fix: auto-fill reasoning_content for moonshot kimi reasoning models

litellm/llms/moonshot/chat/transformation.py

@@ -2,13 +2,15 @@
 Translates from OpenAI's `/v1/chat/completions` to Moonshot AI's `/v1/chat/completions`
 """
 
-from typing import Any, Coroutine, List, Literal, Optional, Tuple, Union, overload
+from typing import Any, Coroutine, List, Literal, Optional, Tuple, Union, cast, overload
 
+import litellm
 from litellm.litellm_core_utils.prompt_templates.common_utils import (
     handle_messages_with_content_list_to_str_conversion,
 )
 from litellm.secret_managers.main import get_secret_str
 from litellm.types.llms.openai import AllMessageValues
+from litellm.utils import supports_reasoning
 
 from ...openai.chat.gpt_transformation import OpenAIGPTConfig
 
@@ -17,17 +19,15 @@ class MoonshotChatConfig(OpenAIGPTConfig):
     @overload
     def _transform_messages(
         self, messages: List[AllMessageValues], model: str, is_async: Literal[True]
-    ) -> Coroutine[Any, Any, List[AllMessageValues]]:
-        ...
+    ) -> Coroutine[Any, Any, List[AllMessageValues]]: ...
 
     @overload
     def _transform_messages(
         self,
         messages: List[AllMessageValues],
         model: str,
         is_async: Literal[False] = False,
-    ) -> List[AllMessageValues]:
-        ...
+    ) -> List[AllMessageValues]: ...
 
     def _transform_messages(
         self, messages: List[AllMessageValues], model: str, is_async: bool = False
@@ -53,22 +53,14 @@ def _transform_messages(
             messages = handle_messages_with_content_list_to_str_conversion(messages)
 
         if is_async:
-            return super()._transform_messages(
-                messages=messages, model=model, is_async=True
-            )
+            return super()._transform_messages(messages=messages, model=model, is_async=True)
         else:
-            return super()._transform_messages(
-                messages=messages, model=model, is_async=False
-            )
+            return super()._transform_messages(messages=messages, model=model, is_async=False)
 
     def _get_openai_compatible_provider_info(
         self, api_base: Optional[str], api_key: Optional[str]
     ) -> Tuple[Optional[str], Optional[str]]:
-        api_base = (
-            api_base
-            or get_secret_str("MOONSHOT_API_BASE")
-            or "https://api.moonshot.ai/v1"
-        )  # type: ignore
+        api_base = api_base or get_secret_str("MOONSHOT_API_BASE") or "https://api.moonshot.ai/v1"  # type: ignore
         dynamic_api_key = api_key or get_secret_str("MOONSHOT_API_KEY")
         return api_base, dynamic_api_key
 
@@ -149,6 +141,48 @@ def map_openai_params(
                 optional_params["temperature"] = 0.3
         return optional_params
 
+    def fill_reasoning_content(self, messages: List[AllMessageValues]) -> List[AllMessageValues]:
+        """
+        Moonshot reasoning models require `reasoning_content` on every assistant
+        message that contains tool_calls (multi-turn tool-calling flows).
+
+        For each such message that is missing the field:
+          1. Promote provider_specific_fields["reasoning_content"] if present and non-empty
+             (this is where LiteLLM stores it from a previous response)
+          2. Otherwise inject a single space — the minimum value the API accepts
+        Messages that already carry the field, or are not assistant/tool-call messages,
+        are appended as-is (no copy made).
+        """
+        result: List[AllMessageValues] = []
+        for msg in messages:
+            if (
+                msg.get("role") == "assistant"
+                and msg.get("tool_calls")
+                and "reasoning_content" not in msg
+            ):
+                patched = dict(cast(dict, msg))
+                provider_fields = patched.get("provider_specific_fields") or {}
+                stored = provider_fields.get("reasoning_content")
+                if stored:
+                    patched["reasoning_content"] = stored
+                    # Remove the promoted key from provider_specific_fields to
+                    # avoid sending the value twice in the serialised request body
+                    cleaned_provider_fields = dict(provider_fields)
+                    cleaned_provider_fields.pop("reasoning_content", None)
+                    patched["provider_specific_fields"] = cleaned_provider_fields
+                else:
+                    litellm.verbose_logger.warning(
+                        "Moonshot reasoning model: assistant tool-call message is missing "
+                        "`reasoning_content`. Injecting a placeholder to satisfy API validation. "
+                        "For best results, preserve `reasoning_content` from the original "
+                        "assistant response when building multi-turn conversation history."
+                    )
+                    patched["reasoning_content"] = " "
+                result.append(cast(AllMessageValues, patched))
+            else:
+                result.append(msg)
+        return result
+
     def transform_request(
         self,
         model: str,
@@ -169,6 +203,10 @@ def transform_request(
                 optional_params=optional_params,
             )
 
