fix(sampling): reject conflicting structural tag constraints (#32525)

python/sglang/srt/sampling/sampling_params.py

@@ -196,9 +196,12 @@ class SamplingParams(msgspec.Struct, kw_only=True, array_like=True):
             self.json_schema,
             self.regex,
             self.ebnf,
+            self.structural_tag,
         ]  # since mutually exclusive, only one can be set
         if sum(x is not None for x in grammars) > 1:
-            raise ValueError("Only one of regex, json_schema, or ebnf can be set.")
+            raise ValueError(
+                "Only one of json_schema, regex, ebnf, or structural_tag can be set."
+            )
 
     def normalize(self, tokenizer):
         # Process stop strings

test/registered/unit/sampling/test_sampling_params.py

@@ -77,6 +77,12 @@ class TestSamplingParamsInit(CustomTestCase):
 class TestSamplingParamsVerify(CustomTestCase):
 
     VOCAB_SIZE = 32000
+    GRAMMAR_VALUES = {
+        "json_schema": '{"type":"object"}',
+        "regex": "abc",
+        "ebnf": 'root ::= "abc"',
+        "structural_tag": '{"structures":[],"triggers":[]}',
+    }
 
     def _make(self, **kwargs):
         """Helper: create SamplingParams with safe defaults, override with kwargs."""
@@ -273,21 +279,25 @@ class TestSamplingParamsVerify(CustomTestCase):
         sp.verify(self.VOCAB_SIZE)
 
     def test_multiple_grammars_raises(self):
-        """Test that verify() rejects setting both json_schema and regex (mutually exclusive)."""
+        """Reject structural_tag combined with any other grammar constraint.
-        sp = self._make(json_schema='{"type":"object"}', regex="abc")
+
-        with self.assertRaises(ValueError):
+        Constraint selection is a fixed if/elif chain, so a constraint left out of
+        this check is silently dropped with no error to the caller.
+        """
+        for other in ("json_schema", "regex", "ebnf"):
+            with self.subTest(other=other):
+                sp = self._make(
+                    structural_tag=self.GRAMMAR_VALUES["structural_tag"],
+                    **{other: self.GRAMMAR_VALUES[other]},
+                )
+                with self.assertRaisesRegex(ValueError, "Only one of"):
                     sp.verify(self.VOCAB_SIZE)
 
     def test_single_grammar_valid(self):
-        """Test that setting only one grammar type is accepted."""
+        """Test that each grammar constraint is valid on its own."""
-        sp = self._make(json_schema='{"type":"object"}')
+        for grammar, value in self.GRAMMAR_VALUES.items():
-        sp.verify(self.VOCAB_SIZE)
+            with self.subTest(grammar=grammar):
-
+                self._make(**{grammar: value}).verify(self.VOCAB_SIZE)
-    def test_all_three_grammars_set_raises(self):
-        """Test that verify() rejects setting json_schema, regex, and ebnf together."""
-        sp = self._make(json_schema="{}", regex="a", ebnf="rule")
-        with self.assertRaises(ValueError):
-            sp.verify(self.VOCAB_SIZE)
 
 
 class TestSamplingParamsNormalize(CustomTestCase):