[Bugfix][Model Runner V2][Spec Decode] Fix off-by-one in bad_words draft-prefix matching

[jyan-R]

Purpose

Fix an off-by-one in the spec-decode branch of _bad_words_kernel (vllm/v1/worker/gpu/sample/bad_words.py), present since the kernel was introduced in #33433.

The sampler passes input_ids gathered at logits_indices, so for a spec-decode request the per-request local layout is: local position 0 = last committed sampled token, local position j = draft token d_{j-1}. The sibling kernels index draft tokens accordingly (_penalties_kernel uses start_idx + prev_pos + 1; thinking budget's _load_effective_token uses + 1 with an explicit comment). _bad_words_kernel read input_ids_ptr + cur_req_first_pos + spec_offset without the + 1, so every draft-region read was shifted back by one token (the boundary token was read twice).

Consequences when bad_words and speculative decoding are combined and a multi-token bad word's prefix window reaches into the draft tokens:

• prefix matches ending in the draft region are missed (the banned token can be sampled/accepted);
• matches spanning the committed/draft boundary fire one verification row late;
• the duplicated boundary read can spuriously match and ban an innocent token.

Single-token bad words and non-spec rows are unaffected. Model Runner V1 has a separate implementation and is unaffected.

Not a duplicate: searched open PRs/issues for bad_words + spec decode / off-by-one / _bad_words_kernel; no existing fix found (closest is #34213, which only touches penalties).

Test Plan

New unit test (first coverage for this kernel), following the pattern of tests/v1/worker/test_gpu_thinking_budget.py:

pytest -v tests/v1/worker/test_gpu_bad_words.py

Scenario: committed output [10, 11], draft tokens [12, 13]. The core regression case is the boundary-spanning prefix ([11, 12, 30] must mask at the row where the prefix completes, not one row later); the other three cases pin the remaining coverage axes: a positive match at draft offset >= 1 ([12, 13, 40]), a no-double-count guard for the boundary token ([11, 11, 50]), and a committed-only baseline ([10, 11, 60]). Assertions compare the full logits tensor.

Test Result

Ran on an A100 SXM4 80GB (torch 2.13.0+cu130, triton 3.7.1, Python 3.11), with the branch installed editable via VLLM_USE_PRECOMPILED (nightly x86_64 wheel 66728feb1):

• With the fix (HEAD): 4 passed.
• With the pre-fix kernel (parent commit's bad_words.py swapped in): 3 failed, 1 passed — exactly the three spec-branch cases fail; the committed-only baseline passes.
• ruff check / ruff format --check pass on both changed files.
• Cross-validation: the kernel's scalar loop was transcribed to pure Python and checked against an independent reference matcher over 20,000 randomized spec-decode scenarios; with the + 1 the kernel matches the reference in all cases, without it ~24% of scenarios diverge.

AI assistance was used for this change (Claude); I have reviewed every changed line.

[github-actions]

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[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.

[Copilot]

Pull request overview

Fixes a speculative-decoding-only off-by-one in the Model Runner V2 Triton _bad_words_kernel so draft-prefix matching reads the correct draft tokens (avoiding missed masks and spurious boundary matches), and adds initial GPU unit coverage for this kernel.

Changes:

• Adjust speculative branch indexing in _bad_words_kernel by + 1 to correctly align input_ids local layout for drafts.
• Add a new CUDA/Triton-backed unit test suite covering committed-only, draft-only, and committed/draft-boundary bad-words prefix cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
vllm/v1/worker/gpu/sample/bad_words.py	Fix speculative-draft token indexing in bad-words prefix matching and document the local input_ids layout.
tests/v1/worker/test_gpu_bad_words.py	New GPU unit tests validating masking behavior across committed vs draft token boundaries for speculative decoding.

Suppressed comments (1)

tests/v1/worker/test_gpu_bad_words.py:62

• idx_mapping_np / expanded_idx_mapping also hard-code the request index as 3, which couples this test to RequestState's internal slot allocation. Compute the request index from the created state and use it consistently for the mappings.

    idx_mapping_np = np.array([3], dtype=np.intp)
    expanded_idx_mapping = torch.tensor(
        [3] * num_logits, dtype=torch.int32, device=DEVICE
    )

---

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[njhill]

Thanks @jyan-R

[github-actions]

✅ @jyan-R, CI is now available for this PR.

• /ci run starts a CI build.
• /ci retry retries failed jobs in the CI build for the current PR head. If the current head has no CI build, it starts a new CI build for the current head containing only jobs that failed in the latest earlier CI build for this PR.
• /ci cancel cancels scheduled or running CI builds for this PR branch.

[njhill]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83984 for commit 04777f0c269c.

[jyan-R]

/ci retry

[github-actions]

✅ Queued 2 failed job(s) for retry in Buildkite CI #83984.

[jyan-R]

CI status note: the remaining red job Multi-Modal Models (Standard 4, other + whisper) failed twice (initial + retry) with an identical, PR-unrelated signature — gemma4 engine-core warmup crashing on a PyTorch-internal NVML assert (CUDACachingAllocator.cpp:1407), preceded by stale torch_compile_cache cubin misses on the node. The AMD mirror of the same suite passed in this build, and the jobs covering this PR's area (v1-sample-plus-logits, v1-others-cpu) are green. Filed #52403 to track the infra failure; not retrying further to avoid burning CI.

[jyan-R]

Build #83984 finished. Final tally: everything green except (a) cpu-language-generation-and-pooling-model-tests — Expired, never picked up by an agent, so it has not actually run; and (b) the whisper job tracked in #52403. Issuing one /ci retry to requeue the expired job; the whisper job is expected to stay red unless the node's stale compile cache cleared (see #52403).
🤖 Posted by Claude Code on behalf of @jyan-R

[jyan-R]

/ci retry

[github-actions]

✅ Queued 2 failed job(s) for retry in Buildkite CI #83984.

[jyan-R]

/ci retry

[github-actions]

✅ Queued 1 failed job(s) for retry in Buildkite CI #83984.