Accept callables in Tool.from_tool() (#3235)

* Accept callables in Tool.from_tool()

* chore: Update SDK documentation

* Add end-to-end client test for decorated function transform

---------

Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com>

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

@@ -106,10 +106,10 @@ Schedule this tool for background execution via docket.
 #### `from_tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool.py#L364" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
-from_tool(cls, tool: Tool) -> TransformedTool
+from_tool(cls, tool: Tool | Callable[..., Any]) -> TransformedTool
 ```
 
-#### `get_span_attributes` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool.py#L395" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `get_span_attributes` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool.py#L412" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 get_span_attributes(self) -> dict[str, Any]

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

@@ -62,7 +62,7 @@ y=2)` will call the parent tool with `x=1` and `y=2`.
 - `RuntimeError`: If called outside a transformed tool context.
 
 
-### `apply_transformations_to_tools` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L977" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `apply_transformations_to_tools` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L979" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 apply_transformations_to_tools(tools: dict[str, Tool], transformations: dict[str, ToolTransformConfig]) -> dict[str, Tool]
@@ -213,7 +213,7 @@ functions.
 #### `from_tool` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L369" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
-from_tool(cls, tool: Tool, name: str | None = None, version: str | NotSetT | None = NotSet, title: str | NotSetT | None = NotSet, description: str | NotSetT | None = NotSet, tags: set[str] | None = None, transform_fn: Callable[..., Any] | None = None, transform_args: dict[str, ArgTransform] | None = None, annotations: ToolAnnotations | NotSetT | None = NotSet, output_schema: dict[str, Any] | NotSetT | None = NotSet, serializer: Callable[[Any], str] | NotSetT | None = NotSet, meta: dict[str, Any] | NotSetT | None = NotSet) -> TransformedTool
+from_tool(cls, tool: Tool | Callable[..., Any], name: str | None = None, version: str | NotSetT | None = NotSet, title: str | NotSetT | None = NotSet, description: str | NotSetT | None = NotSet, tags: set[str] | None = None, transform_fn: Callable[..., Any] | None = None, transform_args: dict[str, ArgTransform] | None = None, annotations: ToolAnnotations | NotSetT | None = NotSet, output_schema: dict[str, Any] | NotSetT | None = NotSet, serializer: Callable[[Any], str] | NotSetT | None = NotSet, meta: dict[str, Any] | NotSetT | None = NotSet) -> TransformedTool
 ```
 
