docs: document OpenAI reasoning summaries (#30882)

libs/partners/openai/langchain_openai/chat_models/base.py

@@ -2132,6 +2132,40 @@ class ChatOpenAI(BaseChatOpenAI):  # type: ignore[override]
 
             "Your name is Bob. How can I help you today, Bob?"
 
+    .. dropdown:: Reasoning output
+
+        OpenAI's Responses API supports `reasoning models <https://platform.openai.com/docs/guides/reasoning?api-mode=responses>`_
+        that expose a summary of internal reasoning processes.
+
+        .. code-block:: python
+
+            from langchain_openai import ChatOpenAI
+
+            reasoning = {
+                "effort": "medium",  # 'low', 'medium', or 'high'
+                "summary": "auto",  # 'detailed', 'auto', or None
+            }
+
+            llm = ChatOpenAI(
+                model="o4-mini", use_responses_api=True, model_kwargs={"reasoning": reasoning}
+            )
+            response = llm.invoke("What is 3^3?")
+
+            print(f"Output: {response.text()}")
+            print(f"Reasoning: {response.additional_kwargs['reasoning']}")
+
+        .. code-block:: none
+
+            Output: 3^3 = 27.
+
+            Reasoning: {
+                'id': 'rs_67fffc44b1c08191b6ca9bead6d832590433145b1786f809',
+                'summary': [
+                    {'text': 'The user wants to know...', 'type': 'summary_text'}
+                ],
+                'type': 'reasoning'
+            }
+
     .. dropdown:: Structured output
 
         .. code-block:: python