diff --git a/src/fastmcp/tools/tool.py b/src/fastmcp/tools/tool.py
index ca868198c..1457bb3e8 100644
--- a/src/fastmcp/tools/tool.py
+++ b/src/fastmcp/tools/tool.py
@@ -101,7 +101,7 @@ class Tool(FastMCPBaseModel, ABC):
         tool: Tool,
         transform_fn: Callable[..., Any] | None = None,
         name: str | None = None,
-        transform_args: dict[str, str | ArgTransform | None] | None = None,
+        transform_args: dict[str, ArgTransform] | None = None,
         description: str | None = None,
         tags: set[str] | None = None,
         annotations: ToolAnnotations | None = None,
diff --git a/src/fastmcp/tools/tool_transform.py b/src/fastmcp/tools/tool_transform.py
index 11d3338a3..b193d4906 100644
--- a/src/fastmcp/tools/tool_transform.py
+++ b/src/fastmcp/tools/tool_transform.py
@@ -168,6 +168,11 @@ class ArgTransform:
                 "Hidden parameters cannot be required since clients cannot provide them."
             )
 
+        if self.required is False:
+            raise ValueError(
+                "Cannot specify 'required=False'. Set a default value instead."
+            )
+
 
 class TransformedTool(Tool):
     """A tool that is transformed from another tool.
diff --git a/tests/tools/test_tool_transform.py b/tests/tools/test_tool_transform.py
index cb5af15b4..017ebdcf6 100644
--- a/tests/tools/test_tool_transform.py
+++ b/tests/tools/test_tool_transform.py
@@ -858,23 +858,14 @@ async def test_arg_transform_required_false():
     def base_tool(required_param: int) -> str:
         return f"value: {required_param}"
 
-    # Make the required parameter optional with a default
-    new_tool = Tool.from_tool(
-        base_tool,
-        transform_args={"required_param": ArgTransform(required=False, default=99)},
-    )
-
-    # Parameter should now be optional (not in required list, has default)
-    assert "required_param" not in new_tool.parameters["required"]
-    assert new_tool.parameters["properties"]["required_param"]["default"] == 99
-
-    # Should work when parameter is not provided (uses default)
-    result = await new_tool.run(arguments={})
-    assert result[0].text == "value: 99"  # type: ignore
-
-    # Should work when parameter is provided
-    result = await new_tool.run(arguments={"required_param": 123})
-    assert result[0].text == "value: 123"  # type: ignore
+    with pytest.raises(
+        ValueError,
+        match="Cannot specify 'required=False'. Set a default value instead.",
+    ):
+        Tool.from_tool(
+            base_tool,
+            transform_args={"required_param": ArgTransform(required=False, default=99)},  # type: ignore
+        )
 
 
 async def test_arg_transform_required_with_rename():