feat: Enable structural tags by default and improve Kimi K3 guidance

components/src/dynamo/common/configuration/groups/runtime_args.py

@@ -40,8 +40,8 @@ class DynamoRuntimeConfig(ConfigBase):
     dyn_reasoning_parser: Optional[str] = None
     dyn_default_thinking_mode: Optional[str] = None
     exclude_tools_when_tool_choice_none: bool = True
-    dyn_enable_structural_tag: bool = False
-    dyn_structural_tag_scope: str = "auto"
+    dyn_enable_structural_tag: bool = True
+    dyn_structural_tag_scope: str = "always"
     dyn_structural_tag_schema: str = "auto"
     custom_jinja_template: Optional[str] = None
     endpoint_types: str
@@ -293,16 +293,17 @@ def add_arguments(self, parser: argparse.ArgumentParser) -> None:
             g,
             flag_name="--dyn-enable-structural-tag",
             env_var="DYN_ENABLE_STRUCTURAL_TAG",
-            default=False,
-            help="Enable structural tag guided decoding for tool calls. "
-            "Named Kimi K3 tool_choice requests always activate their required "
-            "XTML structural tag even when this flag is off.",
+            default=True,
+            help="Enable structural tag guided decoding for tool calls when the configured "
+            "parser and backend support it. Explicitly disabling this flag is "
+            "authoritative for Rust, Python vLLM, and Python SGLang frontend "
+            "preprocessing.",
         )
         add_argument(
             g,
             flag_name="--dyn-structural-tag-scope",
             env_var="DYN_STRUCTURAL_TAG_SCOPE",
-            default="auto",
+            default="always",
             choices=["auto", "always"],
             help="Controls when structural tags are activated. "
             "'auto': for required/named tool_choice, and if any tool has strict=true "
@@ -317,9 +318,10 @@ def add_arguments(self, parser: argparse.ArgumentParser) -> None:
             default="auto",
             choices=["auto", "strict"],
             help="Controls parameter schema strictness inside structural tags. "
-            "'auto': real schema only for tools with strict=true; "
-            "syntactically constrained but schema-unconstrained for all other tools. "
-            "'strict': real parameter schema for all tools.",
+            "'auto': declared parameter schema for tools with strict omitted or true; "
+            "tools with strict=false keep the native tool envelope constrained but use "
+            "schema-unconstrained arguments. "
+            "'strict': declared parameter schema for all tools, overriding strict=false.",
         )
         add_argument(
             g,

components/src/dynamo/common/tests/configuration/test_runtime_args.py

@@ -31,6 +31,35 @@ def _parse_runtime_args(argv: list[str]) -> tuple[DynamoRuntimeConfig, str]:
     return config, parser.format_help()
 
 
+def test_structural_tags_default_on_for_supported_parsers(monkeypatch):
+    monkeypatch.delenv("DYN_ENABLE_STRUCTURAL_TAG", raising=False)
+    monkeypatch.delenv("DYN_STRUCTURAL_TAG_SCOPE", raising=False)
+    monkeypatch.delenv("DYN_STRUCTURAL_TAG_SCHEMA", raising=False)
+
+    config, _ = _parse_runtime_args([])
+
+    assert config.dyn_enable_structural_tag is True
+    assert config.dyn_structural_tag_scope == "always"
+    assert config.dyn_structural_tag_schema == "auto"
+
+
+def test_structural_tag_schema_help_describes_omitted_strict(monkeypatch):
+    monkeypatch.delenv("DYN_STRUCTURAL_TAG_SCHEMA", raising=False)
+
+    _, help_text = _parse_runtime_args([])
+
+    assert "strict omitted or true" in help_text
+    assert "overriding strict=false" in help_text
+
+
+def test_structural_tag_global_opt_out(monkeypatch):
+    monkeypatch.setenv("DYN_ENABLE_STRUCTURAL_TAG", "false")
+
+    config, _ = _parse_runtime_args([])
+
+    assert config.dyn_enable_structural_tag is False
+
+
 def test_fpm_trace_defaults_disabled(monkeypatch):
     monkeypatch.delenv("DYN_FPM_TRACE", raising=False)
 

components/src/dynamo/frontend/prepost.py

@@ -32,9 +32,15 @@
 from dynamo.frontend.vllm_protocol import DeltaFunctionCall, DeltaMessage, DeltaToolCall
 from dynamo.llm.exceptions import InvalidArgument
 
