Refactor Mistral chat transformation to handle list content

- Updated the `_add_reasoning_system_prompt_if_needed` method to convert list content to strings before prepending the reasoning prompt.
- Adjusted tests to verify that system messages with list content are correctly transformed into strings, ensuring original content is preserved.

litellm/llms/mistral/mistral_chat_transformation.py

@@ -252,18 +252,25 @@ 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
+                    # Handle both string and list content - convert everything to string
+                    # since Mistral API expects string content
                     if isinstance(existing_content, str):
                         # String content - prepend reasoning prompt
-                        new_content = f"{reasoning_prompt}\n\n{existing_content}"
+                        content_str = existing_content
                     elif isinstance(existing_content, list):
-                        # List content - prepend reasoning prompt as text block
-                        new_content = [
-                            {"type": "text", "text": reasoning_prompt + "\n\n"}
-                        ] + existing_content
+                        # 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)
                     else:
                         # Fallback for any other type - convert to string
-                        new_content = f"{reasoning_prompt}\n\n{str(existing_content)}"
+                        content_str = str(existing_content)
+                    
+                    # Create the final content with reasoning prompt
+                    new_content = f"{reasoning_prompt}\n\n{content_str}"
                     
                     messages[i] = cast(AllMessageValues, {
                         **msg,

tests/test_litellm/llms/mistral/test_mistral_chat_transformation.py

@@ -163,18 +163,17 @@ class TestMistralReasoningSupport:
         
         result = mistral_config._add_reasoning_system_prompt_if_needed(messages, optional_params)
         
-        # Should modify existing system message with list content
+        # Should modify existing system message with list content converted to string
         assert len(result) == 2
         assert result[0]["role"] == "system"
-        assert isinstance(result[0]["content"], list)
+        assert isinstance(result[0]["content"], str)
         
-        # First item should be the reasoning prompt
-        assert result[0]["content"][0]["type"] == "text"
-        assert "<think>" in result[0]["content"][0]["text"]
+        # Should contain the reasoning prompt
+        assert "<think>" 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"]
+        # 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"]
         
         assert result[1]["role"] == "user"