[Bugfix] Enable DSML structural tag for DeepSeek-V4 with auto + non-strict tools

tests/tool_parsers/test_structural_tag_registry.py

@@ -346,3 +346,17 @@ def test_get_function_parameters_relaxes_function_strict_false():
     )
 
     assert _get_function_parameters(function) is True
+
+
+def test_deepseek_v4_auto_non_strict_gets_structural_tag(
+    sample_tools: list[ChatCompletionToolsParam],
+):
+    # auto + non-strict must still constrain DSML for DeepSeek-V4 so malformed
+    # tool calls don't leak into content / drop as empty responses (#40801).
+    tag = get_model_structural_tag(
+        model="deepseek_v4",
+        tools=sample_tools,
+        tool_choice="auto",
+        reasoning=True,
+    )
+    assert isinstance(tag, StructuralTag)

vllm/tool_parsers/structural_tag_registry.py

@@ -107,7 +107,12 @@ def get_model_structural_tag(
         return None
 
     if tool_choice == "auto" and not _any_tool_strict(tools):
-        return None
+        # DeepSeek-V4: keep DSML grammar guidance on for auto + non-strict so
+        # the model is constrained to valid DSML when it attempts a tool call.
+        # Without this, malformed DSML leaks into content or is dropped as an
+        # empty response (see #40801).
+        if model != "deepseek_v4":
+            return None
 
     dumped_tools = [_dump_tool_for_xgrammar(tool) for tool in tools]
     dumped_tool_choice = _dump_tool_choice_for_xgrammar(tool_choice)