[Bugfix][Qwen3-TTS] Seed residual MTP sampling (#4869)

Signed-off-by: Sy03 <1370724210@qq.com>
Co-authored-by: Yueqian Lin <linyueqian@outlook.com>

tests/entrypoints/openai_api/test_serving_speech.py

@@ -3344,6 +3344,27 @@ class TestTTSAsyncOffloading:
         qwen3_tts_server._build_tts_params.assert_called_once()
         qwen3_tts_server._estimate_prompt_len_async.assert_awaited_once()
 
+    def test_prepare_speech_generation_qwen3_default_seed_sets_tts_local_seed(
+        self, qwen3_tts_server, mocker: MockerFixture
+    ):
+        """Deploy default seed should seed Qwen3 TTS residual MTP sampling."""
+        qwen3_tts_server.engine_client.default_sampling_params_list = [
+            SimpleNamespace(max_tokens=2048, seed=42, extra_args=None)
+        ]
+        qwen3_tts_server._validate_tts_request = mocker.MagicMock(return_value=None)
+        qwen3_tts_server._build_tts_params = mocker.MagicMock(
+            return_value={"text": ["hello"], "task_type": ["CustomVoice"], "speaker": ["Vivian"]}
+        )
+        qwen3_tts_server._estimate_prompt_len_async = mocker.AsyncMock(return_value=512)
+        request = OpenAICreateSpeechRequest(input="hello")
+
+        asyncio.run(qwen3_tts_server._prepare_speech_generation(request))
+
+        stage0_params = qwen3_tts_server.engine_client.generate.call_args.kwargs["sampling_params_list"][0]
+        assert stage0_params.seed == 42
+        assert stage0_params.extra_args["tts_local_seed"] == 42
+        assert qwen3_tts_server.engine_client.default_sampling_params_list[0].extra_args is None
+
     def test_prepare_speech_generation_uses_adapter_model_type_label(
         self,
         voxtral_server,