merge checks into prefill code

Author: matteo

examples/server/utils.hpp

@@ -642,18 +642,18 @@ static json oaicompat_completion_params_parse(
         throw std::runtime_error("Cannot use custom grammar constraints with tools.");
     }
 
-    /* sanity check, max one assistant message at the end of the list */
-    bool last_message_is_assistant = inputs.messages.size() > 0 && inputs.messages.back().role == "assistant";
-    if (last_message_is_assistant && inputs.messages.size() >= 2 && inputs.messages[inputs.messages.size()-2].role == "assistant") {
-        throw std::runtime_error("Cannot have 2 or more assistant messages at the end of the list.");
-    }
-
     /* Prefill assistant message support */
-    bool prefill_assistant_message = last_message_is_assistant;
+    bool prefill_assistant_message = !inputs.messages.empty() && inputs.messages.back().role == "assistant";
     common_chat_msg last_message;
     if (prefill_assistant_message) {
         last_message = inputs.messages.back();
         inputs.messages.pop_back();
+
+        /* sanity check, max one assistant message at the end of the list */
+        if (!inputs.messages.empty() && inputs.messages.back().role == "assistant"){
+            throw std::runtime_error("Cannot have 2 or more assistant messages at the end of the list.");
+        }
+
         inputs.extract_reasoning = false;
         inputs.add_generation_prompt = true;
     }