+        # Moonshot reasoning models: fill in reasoning_content before the API call
+        if supports_reasoning(model=model, custom_llm_provider="moonshot"):
+            messages = self.fill_reasoning_content(messages)
+
         # Call parent transform_request which handles _transform_messages
         return super().transform_request(
             model=model,

litellm/model_prices_and_context_window_backup.json

@@ -22083,6 +22083,7 @@
         "output_cost_per_token": 3e-06,
         "source": "https://platform.moonshot.ai/docs/guide/kimi-k2-5-quickstart",
         "supports_function_calling": true,
+        "supports_reasoning": true,
         "supports_tool_choice": true,
         "supports_video_input": true,
         "supports_vision": true
@@ -22166,6 +22167,7 @@
         "output_cost_per_token": 2.5e-06,
         "source": "https://platform.moonshot.ai/docs/pricing/chat#generation-model-kimi-k2",
         "supports_function_calling": true,
+        "supports_reasoning": true,
         "supports_tool_choice": true,
         "supports_web_search": true
     },
@@ -22180,6 +22182,7 @@
         "output_cost_per_token": 8e-06,
         "source": "https://platform.moonshot.ai/docs/pricing/chat#generation-model-kimi-k2",
         "supports_function_calling": true,
+        "supports_reasoning": true,
         "supports_tool_choice": true,
         "supports_web_search": true
     },

model_prices_and_context_window.json

@@ -22083,6 +22083,7 @@
         "output_cost_per_token": 3e-06,
         "source": "https://platform.moonshot.ai/docs/guide/kimi-k2-5-quickstart",
         "supports_function_calling": true,
+        "supports_reasoning": true,
         "supports_tool_choice": true,
         "supports_video_input": true,
         "supports_vision": true
@@ -22166,6 +22167,7 @@
         "output_cost_per_token": 2.5e-06,
         "source": "https://platform.moonshot.ai/docs/pricing/chat#generation-model-kimi-k2",
         "supports_function_calling": true,
+        "supports_reasoning": true,
         "supports_tool_choice": true,
         "supports_web_search": true
     },
@@ -22180,6 +22182,7 @@
         "output_cost_per_token": 8e-06,
         "source": "https://platform.moonshot.ai/docs/pricing/chat#generation-model-kimi-k2",
         "supports_function_calling": true,
+        "supports_reasoning": true,
         "supports_tool_choice": true,
         "supports_web_search": true
     },

tests/test_litellm/llms/moonshot/test_moonshot_chat_transformation.py

@@ -7,6 +7,7 @@
 
 import os
 import sys
+from unittest.mock import patch
 
 sys.path.insert(
     0, os.path.abspath("../../../../..")
@@ -404,4 +405,149 @@ def test_transform_messages_flattens_text_only_content(self):
 
