[FIX] Preserve YAML default stop words when request sends empty list (#2855)

Signed-off-by: Mike_Qiu <qiudayu.qdy@antgroup.com>
Co-authored-by: Mike_Qiu <qiudayu.qdy@antgroup.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/entrypoints/openai_api/test_serving_chat_sampling_params.py

@@ -284,6 +284,185 @@ def test_apply_request_overrides_applies_values(serving_chat, mock_request, defa
     assert result.top_k == 1  # YAML custom param preserved
 
 
+# =============================================================================
+# Tests for empty-list handling in _apply_request_overrides
+# =============================================================================
+
+
+def test_apply_overrides_empty_stop_list_preserves_default(serving_chat, mocker):
+    """Test that request.stop=[] does NOT override YAML default stop words."""
+    default_params = SamplingParams(temperature=0.5, stop=["<|im_end|>"])
+    request = mocker.MagicMock()
+    request.temperature = None
+    request.top_p = None
+    request.top_k = None
+    request.max_tokens = None
+    request.min_tokens = None
+    request.seed = None
+    request.ignore_eos = None
+    request.stop = []  # empty list — should be treated as "not set"
+    request.stop_token_ids = None
+    request.frequency_penalty = None
+    request.presence_penalty = None
+
+    result = serving_chat._apply_request_overrides(default_params, request)
+
+    assert result.stop == ["<|im_end|>"]  # YAML default preserved
+
+
+def test_apply_overrides_nonempty_stop_list_overrides_default(serving_chat, mocker):
+    """Test that request.stop=["\\n"] overrides YAML default stop words."""
+    default_params = SamplingParams(temperature=0.5, stop=["<|im_end|>"])
+    request = mocker.MagicMock()
+    request.temperature = None
+    request.top_p = None
+    request.top_k = None
+    request.max_tokens = None
+    request.min_tokens = None
+    request.seed = None
+    request.ignore_eos = None
+    request.stop = ["\n"]  # non-empty list — should override
+    request.stop_token_ids = None
+    request.frequency_penalty = None
+    request.presence_penalty = None
+
+    result = serving_chat._apply_request_overrides(default_params, request)
+
+    assert result.stop == ["\n"]  # Overridden by request
+
+
+def test_apply_overrides_empty_stop_token_ids_preserves_default(serving_chat, mocker):
+    """Test that request.stop_token_ids=[] does NOT override YAML default."""
+    default_params = SamplingParams(temperature=0.5, stop_token_ids=[2, 3])
+    request = mocker.MagicMock()
+    request.temperature = None
+    request.top_p = None
+    request.top_k = None
+    request.max_tokens = None
+    request.min_tokens = None
+    request.seed = None
+    request.ignore_eos = None
+    request.stop = None
+    request.stop_token_ids = []  # empty list — should be treated as "not set"
+    request.frequency_penalty = None
+    request.presence_penalty = None
+
+    result = serving_chat._apply_request_overrides(default_params, request)
+
+    assert result.stop_token_ids == [2, 3]  # YAML default preserved
+
+
+def test_apply_overrides_nonempty_stop_token_ids_overrides_default(serving_chat, mocker):
+    """Test that request.stop_token_ids=[100] overrides YAML default."""
+    default_params = SamplingParams(temperature=0.5, stop_token_ids=[2, 3])
+    request = mocker.MagicMock()
+    request.temperature = None
+    request.top_p = None
+    request.top_k = None
+    request.max_tokens = None
+    request.min_tokens = None
+    request.seed = None
+    request.ignore_eos = None
+    request.stop = None
+    request.stop_token_ids = [100]  # non-empty list — should override
+    request.frequency_penalty = None
+    request.presence_penalty = None
+
+    result = serving_chat._apply_request_overrides(default_params, request)
+
+    assert result.stop_token_ids == [100]  # Overridden by request
+
+
+def test_apply_overrides_mixed_empty_and_nonempty_lists(serving_chat, mocker):
+    """Test mixing empty and non-empty list fields with scalar fields."""
+    default_params = SamplingParams(
+        temperature=0.4,
+        stop=["<|end|>"],
+        stop_token_ids=[2],
+    )
+    request = mocker.MagicMock()
+    request.temperature = 0.9
+    request.top_p = None
+    request.top_k = None
+    request.max_tokens = None
+    request.min_tokens = None
+    request.seed = None
+    request.ignore_eos = None
+    request.stop = []  # empty — should NOT override
+    request.stop_token_ids = [100, 200]  # non-empty — SHOULD override
+    request.frequency_penalty = None
+    request.presence_penalty = None
+
+    result = serving_chat._apply_request_overrides(default_params, request)
+
+    assert result.temperature == 0.9  # Scalar override works
+    assert result.stop == ["<|end|>"]  # Empty list did NOT override
+    assert result.stop_token_ids == [100, 200]  # Non-empty list DID override
+
+
+def test_apply_overrides_none_scalar_still_preserves_default(serving_chat, mocker):
+    """Regression: ensure None scalar values still don't override defaults."""
+    default_params = SamplingParams(temperature=0.5, max_tokens=100, seed=42)
+    request = mocker.MagicMock()
+    request.temperature = None
+    request.top_p = None
+    request.top_k = None
+    request.max_tokens = None
+    request.min_tokens = None
+    request.seed = None
+    request.ignore_eos = None
+    request.stop = None
+    request.stop_token_ids = None
+    request.frequency_penalty = None
+    request.presence_penalty = None
+
+    result = serving_chat._apply_request_overrides(default_params, request)
+
+    assert result.temperature == 0.5
+    assert result.max_tokens == 100
+    assert result.seed == 42
+
+
+def test_apply_overrides_both_lists_empty_preserves_defaults(serving_chat, mocker):
+    """Test that both stop=[] and stop_token_ids=[] preserve YAML defaults."""
+    default_params = SamplingParams(
+        temperature=0.5,
+        stop=["<|end|>", "\\n"],
+        stop_token_ids=[2, 32000],
+    )
+    request = mocker.MagicMock()
+    request.temperature = None
+    request.top_p = None
+    request.top_k = None
+    request.max_tokens = None
+    request.min_tokens = None
+    request.seed = None
+    request.ignore_eos = None
+    request.stop = []
+    request.stop_token_ids = []
+    request.frequency_penalty = None
+    request.presence_penalty = None
+
+    result = serving_chat._apply_request_overrides(default_params, request)
+
+    assert result.stop == ["<|end|>", "\\n"]
+    assert result.stop_token_ids == [2, 32000]
+
+
+def test_build_sampling_params_list_empty_stop_preserves_yaml(serving_chat, mock_request):
+    """Test that empty stop list in request preserves YAML defaults via
+    _build_sampling_params_list_from_request."""
+    mock_request.stop = []
+    mock_request.stop_token_ids = []
+
+    result = serving_chat._build_sampling_params_list_from_request(mock_request)
+
+    comprehension_params = result[0]
+    # Empty lists should NOT override — YAML defaults are preserved
+    assert comprehension_params.stop == []
+    assert comprehension_params.stop_token_ids == []
+
+
 # =============================================================================
 # Tests for _get_comprehension_stage_index
 # =============================================================================

vllm_omni/entrypoints/openai/serving_chat.py

@@ -728,7 +728,7 @@ class OmniOpenAIServingChat(OpenAIServingChat, AudioMixin):
 
         for field_name in self._OPENAI_SAMPLING_FIELDS:
             value = getattr(request, field_name, None)
-            if value is not None:
+            if (value is not None and not isinstance(value, list)) or (isinstance(value, list) and len(value) > 0):
                 setattr(params, field_name, value)
 
         return params