Fix async callable object tools

Author: stephanlensky

src/mcp/server/fastmcp/server.py

@@ -466,6 +466,7 @@ async def run_stdio_async(self) -> None:
     async def run_sse_async(self) -> None:
         """Run the server using SSE transport."""
         import uvicorn
+
         starlette_app = self.sse_app()
 
         config = uvicorn.Config(

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
@@ -48,7 +49,7 @@ 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)
@@ -92,3 +93,12 @@ async def run(
             )
         except Exception as e:
             raise ToolError(f"Error executing tool {self.name}: {e}") from e
+
+
+def _is_async_callable(obj: Any) -> Any:
+    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/servers/test_file_server.py

@@ -114,17 +114,13 @@ async def test_read_resource_file(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_delete_file(mcp: FastMCP, test_dir: Path):
-    await mcp.call_tool(
-        "delete_file", arguments={"path": str(test_dir / "example.py")}
-    )
+    await mcp.call_tool("delete_file", arguments={"path": str(test_dir / "example.py")})
     assert not (test_dir / "example.py").exists()
 
 
 @pytest.mark.anyio
 async def test_delete_file_and_check_resources(mcp: FastMCP, test_dir: Path):
-    await mcp.call_tool(
-        "delete_file", arguments={"path": str(test_dir / "example.py")}
-    )
+    await mcp.call_tool("delete_file", arguments={"path": str(test_dir / "example.py")})
     res_iter = await mcp.read_resource("file://test_dir/example.py")
     res_list = list(res_iter)
     assert len(res_list) == 1

tests/server/fastmcp/test_tool_manager.py

@@ -71,6 +71,39 @@ 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):
@@ -137,6 +170,34 @@ 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: