[Bugfix][Structured Outputs] Reject empty structural_tag at request validation

tests/v1/structured_output/test_validation.py

@@ -56,13 +56,18 @@ def test_plain_request_allowed_for_diffusion_models():
     [
         (StructuredOutputsParams(json_object=False), "json_object must be True"),
         (StructuredOutputsParams(json=""), "json cannot be an empty string"),
+        (
+            StructuredOutputsParams(structural_tag=""),
+            "structural_tag cannot be an empty string",
+        ),
     ],
 )
 def test_degenerate_structured_outputs_rejected(structured_outputs, match):
     """json_object=False and an empty json schema pass the `is not None`
     exclusivity check but resolve to no structured-output key, so they must be
-    rejected at request validation (-> 400) instead of reaching and crashing
-    the engine."""
+    rejected at request validation (-> 400). Empty `structural_tag` is rejected
+    for the same reason: `json.loads("")` in `compile_grammar` would otherwise
+    raise JSONDecodeError and surface as a per-request engine error."""
     params = SamplingParams(structured_outputs=structured_outputs)
     with pytest.raises(VLLMValidationError, match=match):
         params._validate_structured_outputs(

vllm/sampling_params.py

@@ -1144,6 +1144,18 @@ def _validate_structured_outputs(
             raise VLLMValidationError(
                 "structured_outputs.regex must not contain a NUL character ('\\x00')"
             )
+        # Note(arpera):
+        # We do NOT check here structured output regex on emptiness because
+        # empty regex is indeed compiles to a valid grammar as well as
+        # whitespace-only regexps, for instance, regex="\n" or regex=" "
+        # are valid patterns and we MUST process them.
+        if (
+            isinstance(self.structured_outputs.structural_tag, str)
+            and self.structured_outputs.structural_tag.strip() == ""
+        ):
+            raise VLLMValidationError(
+                "structured_outputs.structural_tag cannot be an empty string"
+            )
 
         from vllm.v1.structured_output.backend_guidance import (
             has_guidance_unsupported_json_features,