servers and clients
Browse Franklin for concrete examples of tools and services exposed through MCP. Franklin’s MCP directory
Project documentation
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