---
title: function_tool
sidebarTitle: function_tool
---

# `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()