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fastmcp.tools.function_tool

Standalone @tool decorator for FastMCP.

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Functions

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tool

Standalone decorator to mark a function as an MCP tool. Returns the original function with metadata attached. Register with a server using mcp.add_tool(). Args:
  • run_in_thread: Applies to sync tool functions only. When True (default), the sync function is dispatched to a worker thread so it does not block the event loop. Set to False to run the function inline on the event loop thread — useful for libraries with thread affinity (e.g. Windows COM via uiautomation/comtypes/pywin32, tkinter, some GPU/driver bindings). Ignored for async functions. Cannot be combined with timeout on a sync function: inline calls have no cancellation checkpoints, so the timeout would be a silent no-op.

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Classes

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DecoratedTool

Protocol for functions decorated with @tool.

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ToolMeta

Metadata attached to functions by the @tool decorator.

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FunctionTool

Methods:

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from_function

Create a FunctionTool from a function. Args:
  • fn: The function to wrap
  • metadata: ToolMeta object with all configuration. If provided, individual parameters must not be passed.
  • name, title, etc.: Individual parameters for backwards compatibility. Cannot be used together with metadata parameter.

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run

Run the tool with arguments. A tool body may return an InputRequiredResult (SEP-2322) to ask the client for input. Under the stateless multi-round-trip protocol that ask is the full result of this leg, so it is wrapped in an InputRequiredToolResult (a ToolResult subclass) rather than serialized as content; the ask flows through the middleware chain as an ordinary result and the wire handler returns it to the client unmodified.