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function_tool

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

3.3.0b23.8 KB
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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/src/fastmcp/tools/function_tool.py#L437" 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/src/fastmcp/tools/function_tool.py#L59" 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/src/fastmcp/tools/function_tool.py#L68" 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/src/fastmcp/tools/function_tool.py#L91" 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/src/fastmcp/tools/function_tool.py#L111" 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/src/fastmcp/tools/function_tool.py#L285" 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.

register_with_docket <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/function_tool.py#L357" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

python
register_with_docket(self, docket: Docket) -> None

Register this tool with docket for background execution.

Registers the raw function so Docket sees and resolves ALL dependencies — both FastMCP's (CurrentContext, Progress) and Docket-native ones (Retry, Timeout, ConcurrencyLimit).

add_to_docket <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/function_tool.py#L368" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

python
add_to_docket(self, docket: Docket, arguments: dict[str, Any], **kwargs: Any) -> Execution

Schedule this tool for background execution via docket.

FunctionTool splats the arguments dict since .fn expects **kwargs.

Args:

  • docket: The Docket instance
  • arguments: Tool arguments
  • fn_key: Function lookup key in Docket registry (defaults to self.key)
  • task_key: Redis storage key for the result
  • **kwargs: Additional kwargs passed to docket.add()