Fix async callable object tools (#568)

Author: stephanlensky

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

@@ -1,5 +1,6 @@
 from __future__ import annotations as _annotations
 
+import functools
 import inspect
 from collections.abc import Callable
 from typing import TYPE_CHECKING, Any, get_origin
@@ -53,7 +54,7 @@ def from_function(
             raise ValueError("You must provide a name for lambda functions")
 
         func_doc = description or fn.__doc__ or ""
-        is_async = inspect.iscoroutinefunction(fn)
+        is_async = _is_async_callable(fn)
 
         if context_kwarg is None:
             sig = inspect.signature(fn)
@@ -98,3 +99,12 @@ async def run(
             )
         except Exception as e:
             raise ToolError(f"Error executing tool {self.name}: {e}") from e
+
+
+def _is_async_callable(obj: Any) -> bool:
+    while isinstance(obj, functools.partial):
+        obj = obj.func
+
+    return inspect.iscoroutinefunction(obj) or (
+        callable(obj) and inspect.iscoroutinefunction(getattr(obj, "__call__", None))
+    )

tests/server/fastmcp/test_tool_manager.py

@@ -102,6 +102,39 @@ def create_user(user: UserInput, flag: bool) -> dict:
         assert "age" in tool.parameters["$defs"]["UserInput"]["properties"]
         assert "flag" in tool.parameters["properties"]
 
+    def test_add_callable_object(self):
+        """Test registering a callable object."""
+
+        class MyTool:
+            def __init__(self):
+                self.__name__ = "MyTool"
+
+            def __call__(self, x: int) -> int:
+                return x * 2
+
+        manager = ToolManager()
+        tool = manager.add_tool(MyTool())
+        assert tool.name == "MyTool"
+        assert tool.is_async is False
+        assert tool.parameters["properties"]["x"]["type"] == "integer"
+
+    @pytest.mark.anyio
+    async def test_add_async_callable_object(self):
+        """Test registering an async callable object."""
+
+        class MyAsyncTool:
+            def __init__(self):
+                self.__name__ = "MyAsyncTool"
+
+            async def __call__(self, x: int) -> int:
+                return x * 2
+
+        manager = ToolManager()
+        tool = manager.add_tool(MyAsyncTool())
+        assert tool.name == "MyAsyncTool"
+        assert tool.is_async is True
+        assert tool.parameters["properties"]["x"]["type"] == "integer"
+
     def test_add_invalid_tool(self):
         manager = ToolManager()
         with pytest.raises(AttributeError):
@@ -168,6 +201,34 @@ async def double(n: int) -> int:
         result = await manager.call_tool("double", {"n": 5})
         assert result == 10
 
+    @pytest.mark.anyio
+    async def test_call_object_tool(self):
+        class MyTool:
+            def __init__(self):
+                self.__name__ = "MyTool"
+
+            def __call__(self, x: int) -> int:
+                return x * 2
+
+        manager = ToolManager()
+        tool = manager.add_tool(MyTool())
+        result = await tool.run({"x": 5})
+        assert result == 10
+
+    @pytest.mark.anyio
+    async def test_call_async_object_tool(self):
+        class MyAsyncTool:
+            def __init__(self):
+                self.__name__ = "MyAsyncTool"
+
+            async def __call__(self, x: int) -> int:
+                return x * 2
+
+        manager = ToolManager()
+        tool = manager.add_tool(MyAsyncTool())
+        result = await tool.run({"x": 5})
+        assert result == 10
+
     @pytest.mark.anyio
     async def test_call_tool_with_default_args(self):
         def add(a: int, b: int = 1) -> int: