diff --git a/packages/toolbox-core/src/toolbox_core/sync_tool.py b/packages/toolbox-core/src/toolbox_core/sync_tool.py
index ad24e8e3..6e3179ed 100644
--- a/packages/toolbox-core/src/toolbox_core/sync_tool.py
+++ b/packages/toolbox-core/src/toolbox_core/sync_tool.py
@@ -16,6 +16,7 @@
 from asyncio import AbstractEventLoop
 from inspect import Signature
 from threading import Thread
+from types import MappingProxyType
 from typing import Any, Callable, Mapping, TypeVar, Union
 
 from .tool import ToolboxTool
@@ -81,11 +82,11 @@ def __signature__(self) -> Signature:
         return self.__async_tool.__signature__
 
     @property
-    def __annotations__(self) -> dict[str, Any]:  # type: ignore[override]
+    def __annotations__(self) -> MappingProxyType[str, Any]:  # type: ignore[override]
         # Standard Python object attributes like __doc__ are technically "writable".
         # But not defining a setter function makes this a read-only property.
         # Mypy flags this issue in the type checks.
-        return self.__async_tool.__annotations__
+        return MappingProxyType(self.__async_tool.__annotations__)
 
     def __call__(self, *args: Any, **kwargs: Any) -> str:
         """