fix(deepseek): preserve reasoning_content in multi-turn conversations

libs/partners/deepseek/langchain_deepseek/chat_models.py

@@ -261,21 +261,43 @@ def _get_request_payload(
         stop: list[str] | None = None,
         **kwargs: Any,
     ) -> dict:
+        # Get the original messages before conversion to extract reasoning_content
+        messages = self._convert_input(input_).to_messages()
+
+        # Build a map of message index to reasoning_content from additional_kwargs
+        # This is needed because the parent's _get_request_payload doesn't preserve
+        # reasoning_content in the converted message dicts
+        reasoning_content_map: dict[int, str | None] = {}
+        for i, msg in enumerate(messages):
+            if isinstance(msg, AIMessage):
+                reasoning_content = msg.additional_kwargs.get("reasoning_content")
+                if reasoning_content is not None:
+                    reasoning_content_map[i] = reasoning_content
+
         payload = super()._get_request_payload(input_, stop=stop, **kwargs)
-        for message in payload["messages"]:
+        for i, message in enumerate(payload["messages"]):
             if message["role"] == "tool" and isinstance(message["content"], list):
                 message["content"] = json.dumps(message["content"])
-            elif message["role"] == "assistant" and isinstance(
-                message["content"], list
-            ):
-                # DeepSeek API expects assistant content to be a string, not a list.
-                # Extract text blocks and join them, or use empty string if none exist.
-                text_parts = [
-                    block.get("text", "")
-                    for block in message["content"]
-                    if isinstance(block, dict) and block.get("type") == "text"
-                ]
-                message["content"] = "".join(text_parts) if text_parts else ""
+            elif message["role"] == "assistant":
+                # DeepSeek reasoner models require reasoning_content in assistant
+                # messages for multi-turn conversations. Add it from additional_kwargs
+                # if present. Fixes issue #34166.
+                if i in reasoning_content_map:
+                    message["reasoning_content"] = reasoning_content_map[i]
+                # Even if no reasoning_content was captured, DeepSeek API requires
+                # the field to be present (can be empty string) for reasoner models
+                elif "deepseek-reasoner" in self.model_name and "reasoning_content" not in message:
+                    message["reasoning_content"] = ""
+
+                if isinstance(message["content"], list):
+                    # DeepSeek API expects assistant content to be a string, not a list.
+                    # Extract text blocks and join them, or use empty string if none exist.
+                    text_parts = [
+                        block.get("text", "")
+                        for block in message["content"]
+                        if isinstance(block, dict) and block.get("type") == "text"
+                    ]
+                    message["content"] = "".join(text_parts) if text_parts else ""
         return payload
 
     def _create_chat_result(

libs/partners/deepseek/tests/unit_tests/test_chat_models.py

@@ -5,7 +5,7 @@
 from typing import Any, Literal
 from unittest.mock import MagicMock
 
-from langchain_core.messages import AIMessageChunk, ToolMessage
+from langchain_core.messages import AIMessage, AIMessageChunk, HumanMessage, ToolMessage
 from langchain_tests.unit_tests import ChatModelUnitTests
 from openai import BaseModel
 from openai.types.chat import ChatCompletionMessage
@@ -244,6 +244,80 @@ def test_get_request_payload(self) -> None:
         payload = chat_model._get_request_payload([tool_message])
         assert payload["messages"][0]["content"] == "test string"
 
+    def test_get_request_payload_preserves_reasoning_content(self) -> None:
+        """Test that reasoning_content from AIMessage additional_kwargs is preserved.
+
+        This is a regression test for issue #34166 where multi-turn conversations
+        with deepseek-reasoner failed because reasoning_content was not included
+        in assistant messages when resending the conversation history.
+        """
+        chat_model = ChatDeepSeek(
+            model="deepseek-reasoner", api_key=SecretStr("api_key")
+        )
+
+        # Simulate a multi-turn conversation where the first response included
+        # reasoning_content (stored in additional_kwargs)
+        messages = [
+            HumanMessage(content="What is 2 + 2?"),
+            AIMessage(
+                content="The answer is 4.",
+                additional_kwargs={"reasoning_content": "Let me think... 2 + 2 = 4"},
+            ),
+            HumanMessage(content="And what is 3 + 3?"),
+        ]
+
+        payload = chat_model._get_request_payload(messages)
+
+        # Verify the assistant message includes reasoning_content
+        assistant_message = payload["messages"][1]
+        assert assistant_message["role"] == "assistant"
+        assert assistant_message["content"] == "The answer is 4."
+        assert (
+            assistant_message["reasoning_content"] == "Let me think... 2 + 2 = 4"
+        ), "reasoning_content should be preserved from additional_kwargs"
+
+    def test_get_request_payload_adds_empty_reasoning_for_reasoner(self) -> None:
+        """Test that empty reasoning_content is added for reasoner models.
+
+        DeepSeek API requires the reasoning_content field to be present
+        in assistant messages when using reasoner models, even if empty.
+        """
+        chat_model = ChatDeepSeek(
+            model="deepseek-reasoner", api_key=SecretStr("api_key")
+        )
+
+        # AIMessage without reasoning_content
+        messages = [
+            HumanMessage(content="Hello"),
+            AIMessage(content="Hi there!"),
+            HumanMessage(content="How are you?"),
+        ]
+
+        payload = chat_model._get_request_payload(messages)
+
+        # Verify the assistant message has reasoning_content (empty string)
+        assistant_message = payload["messages"][1]
+        assert assistant_message["role"] == "assistant"
+        assert "reasoning_content" in assistant_message
+        assert assistant_message["reasoning_content"] == ""
+
+    def test_get_request_payload_no_reasoning_for_non_reasoner(self) -> None:
+        """Test that reasoning_content is not added for non-reasoner models."""
+        chat_model = ChatDeepSeek(model=MODEL_NAME, api_key=SecretStr("api_key"))
+
+        messages = [
+            HumanMessage(content="Hello"),
+            AIMessage(content="Hi there!"),
+            HumanMessage(content="How are you?"),
+        ]
+
+        payload = chat_model._get_request_payload(messages)
+
+        # Verify the assistant message does NOT have reasoning_content
+        assistant_message = payload["messages"][1]
+        assert assistant_message["role"] == "assistant"
+        assert "reasoning_content" not in assistant_message
+
 
 class SampleTool(PydanticBaseModel):
     """Sample tool schema for testing."""