fix: force xgrammar backend in from_sampling_params

[JMPSequeira]

Problem

In the b12x fork, SamplingParams.validate() — which auto-selects the structured output backend via _validate_structured_outputs — may not be called before Request creation. When this happens, StructuredOutputsParams._backend stays None, and the scheduler skips grammar enforcement entirely. The structured_outputs field is accepted by the API, the grammar is validated and compiled, but the bitmask is never applied to logits during sampling. Constraints are silently ignored.

Reproduction

Send a completion request with a grammar constraint:

{
  "model": "GLM-5.2",
  "prompt": "Say: ",
  "max_tokens": 5,
  "structured_outputs": {"grammar": "root ::= \"hello\""}
}

On the unpatched b12x image, the model outputs freely (e.g. "1. Who they are") instead of being constrained to "hello".

Fix

Force the xgrammar backend in from_sampling_params() when validate() hasn't already selected one. This is a no-op when validate() has run (the backend is already set).

Verification

Tested on the eldritch-fullstack image (v0.11.2.dev279):

1. Simple grammar: root ::= "hello" — output constrained to "hello" ✓
2. Full LLang GBNF grammar: 75 nonterminals, 154K-token vocab (GLM-5.2 tokenizer) — output constrained to valid LLang syntax ✓
3. Model: Qwen3.5-9B-NVFP4 — generated valid LLang code (pattern matching, let-in, pipeline) under constraint ✓

Notes

• This issue was diagnosed and tested using GLM-5.2 (504B) as the analysis and reasoning model.
• The fix is 8 lines, all additive — no existing behavior is changed when validate() has already run.
• The structured_outputs API field, grammar validation, xgrammar compilation, and model runner bitmask application all work correctly in isolation. The only gap was the backend selection not happening before Request creation.

Summary by CodeRabbit

• Bug Fixes
  • Structured outputs now more reliably use the server’s configured backend when a request does not include one.
  • Requests that rely on automatic backend selection will now consistently choose the preferred compatible backend when available, reducing setup-related failures.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d424e1d1-dc01-4d38-a479-e7754347ca9f

📥 Commits

Reviewing files that changed from the base of the PR and between 39032f6 and ef1d8ab.

📒 Files selected for processing (1)

• vllm/v1/structured_output/__init__.py

📝 Walkthrough

Walkthrough

StructuredOutputManager.grammar_init now falls back to the server-configured structured output backend when the request backend is unset, and resolves auto to xgrammar before backend instantiation.

Changes

Structured output backend resolution

Layer / File(s)	Summary
Backend fallback and auto resolution vllm/v1/structured_output/__init__.py	grammar_init uses the server-configured backend when the request backend is unset, and maps auto to xgrammar before backend instantiation.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly states the main fix: selecting the xgrammar backend when structured-output params are not initialized.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@vllm/v1/structured_output/request.py`:
- Around line 45-47: The fallback in StructuredOutputsParams is overwriting the
request-level backend and breaks configured non-auto backends. Update
from_sampling_params() and the backend selection flow so it uses the configured
backend from SamplingParams instead of hard-coding "xgrammar", or move the
fallback into request-local scheduler state. Keep _backend_was_auto semantics
intact and avoid mutating params._backend before validation in request.py.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 0b067feb-9150-46c6-bfdb-f15eddb4e55b

📥 Commits

Reviewing files that changed from the base of the PR and between 2d040b9 and aa95917.

📒 Files selected for processing (1)

• vllm/v1/structured_output/request.py

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@vllm/v1/structured_output/__init__.py`:
- Around line 139-142: `grammar_init()` is incorrectly collapsing `backend ==
"auto"` to `xgrammar`, which overrides the existing auto-resolution behavior
from `SamplingParams.validate()` and can break requests that should fall back to
`outlines` or `guidance`. Update the `GrammarInit`/backend selection path to
preserve `auto` and reuse the same resolution logic already used by
`SamplingParams.validate()`, or move backend determination earlier at request
construction so `grammar_init()` receives the resolved backend instead of
forcing `xgrammar`.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 4348e26e-ff22-4fcb-ad1f-0a84bed07273

📥 Commits

Reviewing files that changed from the base of the PR and between aa95917 and 39032f6.

📒 Files selected for processing (1)

• vllm/v1/structured_output/__init__.py