fix llama_chat_format_single for mistral

common/common.cpp

@@ -2721,7 +2721,7 @@ std::string llama_chat_format_single(const struct llama_model * model,
         const llama_chat_msg & new_msg,
         bool add_ass) {
     std::ostringstream ss;
-    auto fmt_past_msg = llama_chat_apply_template(model, tmpl, past_msg, false);
+    auto fmt_past_msg = past_msg.empty() ? "" : llama_chat_apply_template(model, tmpl, past_msg, false);
     std::vector<llama_chat_msg> chat_new(past_msg);
     // if the past_msg ends with a newline, we must preserve it in the formatted version
     if (add_ass && !fmt_past_msg.empty() && fmt_past_msg.back() == '\n') {

examples/main/main.cpp

@@ -124,6 +124,7 @@ static std::string chat_add_and_format(struct llama_model * model, std::vector<l
     auto formatted = llama_chat_format_single(
         model, g_params->chat_template, chat_msgs, new_msg, role == "user");
     chat_msgs.push_back({role, content});
+    LOG("formatted: %s\n", formatted.c_str());
     return formatted;
 }
 

tests/test-chat-template.cpp

@@ -137,9 +137,25 @@ int main(void) {
         assert(output == expected);
     }
 
-    // test llama_chat_format_single
+
-    std::cout << "\n\n=== llama_chat_format_single ===\n\n";
+    // test llama_chat_format_single for system message
+    std::cout << "\n\n=== llama_chat_format_single (system message) ===\n\n";
     std::vector<llama_chat_msg> chat2;
+    llama_chat_msg sys_msg{"system", "You are a helpful assistant"};
+
+    auto fmt_sys = [&](std::string tmpl) {
+        auto output = llama_chat_format_single(nullptr, tmpl, chat2, sys_msg, false);
+        std::cout << "fmt_sys(" << tmpl << ")\n" << output << "\n-------------------------\n";
+        return output;
+    };
+    assert(fmt_sys("chatml") == "<|im_start|>system\nYou are a helpful assistant<|im_end|>\n");
+    assert(fmt_sys("llama2") == "[INST] You are a helpful assistant\n");
+    assert(fmt_sys("gemma")  == ""); // for gemma, system message is merged with user message
+    assert(fmt_sys("llama3") == "<|start_header_id|>system<|end_header_id|>\n\nYou are a helpful assistant<|eot_id|>");
+
+
+    // test llama_chat_format_single for user message
+    std::cout << "\n\n=== llama_chat_format_single (system message) ===\n\n";
     chat2.push_back({"system", "You are a helpful assistant"});
     chat2.push_back({"user", "Hello"});
     chat2.push_back({"assistant", "I am assistant"});