[antonyamal-3-6-9]

## Purpose

Fixes #54003: Adds a compilation timeout guard (`VLLM_GRAMMAR_COMPILATION_TIMEOUT_S`,

defaulting to 10s) for JSON schema, CFG grammar, and structural tags in the xgrammar
backend.

Previously, `compile_regex_with_timeout` protected regex patterns against exponential DFA

state-space explosion, but JSON schema compilation (compiler.compile_json_schema) and CFG
compilation (compiler.compile_grammar) had no timeout protection. Complex schemas with
recursive or deep combinatorial branching (e.g. nested oneOf/anyOf) could stall
inference workers indefinitely and exhaust worker memory.

This PR brings JSON schema, grammar, and structural tag compilation to parity with regex

timeout handling:
1. Adds VLLM_GRAMMAR_COMPILATION_TIMEOUT_S (default 10s, 0 to disable) in vllm/envs. py.
2. Refactors compile_with_timeout and introduces compile_grammar_with_timeout in
vllm/v1/structured_output/utils.py.
3. Wraps JSON schema, JSON object, grammar, and structural tag compilation and validation
calls in vllm/v1/structured_output/backend_xgrammar.py.
4. Adds dedicated unit and real compiler integration tests in
tests/v1/structured_output/test_grammar_compilation_timeout.py.

## Test Plan

- Unit & regression test suite:
  ```bash
  VLLM_TARGET_DEVICE=cpu .venv/bin/python -m pytest

tests/v1/structured_output/test_grammar_compilation_timeout.py
tests/v1/structured_output/test_regex_compilation_timeout.py
tests/v1/structured_output/test_validation.py -v

• Pre-commit linting and type-checking:
.venv/bin/python -m pre_commit run --files vllm/envs.py
vllm/v1/structured_output/backend_xgrammar.py vllm/v1/structured_output/utils.py
tests/v1/structured_output/test_grammar_compilation_timeout.py

Test Result

1. Pytest Results (30 passed in 14.63s):
   • test_normal_json_schema_compiles_successfully: PASSED
   • test_normal_grammar_compiles_successfully: PASSED
   • test_timeout_raises_value_error: PASSED
   • test_timeout_disabled_when_zero: PASSED
   • test_compilation_error_propagates: PASSED
   • test_spec_included_in_error_message: PASSED
   • test_real_xgrammar_compilation_timeout: PASSED (verifies real compiler.
   compile_json_schema with HuggingFace tokenizer aborts after timeout)
   • All 18 existing validation tests in tests/v1/structured_output/test_validation.py:
   PASSED
2. Pre-commit Checks:
   • ruff check: Passed
   • ruff format: Passed
   • typos: Passed
   • Run mypy for Python 3.10: Passed
   • Check SPDX headers: Passed
   • Validate configuration has default values and docstrings: Passed
   • Sign-off Commit: Passed

Duplicate-Work Check (AGENTS.md)

• gh issue view 54003: Issue filed on 2026-08-27 with 0 PRs linked.
• gh pr list --search "54003 in:body": 0 open PRs.
• gh pr list --search "grammar compilation timeout": Confirmed no active PRs. PR #45172
attempted a broad cross-backend scheduler rewrite but has been inactive with merge
conflicts since June 2026. This PR is a minimal, targeted, and non-conflicting fix rebased
on current origin/main.

AI Assistance Statement (AGENTS.md)

This contribution was authored with AI assistance (Antigravity). Every changed line,
docstring, typing annotation, and test has been reviewed, inspected, and verified end-to-
end by the human author.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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🚀

[arpera]

@antonyamal-3-6-9, first of all thank you for your PR!

DELETED

[antonyamal-3-6-9]

@arpera Thank you for your response. Sorry for the delay. As I can see the message is deleted. Can you please provide a clear feedback, where I can proceed with.

[arpera]

Could you check please first this PR #45172? I am thinking this approach might be more suitable, but if you have concerns about it, please explain them, it would be good to know which approach is better or has kind of drawbacks

[antonyamal-3-6-9]

@arpera

Thanks for pointing me to #45172. After comparing both, I think its centralized deadline approach is a better foundation than adding per-call thread pools. It needs adapting to current main’s grammar_compile_error_reqs / finish_requests flow, and synchronous compilation needs separate handling. Neither approach stops an already-running compilation. Would you prefer that I adapt the focused timeout changes in this PR, or contribute to #45172?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @antonyamal-3-6-9.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork