[Frontend] Accept --reasoning-parser none as the unset default (#61525)

Signed-off-by: Alec Flowers <aflowers@nvidia.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Bugen Zhao <i@bugenzhao.com>

tests/entrypoints/launchers/test_cli_args.py

@@ -300,6 +300,16 @@ def test_passes_with_reasoning_parser(serve_parser):
     validate_parsed_serve_args(args)
 
 
+@pytest.mark.parametrize(
+    ("value", "expected"),
+    [("none", ""), ("None", ""), ("NONE", ""), ("deepseek_r1", "deepseek_r1")],
+)
+def test_reasoning_parser_none_disables_parser(serve_parser, value, expected):
+    """`--reasoning-parser none` disables parsing, as in `vllm-rs serve`."""
+    args = serve_parser.parse_args(args=["--reasoning-parser", value])
+    assert args.reasoning_parser == expected
+
+
 def test_chat_template_validation_for_happy_paths(serve_parser):
     """Ensure validation passes if the chat template exists."""
     args = serve_parser.parse_args(

vllm/engine/arg_utils.py

@@ -184,6 +184,11 @@ def union_dict_and_str(val: str) -> str | dict[str, str] | None:
     return optional_type(json.loads)(val)
 
 
+def parse_reasoning_parser(val: str) -> str:
+    """Map `none` (any case) to the unset default, as `vllm-rs serve` does."""
+    return "" if val.lower() == "none" else val
+
+
 def is_type(type_hint: TypeHint, type: TypeHintT) -> TypeIs[TypeHintT]:
     """Check if the type hint is a specific type."""
     return type_hint is type or get_origin(type_hint) is type
@@ -1114,6 +1119,10 @@ class EngineArgs:
 
         # Structured outputs arguments
         structured_outputs_kwargs = get_kwargs(StructuredOutputsConfig)
+        structured_outputs_kwargs["reasoning_parser"]["type"] = parse_reasoning_parser
+        structured_outputs_kwargs["reasoning_parser"]["help"] += (
+            "\n\n`none` disables reasoning parsing, the same as leaving it unset."
+        )
         structured_outputs_group = parser.add_argument_group(
             title="StructuredOutputsConfig",
             description=StructuredOutputsConfig.__doc__,