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function_tool

docs/python-sdk/fastmcp-tools-function_tool.mdx

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fastmcp.tools.function_tool

Standalone @tool decorator for FastMCP.

Functions

tool <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/function_tool.py#L548" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

python
tool(name_or_fn: str | Callable[..., Any] | None = None) -> Any

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.

Classes

DecoratedTool <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/function_tool.py#L143" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

Protocol for functions decorated with @tool.

ToolMeta <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/function_tool.py#L152" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

Metadata attached to functions by the @tool decorator.

FunctionTool <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/function_tool.py#L198" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

Methods:

from_function <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/function_tool.py#L217" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

python
from_function(cls, fn: Callable[..., Any]) -> FunctionTool

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.

run <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/tools/function_tool.py#L367" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

python
run(self, arguments: dict[str, Any]) -> ToolResult

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.