diff --git a/pydantic_ai_slim/pydantic_ai/models/openai.py b/pydantic_ai_slim/pydantic_ai/models/openai.py
index 6db3742320..73a05c9fde 100644
--- a/pydantic_ai_slim/pydantic_ai/models/openai.py
+++ b/pydantic_ai_slim/pydantic_ai/models/openai.py
@@ -722,6 +722,7 @@ class _MapModelResponseContext:
 
         texts: list[str] = field(default_factory=list)
         tool_calls: list[ChatCompletionMessageFunctionToolCallParam] = field(default_factory=list)
+        reasoning_texts: list[str] = field(default_factory=list)
 
         def map_assistant_message(self, message: ModelResponse) -> chat.ChatCompletionAssistantMessageParam:
             for item in message.parts:
@@ -757,6 +758,9 @@ def _into_message_param(self) -> chat.ChatCompletionAssistantMessageParam:
                 message_param['content'] = None
             if self.tool_calls:
                 message_param['tool_calls'] = self.tool_calls
+            if self.reasoning_texts:
+                if reasoning_field := OpenAIModelProfile.from_profile(self._model.profile).openai_chat_reasoning_field:
+                    message_param[reasoning_field] = '\n\n'.join(self.reasoning_texts)
             return message_param
 
         def _map_response_text_part(self, item: TextPart) -> None:
@@ -773,11 +777,16 @@ def _map_response_thinking_part(self, item: ThinkingPart) -> None:
             This method serves as a hook that can be overridden by subclasses
             to implement custom logic for handling thinking parts.
             """
-            # NOTE: DeepSeek `reasoning_content` field should NOT be sent back per https://api-docs.deepseek.com/guides/reasoning_model,
-            # but we currently just send it in `<think>` tags anyway as we don't want DeepSeek-specific checks here.
-            # If you need this changed, please file an issue.
-            start_tag, end_tag = self._model.profile.thinking_tags
-            self.texts.append('\n'.join([start_tag, item.content, end_tag]))
+            profile = OpenAIModelProfile.from_profile(self._model.profile)
+            if profile.openai_chat_reasoning_field:
+                self.reasoning_texts.append(item.content)
+            else:
+                # Existing fallback logic
+                # NOTE: DeepSeek `reasoning_content` field should NOT be sent back per https://api-docs.deepseek.com/guides/reasoning_model,
+                # but we currently just send it in `<think>` tags anyway as we don't want DeepSeek-specific checks here.
+                # If you need this changed, please file an issue.
+                start_tag, end_tag = self._model.profile.thinking_tags
+                self.texts.append('\n'.join([start_tag, item.content, end_tag]))
 
         def _map_response_tool_call_part(self, item: ToolCallPart) -> None:
             """Maps a `ToolCallPart` to the response context.
diff --git a/pydantic_ai_slim/pydantic_ai/profiles/moonshotai.py b/pydantic_ai_slim/pydantic_ai/profiles/moonshotai.py
index 9b4ffb4bc3..df45f2766d 100644
--- a/pydantic_ai_slim/pydantic_ai/profiles/moonshotai.py
+++ b/pydantic_ai_slim/pydantic_ai/profiles/moonshotai.py
@@ -1,8 +1,11 @@
 from __future__ import annotations as _annotations
 
-from . import ModelProfile
+from .openai import OpenAIModelProfile
 
 
-def moonshotai_model_profile(model_name: str) -> ModelProfile | None:
+def moonshotai_model_profile(model_name: str) -> OpenAIModelProfile | None:
     """Get the model profile for a MoonshotAI model."""
-    return ModelProfile(ignore_streamed_leading_whitespace=True)
+    return OpenAIModelProfile(
+        ignore_streamed_leading_whitespace=True,
+        openai_chat_reasoning_field='reasoning_content',
+    )
diff --git a/pydantic_ai_slim/pydantic_ai/profiles/openai.py b/pydantic_ai_slim/pydantic_ai/profiles/openai.py
index a3cd83d3e8..2118fac18d 100644
--- a/pydantic_ai_slim/pydantic_ai/profiles/openai.py
+++ b/pydantic_ai_slim/pydantic_ai/profiles/openai.py
@@ -51,6 +51,13 @@ class OpenAIModelProfile(ModelProfile):
     See https://github.com/pydantic/pydantic-ai/issues/3245 for more details.
     """
 
+    openai_chat_reasoning_field: str | None = None
+    """The field name to use for sending reasoning content back to the model.
+
+    If set, reasoning content will be sent in this field of the request message
+    instead of being embedded in the content.
+    """
+
     def __post_init__(self):  # pragma: no cover
         if not self.openai_supports_sampling_settings:
             warnings.warn(
diff --git a/tests/models/test_openai.py b/tests/models/test_openai.py
index 004d91ec5a..11dd0742e7 100644
--- a/tests/models/test_openai.py
+++ b/tests/models/test_openai.py
@@ -3116,3 +3116,36 @@ async def test_cache_point_filtering_responses_model():
     assert len(msg['content']) == 2
     assert msg['content'][0]['text'] == 'text before'  # type: ignore[reportUnknownArgumentType]
     assert msg['content'][1]['text'] == 'text after'  # type: ignore[reportUnknownArgumentType]
+
+
+async def test_openai_chat_reasoning_field(allow_model_requests: None):
+    """Test that reasoning content is sent in the configured field when `openai_chat_reasoning_field` is set."""
+    # Mock a response that will trigger a thinking part in history
+    c = completion_message(ChatCompletionMessage(content='world', role='assistant'))
+    mock_client = MockOpenAI.create_mock(c)
+
+    # Configure model with custom profile setting the reasoning field
+    profile = OpenAIModelProfile(openai_chat_reasoning_field='reasoning_content')
+    m = OpenAIChatModel('gpt-4o', provider=OpenAIProvider(openai_client=mock_client), profile=profile)
+
+    # Create a history with a ThinkingPart
+    history = [
+        ModelResponse(
+            parts=[ThinkingPart(content='thinking about it'), TextPart(content='hello')],
+            model_name='gpt-4o',
+            timestamp=datetime(2024, 1, 1, tzinfo=timezone.utc),
+            provider_name='openai',
+        )
+    ]
+
+    agent = Agent(m)
+    await agent.run('next', message_history=history)
+
+    # Verify the request sent to OpenAI has the extra field
+    kwargs = get_mock_chat_completion_kwargs(mock_client)[0]
+    messages = kwargs['messages']
+
+    # The history message (index 0) should have the reasoning_content field
+    assert messages[0]['role'] == 'assistant'
+    assert messages[0]['content'] == 'hello'
+    assert messages[0]['reasoning_content'] == 'thinking about it'