[Bug]: `minItems` and `maxItems` json schema constraint fails on `xgrammar` and did not fallback to `outlines`

[Jason-CKY]

Your current environment

using vllm/vllm-openai:v0.6.6.post1 docker image

Model Input Dumps

No response

🐛 Describe the bug

VLLM server fails to conform guided decoding with JSON schema minItems and maxItems for array types.

from openai import OpenAI
from pydantic import BaseModel

client = OpenAI(
    base_url="http://localhost:8000/v1",
    api_key="-",
)

class Person(BaseModel):
  names: list[str]: Field(..., min_length=2, max_length=2)

print(Person.model_json_schema())  # {'properties': {'names': {'items': {'type': 'string'}, 'maxItems': 2, 'minItems': 2, 'title': 'Names', 'type': 'array'}}, 'required': ['names'], 'title': 'Person', 'type': 'object'}

response = client.chat.completions.create(
  model='aya-23-35b',
  messages=[
    {
      'role': 'user',
      'content': 'Generate 4 names. Respond in json format.'
    }
  ],
  extra_body={"guided_json": Person.model_json_schema()}
)
print(response.choices[0].message.content)

• 2 names should be generated due to JSON constraint, but 4 names is still generated

Fallback to outlines

The default grammar backend is now xgrammar, and it currently still does not support the following keywords for array:

https://github.com/mlc-ai/xgrammar/blob/c1b64920cad24f44f235778c1c00bb52d57da01a/cpp/json_schema_converter.cc#L975-L992

std::string JSONSchemaConverter::VisitArray(
    const picojson::object& schema, const std::string& rule_name
) {
  XGRAMMAR_CHECK(
      (schema.count("type") && schema.at("type").get<std::string>() == "array") ||
      schema.count("items") || schema.count("prefixItems") || schema.count("unevaluatedItems")
  );
  WarnUnsupportedKeywords(
      schema,
      {
          "uniqueItems",
          "contains",
          "minContains",
          "maxContains",
          "minItems",
          "maxItems",
      }
  );

but the vllm fallback code does not check for these keywords, so fallback to outlines did not happen.

vllm/vllm/model_executor/guided_decoding/utils.py

Lines 4 to 35 in 2339d59

	def has_xgrammar_unsupported_json_features(schema: dict) -> bool:
	"""Check if JSON schema contains features unsupported by xgrammar."""
	def check_object(obj: dict) -> bool:
	if not isinstance(obj, dict):
	return False
	# Check for pattern restrictions
	if "pattern" in obj:
	return True
	# Check for numeric ranges
	if obj.get("type") in ("integer", "number") and any(
	key in obj for key in [
	"minimum", "maximum", "exclusiveMinimum",
	"exclusiveMaximum", "multipleOf"
	]):
	return True
	# Recursively check all nested objects and arrays
	for value in obj.values():
	if isinstance(value, dict):
	if check_object(value):
	return True
	elif isinstance(value, list):
	for item in value:
	if isinstance(item, dict) and check_object(item):
	return True
	return False
	return check_object(schema)

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