         # Content should be flattened to a plain string
         assert isinstance(result["messages"][0]["content"], str)
-        assert result["messages"][0]["content"] == "Hello, how are you?"
+        assert result["messages"][0]["content"] == "Hello, how are you?"
+
+    # ------------------------------------------------------------------ #
+    # Tests for fill_reasoning_content                                     #
+    # ------------------------------------------------------------------ #
+
+    def test_reasoning_content_space_injected_when_absent(self):
+        """Assistant tool-call message with no reasoning_content gets a space injected."""
+        config = MoonshotChatConfig()
+
+        messages = [
+            {"role": "user", "content": "What's the weather?"},
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {"id": "call_1", "type": "function", "function": {"name": "get_weather", "arguments": "{}"}}
+                ],
+            },
+            {"role": "tool", "tool_call_id": "call_1", "content": "Sunny, 22°C"},
+        ]
+
+        result = config.fill_reasoning_content(messages)
+
+        assert result[1].get("reasoning_content") == " "
+        # Non-assistant messages are untouched
+        assert "reasoning_content" not in result[0]
+        assert "reasoning_content" not in result[2]
+
+    def test_empty_tool_calls_list_not_injected(self):
+        """Assistant message with tool_calls: [] should not get reasoning_content injected."""
+        config = MoonshotChatConfig()
+
+        original_msg = {
+            "role": "assistant",
+            "content": "Here is the answer.",
+            "tool_calls": [],
+        }
+        messages = [original_msg]
+
+        result = config.fill_reasoning_content(messages)
+
+        assert "reasoning_content" not in result[0]
+        assert result[0] is original_msg
+
+    def test_existing_reasoning_content_not_overwritten(self):
+        """Message that already has reasoning_content is passed through unchanged."""
+        config = MoonshotChatConfig()
+
+        original_msg = {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {"id": "call_1", "type": "function", "function": {"name": "fn", "arguments": "{}"}}
+            ],
+            "reasoning_content": "<actual thinking>",
+        }
+        messages = [original_msg]
+
+        result = config.fill_reasoning_content(messages)
+
+        assert result[0].get("reasoning_content") == "<actual thinking>"
+        # Same object — no copy was made
+        assert result[0] is original_msg
+
+    def test_provider_specific_fields_reasoning_content_promoted(self):
+        """reasoning_content stored in provider_specific_fields is promoted to top level."""
+        config = MoonshotChatConfig()
+
+        messages = [
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {"id": "call_1", "type": "function", "function": {"name": "fn", "arguments": "{}"}}
+                ],
+                "provider_specific_fields": {"reasoning_content": "stored thinking"},
+            }
+        ]
+
+        result = config.fill_reasoning_content(messages)
+
+        assert result[0].get("reasoning_content") == "stored thinking"
+        # The promoted key must be removed from provider_specific_fields to
+        # avoid sending the value twice in the serialised request body
+        assert "reasoning_content" not in (result[0].get("provider_specific_fields") or {})
+
+    def test_reasoning_model_fill_called_from_transform_request(self):
+        """transform_request injects reasoning_content end-to-end for reasoning models."""
+        config = MoonshotChatConfig()
+
+        messages = [
+            {"role": "user", "content": "Call a tool"},
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {"id": "call_1", "type": "function", "function": {"name": "fn", "arguments": "{}"}}
+                ],
+            },
+        ]
+
+        with patch(
+            "litellm.llms.moonshot.chat.transformation.supports_reasoning",
+            return_value=True,
+        ):
+            result = config.transform_request(
+                model="kimi-k2-thinking",
+                messages=messages,
+                optional_params={},
+                litellm_params={},
+                headers={},
+            )
+
+        assert result["messages"][1].get("reasoning_content") == " "
+
+    def test_non_reasoning_model_messages_untouched(self):
+        """For non-reasoning models, transform_request leaves messages unchanged."""
+        config = MoonshotChatConfig()
+
+        messages = [
+            {"role": "user", "content": "Hello"},
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {"id": "call_1", "type": "function", "function": {"name": "fn", "arguments": "{}"}}
+                ],
+            },
+        ]
+
+        with patch(
+            "litellm.llms.moonshot.chat.transformation.supports_reasoning",
+            return_value=False,
+        ):
+            result = config.transform_request(
+                model="moonshot-v1-8k",
+                messages=messages,
+                optional_params={},
+                litellm_params={},
+                headers={},
+            )
+
+        # reasoning_content must not have been injected
+        for msg in result["messages"]:
+            assert "reasoning_content" not in msg