[jethac]

Purpose

A chat request carrying multiple audio clips of differing MEL-frame length crashes Gemma 4 during
_process_audio_input:

AttributeError: 'list' object has no attribute 'squeeze'
  File ".../vllm/model_executor/models/gemma4_mm.py", in _process_audio_input
    input_features = audio_input["input_features_padded"].squeeze(1)

input_features_padded is a MultiModalFieldConfig.batched("audio") field. Its batched() re-pad is a
no-op when the per-clip MEL-frame counts differ, so the field arrives as a list of per-clip tensors
instead of one stacked tensor, and .squeeze(1) fails on the list. Single-clip prompts (a single tensor)
are unaffected, which is why this only triggers with two or more audio clips of unequal duration.

Fix: a shared stack_audio_input_features() helper pads each clip to the batch-max frame count and stacks
into [bn, s_max, f] (building the validity mask alongside), keeping the single-tensor fast path. Both
Gemma4ForConditionalGeneration (gemma4_mm) and the unified variant (gemma4_unified) carried the same
bug and now share the helper.

Note on fix placement: the root cause is that batched("audio") does not re-pad variable-length MEL
features, so this could in principle affect any future consumer of the field. I fixed it at the model
layer (where both Gemma 4 variants converge on one helper) to keep the change tightly scoped, but I am
happy to relocate it to the multimodal field-config / processing layer if you would prefer it solved once
for all consumers.

Test Plan

pytest tests/models/multimodal/processing/test_gemma4_audio_stacking.py

CPU-only, no model weights — the bug is in tensor batching, not the audio encoder.

Test Result

• Before: _process_audio_input raises AttributeError: 'list' object has no attribute 'squeeze' on a
  two-or-more-clip differing-length audio prompt.
• After: clips pad to the batch maximum and stack to [bn, s_max, f]; the new test asserts shapes,
  pad-region zeroing, and mask correctness across the 3-D list, 2-D list, and single-tensor fast paths:

test_list_of_3d_clips_pads_and_stacks PASSED
test_list_of_2d_clips_also_stacks      PASSED
test_single_batched_tensor_fast_path   PASSED
3 passed

---

AI assistance disclosure: this change was developed with AI assistance (Claude); see the Co-authored-by
trailer on the commit. The human author reviewed every changed line, validated the behavior, and ran the
test before submission, per vLLM's AI-Assisted Contributions Policy.

Duplicate-work check

Two other open PRs address the same Gemma 4 audio batching crash in gemma4_mm.py:

• Fix: gemma4 audio batching crash #42573 (ereish64, opened 2026-05-13) — gemma4_mm.py +16/-2, plus a multiaudio.py added at repository root.
• [Bugfix] Fix Gemma4 audio crash with variable-length batched inputs #47459 (fikrikarim, opened 2026-07-02) — gemma4_mm.py +21/-2 with a processing test.

This PR additionally fixes gemma4_unified.py, which neither of the others touches, and covers the audio-tower mask dependency. I have no attachment to which lands: if a maintainer prefers #42573 or #47459, I am happy to close this and port the gemma4_unified.py fix and the mask-dependency note onto whichever is chosen. Flagging the overlap rather than leaving three PRs racing.

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[anshulkulhari7]

The fix looks right, but a heads-up worth surfacing: #44546 ("Gemma4: handle variable-length audio batches", also open, also in gemma4_mm.py) is solving the same bug with essentially the same approach — when input_features_padded arrives as a list of mixed-length clips, both pad each clip to the batch-max frame count via new_zeros, fill, and mask, with a .squeeze(1) fast path for the single batched tensor. The main difference is structural: this PR factors it into a reusable stack_audio_input_features helper (gemma4_mm.py:160) plus unit tests, while #44546 inlines it in _process_audio_input. Probably worth reconciling so only one lands — the extracted helper + test coverage here is the nicer half.

One correctness point worth pulling into the docstring: the zero-padding is only safe because the downstream audio tower honors input_features_mask and never lets the padded frames reach the encoder output. #44546 makes this explicit in its comment ("the audio tower masks padded frames at every SSCP conv ... padding never affects the transcription"); the stack_audio_input_features docstring stops at "padded frames are zero-filled and masked" without stating that the tower actually consumes that mask. Since new_zeros padding would silently corrupt outputs if any encoder path dropped the mask, it'd be good to make that dependency explicit — it's the load-bearing assumption a future refactor could quietly break.

[csantosbh]

Don't let the conflict with #44546 get on the way of merging this; I really don't care which PR gets merged as long as we get the fix in.
Great work @jethac !

[jethac]

Thanks @anshulkulhari7 — agreed, #44546 should own the gemma4_mm.py fix (it's earlier and already approved). This PR is complementary: it also fixes the same variable-length-audio bug in gemma4_unified.py (Gemma4UnifiedForConditionalGeneration, still on the list-unsafe .squeeze(1) around modeling line 426 on main), which #44546 doesn't touch, plus a regression test. Once #44546 lands I'll rebase this down to just the gemma4_unified.py fix + the test so there's no overlap on gemma4_mm.py.

[jethac]

@anshulkulhari7 Addressed in 7bc1b04 — the docstring now states explicitly that the zero-padding is only safe because the audio tower honors input_features_mask and never lets padded frames reach the encoder output. Thanks for flagging it.

[mergify]

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

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

[jethac]

Closing — this was fixed upstream by #50958 ("[Bugfix][Model] Gemma3n/Gemma4: pad variable-length audio batches", merged 2026-08-05), which I should have spotted sooner.

That PR resolves the same defect (.squeeze(1) on input_features_padded failing when per-clip MEL frame counts differ) in both gemma4_mm.py and gemma4_unified.py, via the shared batch_audio_features helper from gemma3n_mm.py — a better factoring than the local stack_audio_input_features here, and it covers Gemma3n too, which this PR did not.

Thanks @anshulkulhari7 for the review back in June; the audio-tower mask dependency you flagged holds in the merged implementation as well.

Also relevant to #42573 and #47459, which target the same bug.