diff --git a/docs/en/samples/pre_post_processing/python/adk/agent.py b/docs/en/samples/pre_post_processing/python/adk/agent.py
index 72a6a979a3..4b6ff4f2fa 100644
--- a/docs/en/samples/pre_post_processing/python/adk/agent.py
+++ b/docs/en/samples/pre_post_processing/python/adk/agent.py
@@ -52,7 +52,7 @@ async def after_tool_callback(
     args: Dict[str, Any],
     tool_context: ToolContext,
     tool_response: Any,
-):
+) -> Optional[Any]:
     """
     Callback fired after a tool execution.
     Enriches response for successful bookings.