[Bugfix][Structured Output][Spec Decode] Constrain bitmask and trim grammar advance at the reasoning boundary

[yuyue0225sc]

Purpose

Fixes #44006.

When VLLM_ENFORCE_STRICT_TOOL_CALLING=1 is combined with MTP/EAGLE speculative
decoding, the engine intermittently rejects token 248069 (Qwen's </think>) and
the request finishes with 500 "Failed to advance FSM ... grammar rejected tokens".

There are two related defects in StructuredOutputManager.grammar_bitmask, a
hardening follow-up to the first fix, and a scheduler-side fix required after
rebasing onto main containing #42452 — all included here.

1. Mid-window reasoning-end loses the bitmask switch

should_fill_bitmask reflects reasoning_ended at the start of the step. When
MTP/EAGLE schedules multiple positions per step, the </think> marker can land in
the middle of a spec window. The previous code prepared the entire window with the
same apply_bitmask value the step started with, so positions after the marker
still got the unconstrained "in-reasoning" mask. The model could then emit a token
listed in structural_tag.excludes (e.g. another </think> after content has
begun), and the next step rejects it via accept_tokens, killing the request.

Fix: for each draft token in the simulated window, call
reasoner.is_reasoning_end_streaming(...) on the running prefix. Once the marker is
observed, flip apply_bitmask=True so all subsequent positions in the same step
(including the bonus row) get the grammar-constrained mask. The marker token itself
is reasoning content, so grammar advancement is skipped for that position.

2. Bonus row inherits stale apply_bitmask from -1 padding

In the async spec-decode path, update_draft_token_ids_in_output pads invalid draft
slots with -1. The previous loop iterated itertools.chain(req_tokens, (-1,)) and
set apply_bitmask=False on the first -1. The trailing -1 (the bonus-token slot)
therefore inherited False whenever any earlier draft was padded, and the bonus row
was filled with the unconstrained mask — re-introducing exactly the same class of
failure as (1) but at the bonus position.

Fix: compute the bonus row as should_fill_bitmask(request) or apply_bitmask, so a
mid-window reasoning-end also reaches the bonus row and -1 padding can no longer
flip it to the unconstrained mask.

3. Harden the mid-window advance against grammar-invalid drafts (0ff3348f2)

Fix (1) introduced a new path: once the marker is observed mid-window, the drafts
that follow it are fed to grammar.accept_tokens. Those drafts were produced before
the grammar became active and are not guaranteed valid. With a permissive grammar
(structural_tag) they are usually accepted, but with a strict-start grammar
(response_format={"type": "json_object"}) the first post-marker draft can be
rejected, and the old code raised AssertionError. Skipping advancement entirely
instead froze the bitmask at the grammar's initial state, letting the opening token
repeat (e.g. {{).

Fix: still attempt to advance through post-marker drafts so the next bitmask row
reflects the advanced state, but tolerate accept_tokens rejection at those
positions instead of asserting; advancements within the window are rolled back at
the end so the persisted grammar state is unchanged.

4. Post-#42452: keep reasoning content out of the scheduler's grammar advance (6b1b0ca8f)

Rebasing onto current main brought in #42452, which makes should_advance return
True at the reasoning-boundary step for structural tags + speculative decoding, so
the FSM can advance through trigger text in the same step. As a side effect the
scheduler now feeds that step's tokens straight into grammar.accept_tokens, and
they still contain reasoning content up to and including the end marker (e.g.
[198, 248069] = "\n</think>"). The structural-tag grammar excludes the marker,
accept_tokens rejects it, and the request dies with FINISHED_ERROR — re-surfacing
#44006 on the rebased branch. On the pre-#42452 base this path was unreachable
because should_advance returned False at the boundary.

Fix: keep #42452's same-step advance, but record the marker's absolute index when
the boundary fires (should_advance) and drop everything up to and including the
marker before the scheduler advances (trim_reasoning_for_advance). A step that is
entirely reasoning content skips the advance, matching the pre-#42452 behavior for
that shape. The sampling side (grammar_bitmask, which never advances through the
marker) and the accept side now agree: reasoning content never reaches the grammar.

Why this is not a duplicate

Checked open PRs:

gh pr list --repo vllm-project/vllm --state open --search "44006 in:body"
gh pr list --repo vllm-project/vllm --state open --search "structured_output speculative bitmask"

No open PR addresses these failure modes. PR #25515 introduced the reasoning_ended
machinery itself; this PR fixes its interaction with multi-position spec windows.
#42452 (merged) fixes FSM advancement for structural tags in should_advance; this
PR is complementary: it fixes the missing bitmask constraints inside the spec window
and trims reasoning content from the tokens that #42452 now routes into
accept_tokens.

Test Plan

Unit tests

tests/v1/spec_decode/test_mtp_structured_output.py (added):

• -1 padding still produces an N+1 row bitmask with terminated FSM state correct
• Mid-window grammar termination after EOS
• Bitmask idempotent across repeated calls
• Bonus row stays constrained after -1-padded drafts (regression for defect 2)
• Mid-window reasoning-end keeps post-marker positions constrained (regression for defect 1)
• Post-marker draft the grammar rejects skips the advance instead of asserting (regression for fix 3)
• validate_tokens → pad with -1 → grammar_bitmask round-trip does not assert
• Boundary step records the reasoning-end marker index (regression for fix 4)
• trim_reasoning_for_advance drops the marker and everything before it, passes
  later steps through unchanged, and trims the [198, </think>] crash shape to
  empty (regression for fix 4)

Also re-ran neighbouring tests to confirm no regression:

pytest tests/v1/spec_decode/test_mtp_structured_output.py -v
pytest tests/v1/structured_output/ -v
pytest tests/v1/spec_decode/test_backup_token_async_spec.py \
       tests/v1/spec_decode/test_rejection_sampler_utils.py \
       tests/v1/spec_decode/test_synthetic_rejection_sampler_utils.py -v

Result:

• test_mtp_structured_output.py: 16 passed (7 cases × xgrammar / guidance, + 2 manager-level cases for fix 4; re-run on the rebased branch)
• tests/v1/structured_output/: 20 passed
• spec-decode rejection-sampler / backup-token subset: 34 passed

End-to-end reproducer

2× H20-96GB, TP=2, Qwen/Qwen3.5-35B-A3B + MTP(k=1) + --reasoning-parser qwen3 +
--tool-call-parser qwen3_coder + VLLM_ENFORCE_STRICT_TOOL_CALLING=1. Fire 50×
the Korean weather tool-calling request from the issue against each branch.

