[Bugfix] Fix Qwen3-TTS no-async-chunk payload splitting (#4870)

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

tests/entrypoints/openai_api/test_serving_speech.py

@@ -3450,6 +3450,48 @@ class TestTTSAsyncOffloading:
 
         mock_coerce.assert_called_once_with(qwen3_tts_server.engine_client.default_sampling_params_list, True)
 
+    def test_prepare_speech_generation_no_async_chunk_stream_uses_final_only(
+        self, qwen3_tts_server, mocker: MockerFixture
+    ):
+        """Full-payload TTS streaming should not request delta multimodal outputs."""
+        qwen3_tts_server.engine_client.model_config.async_chunk = False
+        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)
+        mock_coerce = mocker.patch(
+            "vllm_omni.entrypoints.openai.serving_speech.coerce_param_message_types",
+            return_value=qwen3_tts_server.engine_client.default_sampling_params_list,
+        )
+        request = OpenAICreateSpeechRequest(input="hello", stream_format="audio", response_format="pcm")
+
+        asyncio.run(qwen3_tts_server._prepare_speech_generation(request))
+
+        mock_coerce.assert_called_once_with(qwen3_tts_server.engine_client.default_sampling_params_list, False)
+
+    def test_prepare_speech_generation_no_async_chunk_stream_keeps_delta_for_non_qwen3(
+        self, voxtral_server, mocker: MockerFixture
+    ):
+        """FINAL_ONLY streaming for async_chunk=False is scoped to qwen3_tts only."""
+        voxtral_server.engine_client.model_config.async_chunk = False
+        mocker.patch.object(voxtral_server._get_tts_adapter(), "validate", return_value=None)
+        voxtral_server._build_voxtral_prompt_async = mocker.AsyncMock(
+            return_value={
+                "prompt_token_ids": [1, 2, 3],
+                "additional_information": {"voice": ["test"]},
+            }
+        )
+        mock_coerce = mocker.patch(
+            "vllm_omni.entrypoints.openai.serving_speech.coerce_param_message_types",
+            return_value=voxtral_server.engine_client.default_sampling_params_list,
+        )
+        request = OpenAICreateSpeechRequest(input="hello", voice="test", stream_format="audio", response_format="pcm")
+
+        asyncio.run(voxtral_server._prepare_speech_generation(request))
+
+        mock_coerce.assert_called_once_with(voxtral_server.engine_client.default_sampling_params_list, True)
+
     def test_prepare_speech_generation_qwen3_voicedesign_non_streaming_mode_false(
         self, qwen3_tts_server, mocker: MockerFixture
     ):