diff --git a/src/google/adk/flows/llm_flows/_nl_planning.py b/src/google/adk/flows/llm_flows/_nl_planning.py
index 765623b5b70..79563b6fbd8 100644
--- a/src/google/adk/flows/llm_flows/_nl_planning.py
+++ b/src/google/adk/flows/llm_flows/_nl_planning.py
@@ -72,8 +72,6 @@ class _NlPlanningResponse(BaseLlmResponseProcessor):
   async def run_async(
       self, invocation_context: InvocationContext, llm_response: LlmResponse
   ) -> AsyncGenerator[Event, None]:
-    from ...planners.built_in_planner import BuiltInPlanner
-
     if (
         not llm_response
         or not llm_response.content
@@ -82,11 +80,7 @@ async def run_async(
       return
 
     planner = _get_planner(invocation_context)
-    if (
-        not planner
-        or type(planner).process_planning_response
-        is BuiltInPlanner.process_planning_response
-    ):
+    if not planner:
       return
 
     # Postprocess the LLM response.
diff --git a/src/google/adk/planners/built_in_planner.py b/src/google/adk/planners/built_in_planner.py
index eb665263405..ff7968fc8af 100644
--- a/src/google/adk/planners/built_in_planner.py
+++ b/src/google/adk/planners/built_in_planner.py
@@ -83,4 +83,21 @@ def process_planning_response(
       callback_context: CallbackContext,
       response_parts: List[types.Part],
   ) -> Optional[List[types.Part]]:
-    return
+    """Processes the planning response by returning the model's native content blocks.
+
+    The BuiltInPlanner relies on the model's native thinking feature, where
+    reasoning parts are already marked with thought=True by the model.
+    This method returns the response parts as-is, preserving the standardized
+    content blocks produced by the model.
+
+    Args:
+      callback_context: The callback context of the invocation.
+      response_parts: The LLM response parts.
+
+    Returns:
+      The response parts with the model's native content blocks preserved,
+      or None if there are no parts to process.
+    """
+    if not response_parts:
+      return None
+    return list(response_parts)