diff --git a/tests/entrypoints/openai/test_completion_with_function_calling.py b/tests/entrypoints/openai/test_completion_with_function_calling.py
index 84ad7a0916..799648d399 100644
--- a/tests/entrypoints/openai/test_completion_with_function_calling.py
+++ b/tests/entrypoints/openai/test_completion_with_function_calling.py
@@ -145,7 +145,11 @@ async def test_function_tool_use(client: openai.AsyncOpenAI, model_name: str,
                     "enable_thinking": enable_thinking
                 }
             })
-
+        if enable_thinking:
+            assert chat_completion.choices[0].message.\
+                reasoning_content is not None
+            assert chat_completion.choices[0].message.\
+                reasoning_content != ""
         assert chat_completion.choices[0].message.tool_calls is not None
         assert len(chat_completion.choices[0].message.tool_calls) > 0
     else:
diff --git a/vllm/entrypoints/openai/serving_chat.py b/vllm/entrypoints/openai/serving_chat.py
index a802fbc386..451241d3f9 100644
--- a/vllm/entrypoints/openai/serving_chat.py
+++ b/vllm/entrypoints/openai/serving_chat.py
@@ -1049,6 +1049,7 @@ class OpenAIServingChat(OpenAIServing):
                 message = ChatMessage(
                     role=role,
                     content="",
+                    reasoning_content=reasoning_content,
                     tool_calls=[
                         tool_call_class(function=FunctionCall(
                             name=tool_call.name,