Server:

export VLLM_ENFORCE_STRICT_TOOL_CALLING=1
vllm serve Qwen/Qwen3.5-35B-A3B \
  --tensor-parallel-size 2 \
  --speculative-config '{"method":"mtp","num_speculative_tokens":1}' \
  --reasoning-parser qwen3 \
  --enable-auto-tool-choice \
  --tool-call-parser qwen3_coder \
  --max-model-len 8192 \
  --max-num-seqs 8 \
  --gpu-memory-utilization 0.85 \
  --enable-chunked-prefill \
  --enable-prefix-caching \
  --enable-log-requests \
  --limit-mm-per-prompt '{"image":0,"video":0}' \
  --trust-remote-code \
  --enforce-eager \
  --port 8000

Branch	200 OK	HTTP 500	Failure rate
upstream/main (baseline)	21	29	58%
this PR	50	0	0%

Baseline vllm serve log shows the exact error from the issue:

ERROR ... [backend_xgrammar.py:158] Failed to advance FSM for request ... for token 248069. Please file an issue.
ERROR ... [scheduler.py:1429] Unexpected: grammar rejected tokens [..., 248069] for request ...

After this PR, no such error appears across 50 trials.

Re-ran the same 50-trial reproducer after 0ff3348f2: still 50/50, no regression.

Re-validation after the rebase (fix 4)

Same model/flags on 4× NVIDIA H20-3e 143GB, TP=4, branch rebased onto fe04238
(post-#42452 main):

Branch	200 OK	HTTP 500	Failure rate
rebased, without fix 4	26	24	48%
rebased, with fix 4 (6b1b0ca8f)	50	0	0%

The without-fix-4 failures reproduce the original signature
(grammar rejected tokens [198, 248069]), confirming the accept-side path that
fix 4 closes.

---

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[yuyue0225sc]

This PR fixes #44006, an internal-server-error that affects structured output requests when MTP/EAGLE speculative decoding pads invalid drafts with -1. The change is small (around 20 lines in vllm/v1/structured_output/init.py) and is covered by 5 new regression tests. Local pre-commit passes. Would appreciate a review when time allows. Thanks!

[cjackal]

It seems this PR alone does not fix #44006, I still got the same internal server error for qwen3.5 mtp.

[yuyue0225sc]

It seems this PR alone does not fix #44006, I still got the same internal server error for qwen3.5 mtp.

Thanks for taking the time to retest and report back — you were absolutely right that the original version didn't fix it. I just pushed an updated fix (3941ad2) targeting what I now believe is the actual root cause (a mid-window reasoning boundary, not the bonus row). I'll run an end-to-end repro on Qwen3.5 + MTP tomorrow and share the numbers here. Really appreciate the careful feedback.

[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.

[yuyue0225sc]

Hi @cjackal , opened #44297 to fix this. Reproduced your reproducer on 2×H20, Qwen3.5-35B-A3B + MTP(k=1) +
qwen3_coder + VLLM_ENFORCE_STRICT_TOOL_CALLING=1, 50 trials per branch:

Branch	200 OK	HTTP 500
main baseline	21	29
#44297	50	0

Baseline log shows the same Failed to advance FSM ... for token 248069 you reported; with the PR no such error
across 50 trials.

If you can confirm on your Qwen3.5-397B-A17B-FP8 + MTP=1 setup that the 500s go away, that would be the cleanest
sign-off.

[yuyue0225sc]

Hi @aarnphm @russellb , would either of you have a chance to take a look at this when you have time? This change
touches the reasoning-aware structured-output path introduced in #25515, and only surfaces when speculative decoding
is enabled, so I'd really appreciate your eyes on it given your familiarity with this area.

Brief summary — two related defects in StructuredOutputManager.grammar_bitmask:

1. When MTP/EAGLE schedules multiple positions per step and </think> lands mid-window, the bitmask kept the
   start-of-step apply_bitmask, so post-marker positions got the unconstrained "in-reasoning" mask. Fixed by calling
   reasoner.is_reasoning_end_streaming per simulated draft and flipping apply_bitmask on observation.
2. The bonus row inherited apply_bitmask=False from -1-padded draft slots. Fixed by computing the bonus row as
   should_fill_bitmask(request) or apply_bitmask.

End-to-end on 2×H20, Qwen3.5-35B-A3B + MTP(k=1) + strict tool calling: baseline 29/50 fail → this PR 0/50 fail (same
error reported in #44006). Regression tests added for both failure modes.

Also cc'ing @njhill / @benchislett from the spec-decode side in case you'd like an additional look — no pressure,
just thought you might be interested. Thanks in advance for any feedback!

[yuyue0225sc]

Update: while validating the sibling fix #44993 with response_format={"type":"json_object"}, I found the mid-window advance path added here could raise AssertionError on strict-start grammars. Pushed 0ff3348f2 to harden it (details added as section 3 of the description), plus a regression test. Re-ran the 50-trial reproducer from the issue: still 50/50, no regression.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @yuyue0225sc.

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