[Bugfix][Tool Parser] Use LongCat tags in the LongCat structural tag

tests/tool_parsers/test_structural_tag_registry.py

@@ -48,6 +48,7 @@
 from vllm.tool_parsers.kimi_k2_tool_parser import KimiK2ToolParser
 from vllm.tool_parsers.kimi_k3_tool_parser import KimiK3ToolParser
 from vllm.tool_parsers.llama_tool_parser import Llama3JsonToolParser
+from vllm.tool_parsers.longcat_tool_parser import LongcatFlashToolParser
 from vllm.tool_parsers.mimo_tool_parser import MiMoToolParser
 from vllm.tool_parsers.minimax_m2_tool_parser import MinimaxM2ToolParser
 from vllm.tool_parsers.plamo3_engine_tool_parser import Plamo3EngineToolParser
@@ -850,6 +851,44 @@ class StepParser(DelegatingParser):
     ]
 
 
+@pytest.mark.parametrize(
+    "tool_choice",
+    ["required", {"type": "function", "function": {"name": "get_weather"}}],
+)
+def test_longcat_forced_tool_choice_round_trips_native_tags(
+    sample_tools: list[ChatCompletionToolsParam], tool_choice
+):
+    """Forced LongCat calls must use <longcat_tool_call>, not Hermes tags."""
+
+    class LongcatParser(DelegatingParser):
+        tool_parser_cls = LongcatFlashToolParser
+
+    request = ChatCompletionRequest(
+        messages=[],
+        model="m",
+        tools=[tool.model_dump(exclude_none=True) for tool in sample_tools],
+        tool_choice=tool_choice,
+    )
+    parser = LongcatParser(MagicMock(), tools=sample_tools)
+    request = parser.adjust_request(request)
+
+    grammar = Grammar.from_structural_tag(request.structured_outputs.structural_tag)
+    # Rendered exactly as the LongCat-Flash chat template renders tool calls.
+    output = (
+        '<longcat_tool_call>\n{"name": "get_weather", '
+        '"arguments": {"city": "Dallas"}}\n</longcat_tool_call>'
+    )
+    assert _is_grammar_accept_string(grammar, output)
+    assert not _is_grammar_accept_string(grammar, output.replace("longcat_", ""))
+
+    _, content, tool_calls = parser.parse(output, request, enable_auto_tools=True)
+
+    assert not content
+    assert [(call.name, json.loads(call.arguments)) for call in tool_calls] == [
+        ("get_weather", {"city": "Dallas"})
+    ]
+
+
 def test_tool_parsers_without_structural_tag_support_required_and_named():
     class NonStructuralTagToolParser(ToolParser):
         pass

vllm/tool_parsers/longcat_tool_parser.py

@@ -9,6 +9,8 @@
 
 
 class LongcatFlashToolParser(Hermes2ProToolParser):
+    structural_tag_model = "longcat"
+
     def __init__(self, tokenizer: TokenizerLike, tools: list[Tool] | None = None):
         super().__init__(tokenizer, tools)
 

vllm/tool_parsers/structural_tag_registry.py

@@ -79,7 +79,7 @@
     }
 )
 VLLM_BUILTIN_STRUCTURAL_TAG_MODELS = frozenset(
-    {"glm_4_7", "hermes", "hy_v4", "kimi_k3", "plamo3"}
+    {"glm_4_7", "hermes", "hy_v4", "kimi_k3", "longcat", "plamo3"}
 )
 SUPPORTED_STRUCTURAL_TAG_MODELS = (
     XGRAMMAR_BUILTIN_STRUCTURAL_TAG_MODELS | VLLM_BUILTIN_STRUCTURAL_TAG_MODELS
@@ -346,15 +346,19 @@ def get_function_parameters(function) -> dict[str, Any] | bool:
     return function.parameters if function.parameters is not None else True
 
 
-def _hermes_tool_tags(tools: list[FunctionToolParam]) -> list[TagFormat]:
+def _hermes_tool_tags(
+    tools: list[FunctionToolParam],
+    start_tag: str,
+    end_tag: str,
+) -> list[TagFormat]:
     arguments_field_prefix = '", "arguments": '
     formats = [
         # <tool_call>
         # {"name": "t1", "arguments": {"q": "v"}}
         # </tool_call>
-        ('<tool_call>\n{"name": "', "}\n</tool_call>"),
+        (start_tag + '\n{"name": "', "}\n" + end_tag),
         # <tool_call>{"name": "t1", "arguments": {"q": "v"}}</tool_call>
-        ('<tool_call>{"name": "', "}</tool_call>"),
+        (start_tag + '{"name": "', "}" + end_tag),
     ]
 
     return [
@@ -370,42 +374,64 @@ def _hermes_tool_tags(tools: list[FunctionToolParam]) -> list[TagFormat]:
     ]
 
 
-@register_vllm_structural_tag("hermes")
-def get_hermes_structural_tag(
+def _hermes_style_structural_tag(
     tools: list[FunctionToolParam],
-    builtin_tools: list[BuiltinToolParam],
     tool_choice: SimplifiedToolChoice,
-    reasoning: bool,
-    token_suffix: str = "",
+    start_tag: str,
+    end_tag: str,
 ) -> StructuralTag:
-    del builtin_tools, reasoning, token_suffix
-
-    tool_call_trigger = "<tool_call>"
-
+    tags = _hermes_tool_tags(tools, start_tag, end_tag)
     if tool_choice == "auto":
-        tags = _hermes_tool_tags(tools)
         suffix_tag = (
-            TriggeredTagsFormat(triggers=[tool_call_trigger], tags=tags)
+            TriggeredTagsFormat(triggers=[start_tag], tags=tags)
             if tags
             else AnyTextFormat()
         )
     elif tool_choice == "forced":
         suffix_tag = TagsWithSeparatorFormat(
-            tags=_hermes_tool_tags(tools),
+            tags=tags,
             separator="",
             at_least_one=True,
             stop_after_first=True,
         )
     else:
         suffix_tag = TagsWithSeparatorFormat(
-            tags=_hermes_tool_tags(tools),
+            tags=tags,
             separator="",
             at_least_one=True,
         )
 
     return StructuralTag(format=suffix_tag)
 
 
+@register_vllm_structural_tag("hermes")
+def get_hermes_structural_tag(
+    tools: list[FunctionToolParam],
+    builtin_tools: list[BuiltinToolParam],
+    tool_choice: SimplifiedToolChoice,
+    reasoning: bool,
+    token_suffix: str = "",
+) -> StructuralTag:
+    del builtin_tools, reasoning, token_suffix
+    return _hermes_style_structural_tag(
+        tools, tool_choice, "<tool_call>", "</tool_call>"
+    )
+
+
+@register_vllm_structural_tag("longcat")
+def get_longcat_structural_tag(
+    tools: list[FunctionToolParam],
+    builtin_tools: list[BuiltinToolParam],
+    tool_choice: SimplifiedToolChoice,
+    reasoning: bool,
+    token_suffix: str = "",
+) -> StructuralTag:
+    del builtin_tools, reasoning, token_suffix
+    return _hermes_style_structural_tag(
+        tools, tool_choice, "<longcat_tool_call>", "</longcat_tool_call>"
+    )
+
+
 def _minimax_tool_tags(tools: list[FunctionToolParam]) -> list[TagFormat]:
     return [
         TagFormat(