+from .structural_tag_policy import effective_tool_strict, should_attempt_structural_tag
 from .thinking import apply_default_thinking_mode_to_template_kwargs
 from .utils import legacy_guided_decoding
 
+try:
+    from vllm.tool_parsers.structural_tag_registry import get_model_structural_tag
+except ImportError:  # Older supported vLLM releases do not expose this registry.
+    get_model_structural_tag = None
+
 if TYPE_CHECKING:
     from vllm.config import ModelConfig
 
@@ -204,35 +210,47 @@ def _should_build_tool_call_guidance(
     # enforced guarantee. sglang_prepost.py has the same gap.
     if tool_choice == "none":
         return False
-    if _is_forced_tool_choice(tool_choice):
-        return True
-    if structural_tag_mode != "on":
-        return False
-    if tool_choice != "auto":
-        return False
-    if structural_tag_scope == "always":
-        return True
     # An explicit single-call request is enough to attempt structural-tag
     # guidance. Forced-choice JSON guidance also bounds its array schema below.
     explicit_single_call = (
         "parallel_tool_calls" in request.model_fields_set
         and request.parallel_tool_calls is False
     )
-    return explicit_single_call or any(_tool_is_strict(tool) for tool in request.tools)
+    tool_choice_kind = (
+        "required"
+        if tool_choice == "required"
+        else "named"
+        if _is_named_tool_choice(tool_choice)
+        else "auto"
+        if tool_choice == "auto"
+        else "other"
+    )
+    attempt_structural_tag = should_attempt_structural_tag(
+        mode=structural_tag_mode,
+        scope=structural_tag_scope,
+        tool_choice_kind=tool_choice_kind,
+        has_tools=True,
+        any_explicit_strict=any(_tool_is_strict(tool) for tool in request.tools),
+        parallel_tool_calls_explicitly_false=explicit_single_call,
+    )
+    # Forced choices retain the generic JSON fallback when structural tags are
+    # disabled or unavailable.
+    return attempt_structural_tag or _is_forced_tool_choice(tool_choice)
 
 
 def _request_for_vllm_structural_tag(
     request: ChatCompletionRequest,
     *,
     structural_tag_schema: str,
 ) -> ChatCompletionRequest:
-    strict_schema = structural_tag_schema == "strict"
     tools = [
         tool.model_copy(
             update={
                 "function": tool.function.model_copy(
                     update={
-                        "strict": True if strict_schema else tool.function.strict,
+                        "strict": effective_tool_strict(
+                            tool.function.strict, structural_tag_schema
+                        ),
                     }
                 )
             }
@@ -264,7 +282,31 @@ def build_tool_call_guided_decoding(
             request,
             structural_tag_schema=structural_tag_schema,
         )
-        structural_tag = tool_parser.get_structural_tag(request_for_tag)
+        try:
+            structural_tag_model = getattr(tool_parser, "structural_tag_model", None)
+            structural_tag = (
+                get_model_structural_tag(
+                    model=structural_tag_model,
+                    tools=request_for_tag.tools,
+                    tool_choice=request_for_tag.tool_choice,
+                    reasoning=False,
+                )
+                if structural_tag_model is not None
+                and get_model_structural_tag is not None
+                else tool_parser.get_structural_tag(request_for_tag)
+            )
+        except (
+            AttributeError,
+            KeyError,
+            NotImplementedError,
+            RuntimeError,
+            TypeError,
+            ValueError,
+        ):
+            # Parser implementations are third-party capability providers. A
+            # failed structural-tag build must preserve the pre-existing JSON
+            # or unconstrained compatibility path for the request.
+            structural_tag = None
         if structural_tag is not None:
             tag_value = (
                 structural_tag.model_dump()