 Create a transformed tool from a parent tool.
@@ -293,7 +293,7 @@ async def custom_output(**kwargs) -> ToolResult:
 ```
 
 
-### `ToolTransformConfig` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L923" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `ToolTransformConfig` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L925" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 
 Provides a way to transform a tool.
@@ -301,7 +301,7 @@ Provides a way to transform a tool.
 
 **Methods:**
 
-#### `apply` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L956" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `apply` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/src/fastmcp/tools/tool_transform.py#L958" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 apply(self, tool: Tool) -> TransformedTool

src/fastmcp/tools/tool.py

@@ -363,7 +363,7 @@ class Tool(FastMCPComponent):
     @classmethod
     def from_tool(
         cls,
-        tool: Tool,
+        tool: Tool | Callable[..., Any],
         *,
         name: str | None = None,
         title: str | NotSetT | None = NotSet,
@@ -378,6 +378,8 @@ class Tool(FastMCPComponent):
     ) -> TransformedTool:
         from fastmcp.tools.tool_transform import TransformedTool
 
+        tool = cls._ensure_tool(tool)
+
         return TransformedTool.from_tool(
             tool=tool,
             transform_fn=transform_fn,
@@ -392,6 +394,21 @@ class Tool(FastMCPComponent):
             meta=meta,
         )
 
+    @classmethod
+    def _ensure_tool(cls, tool: Tool | Callable[..., Any]) -> Tool:
+        """Coerce a callable into a Tool, respecting @tool decorator metadata."""
+        if isinstance(tool, Tool):
+            return tool
+
+        from fastmcp.decorators import get_fastmcp_meta
+        from fastmcp.tools.function_tool import FunctionTool, ToolMeta
+
+        fmeta = get_fastmcp_meta(tool)
+        if isinstance(fmeta, ToolMeta):
+            return FunctionTool.from_function(tool, metadata=fmeta)
+
+        return cls.from_function(tool)
+
     def get_span_attributes(self) -> dict[str, Any]:
         return super().get_span_attributes() | {
             "fastmcp.component.type": "tool",

src/fastmcp/tools/tool_transform.py

@@ -368,7 +368,7 @@ class TransformedTool(Tool):
     @classmethod
     def from_tool(
         cls,
-        tool: Tool,
+        tool: Tool | Callable[..., Any],
         name: str | None = None,
         version: str | NotSetT | None = NotSet,
         title: str | NotSetT | None = NotSet,
@@ -456,6 +456,8 @@ class TransformedTool(Tool):
                 )
             ```
         """
+        tool = Tool._ensure_tool(tool)
+
         if (
             serializer is not NotSet
             and serializer is not None

tests/tools/tool_transform/test_tool_transform.py

@@ -9,7 +9,7 @@ from pydantic import BaseModel, Field
 
 from fastmcp import FastMCP
 from fastmcp.client.client import Client
-from fastmcp.tools import Tool, forward, forward_raw
+from fastmcp.tools import Tool, forward, forward_raw, tool
 from fastmcp.tools.function_tool import FunctionTool
 from fastmcp.tools.tool import ToolResult
 from fastmcp.tools.tool_transform import (
@@ -41,6 +41,60 @@ def test_tool_from_tool_no_change(add_tool):
     assert new_tool.description == add_tool.description
 
 
+def test_from_tool_accepts_decorated_function():
+    @tool
+    def search(q: str, limit: int = 10) -> list[str]:
+        """Search for items."""
+        return [f"Result {i} for {q}" for i in range(limit)]
+
+    transformed = Tool.from_tool(
+        search,
+        name="find_items",
+        transform_args={"q": ArgTransform(name="query")},
+    )
+    assert isinstance(transformed, TransformedTool)
+    assert transformed.name == "find_items"
+    assert "query" in transformed.parameters["properties"]
+    assert "q" not in transformed.parameters["properties"]
+
+
+def test_from_tool_accepts_plain_function():
+    def search(q: str, limit: int = 10) -> list[str]:
+        return [f"Result {i} for {q}" for i in range(limit)]
+
+    transformed = Tool.from_tool(
+        search,
+        name="find_items",
+        transform_args={"q": ArgTransform(name="query")},
+    )
+    assert isinstance(transformed, TransformedTool)
+    assert transformed.name == "find_items"
+    assert "query" in transformed.parameters["properties"]
+
+
+def test_from_tool_decorated_function_preserves_metadata():
+    @tool(description="Custom description")
+    def search(q: str) -> list[str]:
+        """Original description."""
+        return []
+
+    transformed = Tool.from_tool(search)
+    assert transformed.parent_tool.description == "Custom description"
+
+
+async def test_from_tool_decorated_function_runs(add_tool):
+    @tool
+    def add(x: int, y: int = 10) -> int:
+        return x + y
+
+    transformed = Tool.from_tool(
+        add,
+        transform_args={"x": ArgTransform(name="a")},
+    )
+    result = await transformed.run(arguments={"a": 3, "y": 5})
+    assert result.structured_content == {"result": 8}
+
+
 async def test_renamed_arg_description_is_maintained(add_tool):
     new_tool = Tool.from_tool(
         add_tool, transform_args={"old_x": ArgTransform(name="new_x")}
@@ -492,6 +546,29 @@ def test_function_with_kwargs_can_add_params(add_tool):
     assert "new_x" in tool.parameters["properties"]
 
 
+async def test_from_tool_decorated_function_via_client():
+    @tool
+    def search(q: str, limit: int = 10) -> list[str]:
+        """Search for items."""
+        return [f"Result {i} for {q}" for i in range(limit)]
+
+    better_search = Tool.from_tool(
+        search,
+        name="find_items",
+        transform_args={
+            "q": ArgTransform(name="query", description="The search terms"),
+        },
+    )
+
+    mcp = FastMCP("Server")
+    mcp.add_tool(better_search)
+
+    async with Client(mcp) as client:
+        result = await client.call_tool("find_items", {"query": "hello", "limit": 3})
+        assert isinstance(result.content[0], TextContent)
+        assert "Result 0 for hello" in result.content[0].text
+
+
 class TestProxy:
     @pytest.fixture
     def mcp_server(self) -> FastMCP: