diff --git a/src/google/adk/tools/_automatic_function_calling_util.py b/src/google/adk/tools/_automatic_function_calling_util.py
index 97d89cb6..89f0d33c 100644
--- a/src/google/adk/tools/_automatic_function_calling_util.py
+++ b/src/google/adk/tools/_automatic_function_calling_util.py
@@ -230,6 +230,7 @@ def build_function_declaration(
       )
       new_func.__signature__ = new_sig
       new_func.__doc__ = func.__doc__
+      new_func.__annotations__ = func.__annotations__
 
   return (
       from_function_with_options(func, variant)