fastmcp: allow passing Tool directly to FastMCP constructor

src/mcp/server/fastmcp/server.py

@@ -10,7 +10,7 @@
     asynccontextmanager,
 )
 from itertools import chain
-from typing import Any, Generic, Literal
+from typing import Any, Generic, Literal, overload
 
 import anyio
 import pydantic_core
@@ -37,7 +37,7 @@
 from mcp.server.fastmcp.exceptions import ResourceError
 from mcp.server.fastmcp.prompts import Prompt, PromptManager
 from mcp.server.fastmcp.resources import FunctionResource, Resource, ResourceManager
-from mcp.server.fastmcp.tools import ToolManager
+from mcp.server.fastmcp.tools import Tool, ToolManager
 from mcp.server.fastmcp.utilities.logging import configure_logging, get_logger
 from mcp.server.fastmcp.utilities.types import Image
 from mcp.server.lowlevel.helper_types import ReadResourceContents
@@ -315,27 +315,42 @@ async def read_resource(self, uri: AnyUrl | str) -> Iterable[ReadResourceContent
             logger.error(f"Error reading resource {uri}: {e}")
             raise ResourceError(str(e))
 
+    @overload
+    def add_tool(self, fn: Tool) -> None: ...
+
+    @overload
     def add_tool(
         self,
         fn: AnyFunction,
         name: str | None = None,
         description: str | None = None,
         annotations: ToolAnnotations | None = None,
+    ) -> None: ...
+
+    def add_tool(
+        self,
+        fn: AnyFunction | Tool,
+        name: str | None = None,
+        description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> None:
         """Add a tool to the server.
 
         The tool function can optionally request a Context object by adding a parameter
         with the Context type annotation. See the @tool decorator for examples.
 
         Args:
-            fn: The function to register as a tool
+            fn: The function to register as a tool or a Tool instance
             name: Optional name for the tool (defaults to function name)
             description: Optional description of what the tool does
             annotations: Optional ToolAnnotations providing additional tool information
         """
-        self._tool_manager.add_tool(
-            fn, name=name, description=description, annotations=annotations
-        )
+        if isinstance(fn, Tool):
+            self._tool_manager.add_tool(fn)
+        else:
+            self._tool_manager.add_tool(
+                fn, name=name, description=description, annotations=annotations
+            )
 
     def tool(
         self,

src/mcp/server/fastmcp/tools/tool_manager.py

@@ -1,7 +1,7 @@
 from __future__ import annotations as _annotations
 
 from collections.abc import Callable
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, overload
 
 from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.tools.base import Tool
@@ -31,17 +31,35 @@ def list_tools(self) -> list[Tool]:
         """List all registered tools."""
         return list(self._tools.values())
 
+    @overload
+    def add_tool(
+        self,
+        fn: Tool,
+    ) -> Tool: ...
+
+    @overload
     def add_tool(
         self,
         fn: Callable[..., Any],
         name: str | None = None,
         description: str | None = None,
         annotations: ToolAnnotations | None = None,
+    ) -> Tool: ...
+
+    def add_tool(
+        self,
+        fn: Callable[..., Any] | Tool,
+        name: str | None = None,
+        description: str | None = None,
+        annotations: ToolAnnotations | None = None,
     ) -> Tool:
         """Add a tool to the server."""
-        tool = Tool.from_function(
-            fn, name=name, description=description, annotations=annotations
-        )
+        if isinstance(fn, Tool):
+            tool = fn
+        else:
+            tool = Tool.from_function(
+                fn, name=name, description=description, annotations=annotations
+            )
         existing = self._tools.get(tool.name)
         if existing:
             if self.warn_on_duplicate_tools:

tests/server/fastmcp/test_tool_manager.py

@@ -6,7 +6,8 @@
 
 from mcp.server.fastmcp import Context, FastMCP
 from mcp.server.fastmcp.exceptions import ToolError
-from mcp.server.fastmcp.tools import ToolManager
+from mcp.server.fastmcp.tools import Tool, ToolManager
+from mcp.server.fastmcp.utilities.func_metadata import ArgModelBase, FuncMetadata
 from mcp.server.session import ServerSessionT
 from mcp.shared.context import LifespanContextT
 from mcp.types import ToolAnnotations
@@ -31,6 +32,32 @@ def add(a: int, b: int) -> int:
         assert tool.parameters["properties"]["a"]["type"] == "integer"
         assert tool.parameters["properties"]["b"]["type"] == "integer"
 
+    def test_add_tool_directly(self):
+        manager = ToolManager()
+
+        def add(a: int, b: int) -> int:
+            return a + b
+
+        class AddArguments(ArgModelBase):
+            a: int
+            b: int
+
+        fn_metadata = FuncMetadata(arg_model=AddArguments)
+
+        original_tool = Tool(
+            name="add",
+            description="Add two numbers.",
+            fn=add,
+            fn_metadata=fn_metadata,
+            is_async=False,
+            parameters=AddArguments.model_json_schema(),
+            context_kwarg=None,
+            annotations=None,
+        )
+        manager.add_tool(original_tool)
+        saved_tool = manager.get_tool("add")
+        assert saved_tool == original_tool
+
     @pytest.mark.anyio
     async def test_async_function(self):
         """Test registering and running an async function."""