Update lmdeploy/serve/openai/api_server.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: windreamer

lmdeploy/serve/openai/api_server.py

@@ -409,10 +409,11 @@ async def chat_completions_v1(request: ChatCompletionRequest, raw_request: Reque
             schema_json = json.dumps(response_format, ensure_ascii=False)
             format_section = f'\n\n# Response Formats\n\n{schema_json}'
             try:
-                messages = request.messages
-                if isinstance(messages, str):
-                    messages += format_section
+                if isinstance(request.messages, str):
+                    # For string prompts, append the format section directly to request.messages
+                    request.messages += format_section
                 else:
+                    messages = request.messages
                     for msg in messages:
                         if msg['role'] == 'system':
                             msg['content'] += format_section