Prefilling assistant message in openai compatible API

Author: matteo

examples/server/utils.hpp

@@ -642,9 +642,26 @@ static json oaicompat_completion_params_parse(
         throw std::runtime_error("Cannot use custom grammar constraints with tools.");
     }
 
+    /* Prefill assistant message support */
+    bool prefill_assistant_message = inputs.messages.size() > 0 && inputs.messages[inputs.messages.size()-1].role == "assistant";
+    common_chat_msg last_message;
+    if (prefill_assistant_message)
+    {
+    	last_message = inputs.messages.back();
+        inputs.messages.pop_back();
+        inputs.extract_reasoning = false;
+        inputs.add_generation_prompt = true;
+    }
+
     // Apply chat template to the list of messages
     auto chat_params = common_chat_templates_apply(tmpls, inputs);
 
+    /* Append assistant prefilled message */
+    if (prefill_assistant_message)
+    {
+         chat_params.prompt += last_message.content;
+    }
+
     llama_params["chat_format"]      = static_cast<int>(chat_params.format);
     llama_params["prompt"]           = chat_params.prompt;
     if (!chat_params.grammar.empty()) {