[Frontend] Normalize reasoning_content to reasoning for client compatibility

tests/tool_use/test_chat_completion_request_validations.py

@@ -61,3 +61,58 @@ def test_chat_completion_request_with_tool_choice_but_no_tools(tool_choice):
                 "tools": None,
             }
         )
+
+
+def test_reasoning_content_normalized_to_reasoning():
+    request = ChatCompletionRequest.model_validate(
+        {
+            "messages": [
+                {"role": "user", "content": "What is 2+2?"},
+                {
+                    "role": "assistant",
+                    "content": "4",
+                    "reasoning_content": "2+2 equals 4",
+                },
+                {"role": "user", "content": "Are you sure?"},
+            ],
+            "model": "facebook/opt-125m",
+        }
+    )
+    assistant_msg = request.messages[1]
+    assert assistant_msg.get("reasoning") == "2+2 equals 4"
+    assert "reasoning_content" not in assistant_msg
+
+
+def test_reasoning_takes_precedence_over_reasoning_content():
+    request = ChatCompletionRequest.model_validate(
+        {
+            "messages": [
+                {"role": "user", "content": "What is 2+2?"},
+                {
+                    "role": "assistant",
+                    "content": "4",
+                    "reasoning": "from reasoning field",
+                    "reasoning_content": "from reasoning_content field",
+                },
+            ],
+            "model": "facebook/opt-125m",
+        }
+    )
+    assistant_msg = request.messages[1]
+    assert assistant_msg.get("reasoning") == "from reasoning field"
+    assert "reasoning_content" not in assistant_msg
+
+
+def test_no_reasoning_fields_unchanged():
+    request = ChatCompletionRequest.model_validate(
+        {
+            "messages": [
+                {"role": "user", "content": "Hello"},
+                {"role": "assistant", "content": "Hi"},
+            ],
+            "model": "facebook/opt-125m",
+        }
+    )
+    assistant_msg = request.messages[1]
+    assert assistant_msg.get("reasoning") is None
+    assert "reasoning_content" not in assistant_msg

vllm/entrypoints/openai/chat_completion/protocol.py

@@ -413,13 +413,14 @@ class ChatCompletionRequest(OpenAIBaseModel):
 
     @model_validator(mode="before")
     @classmethod
-    def _materialize_tool_calls_before(cls, data: Any) -> Any:
-        """Eagerly convert tool_calls generators/iterators to lists.
-
-        Must run before Pydantic field validation so that one-shot
-        generators are not consumed during union type matching of
-        ChatCompletionAssistantMessageParam (which types tool_calls
-        as Iterable[...]).
+    def _normalize_messages_before(cls, data: Any) -> Any:
+        """Pre-process message dicts before Pydantic field validation.
+
+        Performs two normalizations in a single pass:
+        - Converts tool_calls generators/iterators to lists so one-shot
+          generators are not consumed during union type matching.
+        - Renames the deprecated ``reasoning_content`` field to
+          ``reasoning`` so downstream code only needs to check one field.
         """
         if not isinstance(data, dict):
             return data
@@ -432,6 +433,9 @@ def _materialize_tool_calls_before(cls, data: Any) -> Any:
             tool_calls = msg.get("tool_calls")
             if tool_calls is not None and not isinstance(tool_calls, list):
                 msg["tool_calls"] = list(tool_calls)
+            reasoning_content = msg.pop("reasoning_content", None)
+            if reasoning_content is not None and msg.get("reasoning") is None:
+                msg["reasoning"] = reasoning_content
         return data
 
     @model_validator(mode="after")