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

---
title: tool_transform
sidebarTitle: tool_transform
---

# `fastmcp.tools.tool_transform`

## Classes

### `ArgTransform` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L85"><Icon icon="github" size="14" /></a></sup>


Configuration for transforming a parent tool's argument.

This class allows fine-grained control over how individual arguments are transformed
when creating a new tool from an existing one. You can rename arguments, change their
descriptions, add default values, or hide them from clients while passing constants.

**Examples:**

Rename argument 'old_name' to 'new_name'
```python
ArgTransform(name="new_name")
```

Change description only
```python
ArgTransform(description="Updated description")
```

Add a default value (makes argument optional)
```python
ArgTransform(default=42)
```

Add a default factory (makes argument optional)
```python
ArgTransform(default_factory=lambda: time.time())
```

Change the type
```python
ArgTransform(type=str)
```

Hide the argument entirely from clients
```python
ArgTransform(hide=True)
```

Hide argument but pass a constant value to parent
```python
ArgTransform(hide=True, default="constant_value")
```

Hide argument but pass a factory-generated value to parent
```python
ArgTransform(hide=True, default_factory=lambda: uuid.uuid4().hex)
```

Make an optional parameter required (removes any default)
```python
ArgTransform(required=True)
```

Combine multiple transformations
```python
ArgTransform(name="new_name", description="New desc", default=None, type=int)
```


### `TransformedTool` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L199"><Icon icon="github" size="14" /></a></sup>


A tool that is transformed from another tool.

This class represents a tool that has been created by transforming another tool.
It supports argument renaming, schema modification, custom function injection,
and provides context for the forward() and forward_raw() functions.

The transformation can be purely schema-based (argument renaming, dropping, etc.)
or can include a custom function that uses forward() to call the parent tool
with transformed arguments.


**Methods:**

#### `from_tool` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L280"><Icon icon="github" size="14" /></a></sup>

```python
from_tool(cls, tool: Tool, name: str | None = None, description: str | None = None, tags: set[str] | None = None, transform_fn: Callable[..., Any] | None = None, transform_args: dict[str, ArgTransform] | None = None, annotations: ToolAnnotations | None = None, serializer: Callable[[Any], str] | None = None, enabled: bool | None = None) -> TransformedTool
```

Create a transformed tool from a parent tool.

**Args:**
- `tool`: The parent tool to transform.
- `transform_fn`: Optional custom function. Can use forward() and forward_raw()
to call the parent tool. Functions with **kwargs receive transformed
argument names.
- `name`: New name for the tool. Defaults to parent tool's name.
- `transform_args`: Optional transformations for parent tool arguments.
Only specified arguments are transformed, others pass through unchanged\:
- Simple rename (str)
- Complex transformation (rename/description/default/drop) (ArgTransform)
- Drop the argument (None)
- `description`: New description. Defaults to parent's description.
- `tags`: New tags. Defaults to parent's tags.
- `annotations`: New annotations. Defaults to parent's annotations.
- `serializer`: New serializer. Defaults to parent's serializer.

**Returns:**
- TransformedTool with the specified transformations.

**Examples:**

# Transform specific arguments only
```python
Tool.from_tool(parent, transform_args={"old": "new"})  # Others unchanged
```

# Custom function with partial transforms
```python
async def custom(x: int, y: int) -> str:
    result = await forward(x=x, y=y)
    return f"Custom: {result}"

Tool.from_tool(parent, transform_fn=custom, transform_args={"a": "x", "b": "y"})
```

# Using **kwargs (gets all args, transformed and untransformed)
```python
async def flexible(**kwargs) -> str:
    result = await forward(**kwargs)
    return f"Got: {kwargs}"

Tool.from_tool(parent, transform_fn=flexible, transform_args={"a": "x"})
```