diff --git a/python/semantic_kernel/agents/chat_completion/chat_completion_agent.py b/python/semantic_kernel/agents/chat_completion/chat_completion_agent.py
index 7a42e664be4f..fa5175a7ec7a 100644
--- a/python/semantic_kernel/agents/chat_completion/chat_completion_agent.py
+++ b/python/semantic_kernel/agents/chat_completion/chat_completion_agent.py
@@ -89,7 +89,10 @@ async def _on_new_message(self, new_message: str | ChatMessageContent) -> None:
             or "thread_id" not in new_message.metadata
             or new_message.metadata["thread_id"] != self._id
         ):
-            self._chat_history.add_message(new_message)
+            if isinstance(self._chat_history, ChatHistoryReducer):
+                await self._chat_history.add_message_async(new_message)
+            else:
+                self._chat_history.add_message(new_message)
 
     async def get_messages(self) -> AsyncIterable[ChatMessageContent]:
         """Retrieve the current chat history.
diff --git a/python/tests/unit/agents/chat_completion/test_chat_completion_agent.py b/python/tests/unit/agents/chat_completion/test_chat_completion_agent.py
index b79af78c2c39..40796a0fa3ce 100644
--- a/python/tests/unit/agents/chat_completion/test_chat_completion_agent.py
+++ b/python/tests/unit/agents/chat_completion/test_chat_completion_agent.py
@@ -164,6 +164,19 @@ def test_initialize_with_reducer_chat_history():
     assert isinstance(thread._chat_history, ChatHistoryTruncationReducer)
 
 
+async def test_thread_with_reducer_auto_reduces_new_messages():
+    reducer = ChatHistoryTruncationReducer(target_count=1, auto_reduce=True)
+    thread = ChatHistoryAgentThread(chat_history=reducer, thread_id="reducer_test_thread")
+
+    await thread.on_new_message(ChatMessageContent(role=AuthorRole.USER, content="first"))
+    await thread.on_new_message(ChatMessageContent(role=AuthorRole.ASSISTANT, content="second"))
+
+    messages = [message async for message in thread.get_messages()]
+
+    assert len(messages) == 1
+    assert messages[0].content == "second"
+
+
 async def test_get_response(kernel_with_ai_service: tuple[Kernel, ChatCompletionClientBase]):
     kernel, _ = kernel_with_ai_service
     agent = ChatCompletionAgent(