Enhance Mistral chat transformation to preserve content types

- Updated the `_add_reasoning_system_prompt_if_needed` method to maintain the original format of list content when prepending the reasoning prompt.
- Adjusted tests to verify that both string and list content types are correctly handled, ensuring the reasoning prompt is added without altering the content structure.

litellm/llms/mistral/mistral_chat_transformation.py

@@ -252,25 +252,18 @@ Then provide a clear, concise answer based on your reasoning."""
                     existing_content = msg.get("content", "")
                     reasoning_prompt = self._get_mistral_reasoning_system_prompt()
                     
-                    # Handle both string and list content - convert everything to string
-                    # since Mistral API expects string content
+                    # Handle both string and list content, preserving original format
                     if isinstance(existing_content, str):
                         # String content - prepend reasoning prompt
-                        content_str = existing_content
+                        new_content = f"{reasoning_prompt}\n\n{existing_content}"
                     elif isinstance(existing_content, list):
-                        # List content - convert to string first
-                        content_str = ""
-                        for item in existing_content:
-                            if isinstance(item, dict) and item.get("type") == "text":
-                                content_str += item.get("text", "")
-                            else:
-                                content_str += str(item)
+                        # List content - prepend reasoning prompt as text block
+                        new_content = [
+                            {"type": "text", "text": reasoning_prompt + "\n\n"}
+                        ] + existing_content
                     else:
                         # Fallback for any other type - convert to string
-                        content_str = str(existing_content)
-                    
-                    # Create the final content with reasoning prompt
-                    new_content = f"{reasoning_prompt}\n\n{content_str}"
+                        new_content = f"{reasoning_prompt}\n\n{str(existing_content)}"
                     
                     messages[i] = cast(AllMessageValues, {
                         **msg,

tests/test_litellm/llms/mistral/test_mistral_chat_transformation.py

@@ -163,23 +163,56 @@ class TestMistralReasoningSupport:
         
         result = mistral_config._add_reasoning_system_prompt_if_needed(messages, optional_params)
         
-        # Should modify existing system message with list content converted to string
+        # Should modify existing system message preserving list format
         assert len(result) == 2
         assert result[0]["role"] == "system"
-        assert isinstance(result[0]["content"], str)
+        assert isinstance(result[0]["content"], list)
         
-        # Should contain the reasoning prompt
-        assert "<think>" in result[0]["content"]
+        # First item should be the reasoning prompt
+        assert result[0]["content"][0]["type"] == "text"
+        assert "<think>" in result[0]["content"][0]["text"]
         
-        # Original content should be preserved (converted from list to string)
-        assert "You are a helpful assistant." in result[0]["content"]
-        assert "You always provide detailed explanations." in result[0]["content"]
+        # Original content should be preserved
+        assert "You are a helpful assistant." in result[0]["content"][1]["text"]
+        assert "You always provide detailed explanations." in result[0]["content"][2]["text"]
         
         assert result[1]["role"] == "user"
         
         # Should remove the internal flag
         assert "_add_reasoning_prompt" not in optional_params
 
+    def test_add_reasoning_system_prompt_preserves_content_types(self):
+        """Test that reasoning prompt preserves original content types (string vs list)."""
+        mistral_config = MistralConfig()
+        
+        # Test with string content
+        string_messages = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "Hello"}
+        ]
+        string_params = {"_add_reasoning_prompt": True}
+        
+        string_result = mistral_config._add_reasoning_system_prompt_if_needed(string_messages, string_params)
+        assert isinstance(string_result[0]["content"], str)
+        assert "<think>" in string_result[0]["content"]
+        assert "You are helpful." in string_result[0]["content"]
+        
+        # Test with list content
+        list_messages = [
+            {
+                "role": "system", 
+                "content": [{"type": "text", "text": "You are helpful."}]
+            },
+            {"role": "user", "content": "Hello"}
+        ]
+        list_params = {"_add_reasoning_prompt": True}
+        
+        list_result = mistral_config._add_reasoning_system_prompt_if_needed(list_messages, list_params)
+        assert isinstance(list_result[0]["content"], list)
+        assert list_result[0]["content"][0]["type"] == "text"
+        assert "<think>" in list_result[0]["content"][0]["text"]
+        assert "You are helpful." in list_result[0]["content"][1]["text"]
+
     def test_add_reasoning_system_prompt_no_flag(self):
         """Test that no modification happens when _add_reasoning_prompt flag is not set."""
         mistral_config = MistralConfig()