tts: default language to auto, NULL lang selects auto, reject NULL text

src/pipeline-tts.cpp

@@ -439,8 +439,12 @@ qt_status pipeline_tts_synthesize(PipelineTTS *                pt,
     TtsPerf perf = {};
     Timer   t_total;
 
+    // NULL lang selects automatic language: the prompt carries no
+    // language id and the model infers it from the text.
+    const char * lang = params->lang ? params->lang : "auto";
+
     Timer t_build;
-    if (!prompt_builder_build(pt, tok, params->text, params->lang, instruct, speaker, ref_spk_emb_ptr, ref_text,
+    if (!prompt_builder_build(pt, tok, params->text, lang, instruct, speaker, ref_spk_emb_ptr, ref_text,
                               ref_codes_T > 0 ? ref_codes.data() : NULL, ref_codes_T, &prompt)) {
         return QT_STATUS_GENERATE_FAILED;
     }

src/qwen.cpp

@@ -313,6 +313,14 @@ enum qt_status qt_synthesize(struct qt_context * q, const struct qt_tts_params *
         return QT_STATUS_INVALID_PARAMS;
     }
 
+    if (!params->text || !params->text[0]) {
+        qt_set_error("qt_synthesize: params->text is NULL or empty");
+        if (out) {
+            qt_audio_free(out);
+        }
+        return QT_STATUS_INVALID_PARAMS;
+    }
+
     // Mode validation. Mirrors the upstream Python which raises
     // ValueError when generate_voice_design is called on a non
     // voice_design model and the same shape applies to

src/qwen.h

@@ -197,8 +197,10 @@ QT_API void qt_log_set(qt_log_cb cb, void * user_data);
 struct qt_tts_params {
     int abi_version;
 
-    // Input text and language hint. lang accepts the upstream
-    // qwen3-tts language names ("english", "chinese", "auto", ...).
+    // Input text and language hint. text is required and non empty.
+    // lang accepts the upstream qwen3-tts language names ("english",
+    // "chinese", "auto", ...). NULL selects "auto": the prompt carries
+    // no language id and the model infers it from the text.
     // instruct is the style instruction string; required for
     // voice_design, optional for custom_voice, rejected for base.
     // speaker is the named speaker for custom_voice models, rejected