---
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#L375" 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().


## Classes

### `DecoratedTool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/function_tool.py#L56" 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#L65" 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#L87" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

**Methods:**

#### `to_mcp_tool` <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>

```python
to_mcp_tool(self, **overrides: Any) -> mcp.types.Tool
```

Convert the FastMCP tool to an MCP tool.

Extends the base implementation to add task execution mode if enabled.


#### `from_function` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/function_tool.py#L110" 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#L253" 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#L298" 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.

FunctionTool registers the underlying function, which has the user's
Depends parameters for docket to resolve.


#### `add_to_docket` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/function_tool.py#L308" 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()

