Optimize response string construction

Use list accumulation and join in get_response_string to avoid O(n^2) string concatenation and add a brief comment explaining the performance rationale.

litellm/utils.py

@@ -4437,17 +4437,20 @@ def get_response_string(response_obj: Union[ModelResponse, ModelResponseStream])
         responses_api_response = getattr(response_obj, "response", None)
         if responses_api_response and hasattr(responses_api_response, "output"):
             output_list = responses_api_response.output
-            response_str = ""
+            # Use list accumulation to avoid O(n^2) string concatenation:
+            # repeatedly doing `response_str += part` copies the full string each time
+            # because Python strings are immutable, so total work grows with n^2.
+            response_output_parts: List[str] = []
             for output_item in output_list:
                 # Handle output items with content array
                 if hasattr(output_item, "content"):
                     for content_part in output_item.content:
                         if hasattr(content_part, "text"):
-                            response_str += content_part.text
+                            response_output_parts.append(content_part.text)
                 # Handle output items with direct text field
                 elif hasattr(output_item, "text"):
-                    response_str += output_item.text
-            return response_str
+                    response_output_parts.append(output_item.text)
+            return "".join(response_output_parts)
 
     # Handle Responses API text delta events
     if hasattr(response_obj, "type") and hasattr(response_obj, "delta"):
@@ -4461,16 +4464,17 @@ def get_response_string(response_obj: Union[ModelResponse, ModelResponseStream])
         response_obj.choices
     )
 
-    response_str = ""
+    # Use list accumulation to avoid O(n^2) string concatenation across choices
+    response_parts: List[str] = []
     for choice in _choices:
         if isinstance(choice, Choices):
             if choice.message.content is not None:
-                response_str += choice.message.content
+                response_parts.append(str(choice.message.content))
         elif isinstance(choice, StreamingChoices):
             if choice.delta.content is not None:
-                response_str += choice.delta.content
+                response_parts.append(str(choice.delta.content))
 
-    return response_str
+    return "".join(response_parts)
 
 
 def get_api_key(llm_provider: str, dynamic_api_key: Optional[str]):