fastmcp: allow passing Tool directly to .add_tool

src/mcp/server/fastmcp/server.py

@@ -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
@@ -315,6 +315,10 @@ async def read_resource(self, uri: AnyUrl | str) -> Iterable[ReadResourceContent
             logger.error(f"Error reading resource {uri}: {e}")
             raise ResourceError(str(e))
 
+    def add_tool_instance(self, tool: Tool) -> None:
+        """Add a Tool instance to the server."""
+        self._tool_manager.add_tool_instance(tool)
+
     def add_tool(
         self,
         fn: AnyFunction,

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

@@ -31,6 +31,16 @@ def list_tools(self) -> list[Tool]:
         """List all registered tools."""
         return list(self._tools.values())
 
+    def add_tool_instance(self, tool: Tool) -> Tool:
+        """Add a Tool instance to the server."""
+        existing = self._tools.get(tool.name)
+        if existing:
+            if self.warn_on_duplicate_tools:
+                logger.warning(f"Tool already exists: {tool.name}")
+            return existing
+        self._tools[tool.name] = tool
+        return tool
+
     def add_tool(
         self,
         fn: Callable[..., Any],
@@ -42,13 +52,7 @@ def add_tool(
         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:
-                logger.warning(f"Tool already exists: {tool.name}")
-            return existing
-        self._tools[tool.name] = tool
-        return tool
+        return self.add_tool_instance(tool)
 
     async def call_tool(
         self,

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_instance(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_instance(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."""