fix: reject string schemas that mix pattern/format with length bounds (#49996)

Signed-off-by: Yufeng He <40085740+he-yufeng@users.noreply.github.com>

tests/v1/structured_output/test_utils.py

@@ -14,6 +14,12 @@ pytestmark = pytest.mark.cpu_test
 def unsupported_string_schemas():
     return [
         {"type": "string", "format": "non_existing_format"},
+        # pattern/format is compiled but length bounds are silently dropped,
+        # so the combination must be rejected instead of producing quietly
+        # wrong output
+        {"type": "string", "pattern": "^a+$", "maxLength": 2},
+        {"type": "string", "pattern": "^a+$", "minLength": 3},
+        {"type": "string", "format": "email", "maxLength": 10},
     ]
 
 

vllm/v1/structured_output/backend_xgrammar.py

@@ -262,6 +262,20 @@ def has_xgrammar_unsupported_json_features(schema: dict[str, Any]) -> bool:
         ):
             return True
 
+        # A string mixing a generative constraint (pattern or format) with
+        # explicit length bounds. xgrammar compiles the pattern/format side
+        # and silently drops minLength/maxLength from the grammar, so output
+        # can violate the bound without any error surfacing. Verified against
+        # the compiled EBNF: pattern/format grammars come out byte-identical
+        # with and without the length keywords, while maxLength alone lowers
+        # to {0, N} correctly.
+        if (
+            obj.get("type") == "string"
+            and ("pattern" in obj or "format" in obj)
+            and ("minLength" in obj or "maxLength" in obj)
+        ):
+            return True
+
         # Unsupported keywords for objects
         if obj.get("type") == "object" and any(
             key in obj for key in ("patternProperties", "propertyNames")