[Bug]: Remove fallback to outlines for int/number range and pattern constraints in guided_json

[csy1204]

Your current environment

• vllm 0.8.3
• xgrammar

🐛 Describe the bug

Hi team,

I’d like to propose removing the current fallback mechanism to outlines for int/number range constraints and pattern-based validations in Guided JSON mode.

vllm/vllm/model_executor/guided_decoding/utils.py

Lines 6 to 23 in ee378f3

	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

Previously, vLLM defaulted to Outlines when encountering complex JSON schema elements (like pattern, minimum, or maximum) due to limitations in xgrammar. #10899

However, xgrammar has now added full support for these schema constraints, which makes this fallback no longer necessary.

• [Feature] Support Regex for GrammarCompiler mlc-ai/xgrammar#185
• Float range support for xgrammar mlc-ai/xgrammar#289
• https://github.com/mlc-ai/xgrammar/blob/8fa47978e37970865a6630a9533f2e1db7dc8f46/cpp/json_schema_converter.cc#L1645-L1655

The main motivation behind this PR is not only to simplify and unify the backend behavior, but also to address a serious performance concern. When fallback to Outlines occurs in production, we've experienced:

• Significantly higher memory usage, leading to OOM errors, and
• Much slower response times, which in some cases rendered Guided JSON unusable for real-time services.

Additionally, this proposal is strongly inspired by the recent PR [Bugfix][v1] xgrammar structured output supports Enum, which brought long-awaited support for enum in xgrammar. Just as that change allowed us to move away from unnecessary fallbacks for enum types, we believe it's now equally reasonable and timely to eliminate fallbacks related to numeric ranges and regex patterns as well.

This change would allow Guided JSON to be more reliable and performant under real-world workloads, especially when schemas include numeric ranges or regex patterns.

Looking forward to your feedback!

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[russellb]

happy to remove the fallback for any features that have been added to xgrammar. Are you interested in writing the PR?

[russellb]

I will review it if you write the PR. If you're not comfortable doing it, or don't have time, I'll put it on my TODO list. Just let me know!

[csy1204]

@russellb I'm so happy to contribute this huge project. Please review my PR Thanks! #16725

[tcapelle]

I would also suggest to update the dosc about using outlines backed: https://docs.vllm.ai/en/latest/features/structured_outputs.html#experimental-automatic-parsing-openai-api