server: use random media marker (#21962)

* server: use random media marker

* nits

* remove legacy <__image__> token

* revert special char in random

tools/server/server-common.cpp

@@ -84,6 +84,14 @@ std::string gen_tool_call_id() {
     return random_string();
 }
 
+static std::string media_marker = "";
+const char * get_media_marker() {
+    if (media_marker.empty()) {
+        media_marker = "<__media_" + random_string() + "__>";
+    }
+    return media_marker.c_str();
+}
+
 //
 // lora utils
 //
@@ -975,7 +983,7 @@ json oaicompat_chat_params_parse(
                 handle_media(out_files, image_url, opt.media_path);
 
                 p["type"] = "media_marker";
-                p["text"] = mtmd_default_marker();
+                p["text"] = get_media_marker();
                 p.erase("image_url");
 
             } else if (type == "input_audio") {
@@ -996,7 +1004,7 @@ json oaicompat_chat_params_parse(
                 // TODO: add audio_url support by reusing handle_media()
 
                 p["type"] = "media_marker";
-                p["text"] = mtmd_default_marker();
+                p["text"] = get_media_marker();
                 p.erase("input_audio");
 
             } else if (type != "text") {
@@ -1460,7 +1468,7 @@ json convert_transcriptions_to_chatcmpl(
     if (!language.empty()) {
         prompt += string_format(" (language: %s)", language.c_str());
     }
-    prompt += mtmd_default_marker();
+    prompt += get_media_marker();
 
     json chatcmpl_body = inp_body; // copy all fields
     chatcmpl_body["messages"] = json::array({

tools/server/server-common.h

@@ -92,6 +92,9 @@ std::string random_string();
 std::string gen_chatcmplid();
 std::string gen_tool_call_id();
 
+// get a random marker; note: each time the server restarts, the marker will be different
+const char * get_media_marker();
+
 //
 // lora utils
 //

tools/server/server-context.cpp

@@ -708,6 +708,7 @@ private:
             mparams.warmup           = params_base.warmup;
             mparams.image_min_tokens = params_base.image_min_tokens;
             mparams.image_max_tokens = params_base.image_max_tokens;
+            mparams.media_marker     = get_media_marker();
 
             mctx = mtmd_init_from_file(mmproj_path.c_str(), model, mparams);
             if (mctx == nullptr) {