[Bugfix] Make xgrammar an import-time optional dependency

[ybwbqg9379]

Purpose

Fixes #56559. On platforms without an xgrammar wheel (e.g. s390x), from vllm import LLM and vllm serve crash at import time even for workloads that never use structured output.

On current main the first failure is not backend_xgrammar.py (already lazy-loaded) but vllm/parser/harmony.py, pulled in via vllm/v1/structured_output/__init__.py -> vllm.parser; vllm/tool_parsers/structural_tag_registry.py has the same unconditional imports and the OpenAI server imports both. Once those are fixed, the next failure is the dataclass field annotations in backend_xgrammar.py (xgr.GrammarMatcher), which evaluate the LazyLoader at class-definition time.

Changes:

• harmony.py, structural_tag_registry.py: wrap the xgrammar imports in try/except ImportError and bind the names to PlaceholderModule("xgrammar"), so structural tag builders raise a clear ImportError on first use instead of breaking import. Module-level xgrammar objects (_JSON_CONTENT, _ANY_CONTENT, the runtime-evaluated StructuralTagBuilder alias) are made lazy so nothing touches the placeholders at import time.
• backend_xgrammar.py: from __future__ import annotations to defer the field annotations (same line as [Bugfix] Defer xgrammar annotations to fix startup crash when xgrammar is unavailable #56561, which is necessary but not sufficient on its own).
• tests/standalone_tests/lazy_imports.py: add xgrammar to the modules that must not be imported by import vllm (already run in CI).
• tests/tool_parsers/test_structural_tag_registry.py: subprocess regression test that blocks xgrammar, imports LLM, and asserts a builder raises ImportError mentioning xgrammar.

Why this is not a duplicate

Searched open PRs for 56559, "xgrammar import", "xgrammar s390x". #56561 only adds the __future__ line to backend_xgrammar.py; with xgrammar blocked, from vllm import LLM on main still fails earlier in harmony.py, so that PR alone does not fix the issue. This PR includes that line and fixes the remaining import chain; see my comment on the issue.

Test Plan

.venv/bin/python tests/standalone_tests/lazy_imports.py
.venv/bin/python -m pytest tests/parser/test_harmony.py tests/tool_parsers/test_structural_tag_registry.py
.venv/bin/python -m pytest tests/v1/structured_output/test_backend_xgrammar_stop_tokens.py tests/v1/structured_output/test_utils.py tests/v1/structured_output/test_validation.py
pre-commit run --files # incl. mypy-3.10 / mypy-3.12
Manual: with sys.modules["xgrammar"] = None, from vllm import LLM, import vllm.entrypoints.openai.api_server and import vllm.entrypoints.cli.main all succeed on this branch and fail on main.

Test Result

• lazy_imports.py: passes (fails on main after adding xgrammar to the list)
• harmony + structural_tag_registry: 147 passed; new test: 1 passed
• structured_output: 25 passed
• pre-commit (ruff, typos, mypy 3.10/3.12, SPDX, lazy-import checks): all passed

Model output

No change to model execution. Sanity check on an RTX 5090 with Qwen/Qwen2.5-1.5B-Instruct and structured_outputs_config={"backend": "xgrammar"}: JSON-schema constrained generation returns valid JSON, and a hermes structural tag from get_model_structural_tag yields <tool_call>{"name": "get_weather", "arguments": {"city": "Tokyo"}}</tool_call>.

[claude]

Claude Code Review

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ybwbqg9379.

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

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ybwbqg9379.

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

[ybwbqg9379]

Rebased onto main (71fc70d) to clear the conflict; the branch is a single commit again.

The conflict was confined to vllm/tool_parsers/structural_tag_registry.py and its test, from #57272 (XGrammar 0.2.7), #56268 (--tool-strict-level), #57194 and #52136. Two changes were needed on top of the original patch:

• dropped builtin_structural_tag from the guarded import and from the placeholder list, since [Refactor] Remove unused interface methods #57194 removed the vLLM-side deepseek_v41 builder that used it;
• kept the new from vllm.tool_parsers.tool_strict_level import ToolStrictLevel.

git range-diff confirms nothing else changed. harmony.py, backend_xgrammar.py and lazy_imports.py applied unchanged.

Re-verified after the rebase: tests/tool_parsers/test_structural_tag_registry.py 165 passed; tests/tool_parsers 1089 passed (the 15 errors in test_llama3_json_tool_parser.py are HF 401s on the gated meta-llama/Llama-3.2-1B-Instruct, not related); tests/v1/structured_output 128 passed, 1 xfailed; tests/standalone_tests/lazy_imports.py passes; pre-commit and mypy-3.12 clean.

One check beyond the test in this PR: with sys.modules["xgrammar"] = None, both from vllm import LLM and vllm.entrypoints.openai.api_server.build_app import cleanly, and the structural tag builders raise ImportError naming xgrammar on first use.

Note for whoever reviews: vllm/parser/cohere_command.py also imports xgrammar unguarded, but it predates this PR's base and is only imported lazily inside a function in parser_manager.py, so it is not on the import vllm path. I left it alone rather than widen the diff.

[ybwbqg9379]

@aarnphm @russellb — ping after the Sep 18 rebase; the branch is clean against main again.

Same CI situation as any first-time contributor: pre-run-check fails because I have no
merged PRs yet, so pre-commit never runs. A ready label or /ci run would give this
a real signal.

Also flagging overlap: #56561 fixes the same class of failure but only in
backend_xgrammar.py. This PR covers that file plus vllm/parser/harmony.py and
vllm/tool_parsers/structural_tag_registry.py, which are also on the import vllm path,
so the narrower one alone would not make import vllm work without xgrammar.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ybwbqg9379.

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

[sfeng33]

Thanks for the PR. I'm hesitant to take this as-is, for two reasons:

xgrammar is a hard dependency on s390x today. requirements/common.txt pins xgrammar == 0.2.7 with an explicit s390x marker, and docker/Dockerfile.s390x (which our s390x CI builds) installs it from sdist using the gcc-toolset-14 / cmake toolchain in that image. Both xgrammar and apache-tvm-ffi publish sdists. So the "cannot be built on s390x" premise in #56559 doesn't match how vLLM's own s390x image works; it looks like an environment issue in the reporter's UBI 9 / Spyre setup.

Making xgrammar import-optional is a policy decision, not a bugfix. Structural tags now underpin tool-call parsing, and the set of modules importing xgrammar keeps growing (vllm/parser/abstract_parser.py was added on main after this PR, so the branch no longer achieves its goal). Per-file PlaceholderModule guards plus a lazy_imports.py check would make "import vllm works without xgrammar" a contract we have to maintain everywhere, and that needs sign-off from the structured-output owners first. If we do go that way, it should be done at a single choke point (e.g. the vllm.parser import in vllm/v1/structured_output/__init__.py) and paired with dropping the requirement marker for the affected platforms.

The from __future__ import annotations line in backend_xgrammar.py is fine on its own as hygiene (matches backend_outlines.py), but it shouldn't be described as fixing #56559.