[Bugfix][Spec Decode] Restrict embedding-width share guard to EAGLE drafts by evantakahashi · Pull Request #47953 · vllm-project/vllm

evantakahashi · 2026-07-08T03:20:35Z

Purpose

Fixes #47794 — Gemma4 MTP fails engine initialization on nightly with a linear shape mismatch ([s47, 3840] x [5632, 1024]).

Root cause: #43957 added an embedding-width guard to _maybe_share_embeddings that disables target↔draft embedding sharing when the widths differ. That is correct for EAGLE drafts, which consume input_embeds in their decoder layers at their own hidden size (the Eagle3MiniMaxM2 XPU case #43957 fixed). It is wrong for MTP drafts: Gemma4 MTP's pre_projection is built as Linear(2 * backbone_hidden_size, hidden_size) and its draft-width embed_tokens exists only to populate the tied draft-dim lm_head — gemma4_mtp.py explicitly documents that sharing replaces it with the backbone-width table. With sharing disabled, the concat width becomes 1024 + 2816 = 3840 instead of 2816 + 2816 = 5632, crashing at init.

Fix: apply the width guard only to EAGLE drafts, keyed on the has_own_embed_tokens attribute — the existing EAGLE/MTP discriminator in this method. MTP drafts return to unconditional sharing (pre-#43957 behavior); the #43957 EAGLE behavior is preserved.

Relationship to existing PRs

I claimed #47794 in the issue at 04:13 UTC with this diagnosis and fix plan (comment); #47833 was opened at 07:17 UTC without engaging that claim. Beyond the overlap, this PR adds runtime verification driving the real Gemma4MTP module (see below), which #47833 notes it could not do, plus an additional MTP dims-match regression case. Happy to consolidate whichever way maintainers prefer.

Test Plan

pytest tests/v1/spec_decode/test_share_embeddings.py -v

Runtime A/B verification (CPU, no GPU needed): instantiated the real Gemma4MTP module (0 decoder layers, draft width 8, backbone width 32) plus a target with a backbone-width embedding table, ran the real SpecDecodeBaseProposer._maybe_share_embeddings, then the real Gemma4MTP.forward — the exact seam that crashed in the issue.

Test Result

On main (unfixed): new test test_mtp_shares_embeddings_when_dims_differ FAILS; runtime harness reports embeddings shared: False and forward crashes with mat1 and mat2 shapes cannot be multiplied (4x40 and 64x8) — the scaled analogue of the issue's 3840 x 5632 mismatch.
With this PR: all 4 tests PASS; harness reports embeddings shared: True, forward OK: draft_hidden=(4, 8) backbone_hidden=(4, 32), and the tied draft-dim lm_head is left intact.
EAGLE regression coverage: dims-differ stays unshared ([XPU] Fix Eagle3 initialization on XPU #43957 behavior), dims-match shares.
pre-commit (ruff check/format, mypy hook, typos) passes on changed files.
Not run: end-to-end GPU serve with the 26B checkpoint (no hardware) — @realmorita, could you confirm on your setup?

AI assistance (Claude) was used for the analysis, fix, tests, and verification; submitted after human review.

🤖 Generated with Claude Code

…rafts MTP drafts project target-width embeddings (e.g. Gemma4 MTP's pre_projection takes 2 * backbone_hidden_size) and rely on sharing to replace their draft-width embed_tokens, so the width guard from vllm-project#43957 must only apply to EAGLE drafts. Fixes vllm-project#47794 Signed-off-by: Evan Takahashi <evan10takahashi@gmail.com>

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github-actions · 2026-07-08T03:20:44Z

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benchislett

I don't think we need a whole new test file for this. Please remove. Fix looks good to me

Signed-off-by: Evan Takahashi <evan10takahashi@gmail.com>

evantakahashi · 2026-07-09T05:06:37Z

Done — removed the test file in e1366d0. Thanks for the review!

realmorita · 2026-07-09T05:45:58Z

Confirmed on my setup with the 26B checkpoint. Engine initialization succeeded, and a short chat completion request completed successfully.

Thanks @evantakahashi for the quick fix!

anencore94 · 2026-07-16T04:38:33Z

Same issue here. Look forward to be merged soon !

Mazyod · 2026-07-18T06:17:56Z

Validated this patch end-to-end on a real production config (Gemma-4-31B fp8_block + MTP draft, V1 model runner) on v0.25.1: it boots and serves. 🙏

We hit #47794 in production: Gemma-4-31B (compressed-tensors / fp8-block main) + its MTP assistant draft, VLLM_USE_V2_MODEL_RUNNER=0, --reasoning-parser gemma4, --speculative-config '{"method":"mtp",...,"num_speculative_tokens":2}'. Boot crashes in the dummy run at gemma4_mtp.py pre_projection:

RuntimeError: mat1 and mat2 shapes cannot be multiplied (2496x6400 and 10752x1024)

The draft's 1024-wide input embedding concatenated with the 5376-wide backbone hidden gives 6400, but pre_projection is Linear(2 * backbone_hidden_size = 10752, ...). We independently bisected it to the embedding-share guard added in #43957 (git log <pre-#43957>..v0.25.1 -- vllm/v1/spec_decode/llm_base_proposer.py is exactly that one commit): for this checkpoint draft_dim (1024) != target_dim (5376), so the guard flips share_embeddings=False and the draft keeps its own 1024-wide input embedding, breaking the projection.

Applied this PR's change on top of vllm/vllm-openai:v0.25.1 and re-ran the exact serve command, nothing else touched:

Before: crash at boot (dummy_run into pre_projection).
After: boots cleanly and serves. The log shows Detected MTP model. Sharing target model embedding weights with the draft model. with no subsequent Keeping separate embedding weights line, and a test completion returns normally.

So the fix resolves the crash on a different config (fp8-block main, V1 runner, enforce-eager) than the issue's original repro. The EAGLE vs MTP distinction via has_own_embed_tokens reads correctly to us: the MTP draft's own 1024-wide lm_head stays intact while only the input embedding becomes the shared 5376-wide one. Thanks for the fix!

lucianommartins · 2026-07-18T23:39:36Z

Hi @ywang96 @Isotr0py @benchislett @LucasWilkinson - looks like #43957 caused catastrophic side effects - Gemma4 MTP is broken as reported by #48848

could we please expedite this merge? or if you think it is not ready to be merged, can we revert #43957?

eugr · 2026-07-20T20:40:57Z

@mgoin - fyi, gemma 4 MTP is still broken in vllm nightly

Patch 0001 was retired at the v0.26.0 gate on a leave-one-out probe that passed. On 2026-08-06 the resulting unpatched image crash-looped a production Gemma-4-31B + MTP deployment with that patch's exact signature: `a and b must have same reduction dim, [s47, 6400] X [10752, 512]` — the 1024-wide draft embedding concatenated with the 5376-wide backbone hidden state, fed to a pre_projection expecting 2 * 5376. The mechanical check disagreed with that verdict at the time and was right: git merge-base --is-ancestor b2b8f67 v0.26.0 # fails Upstream vllm-project#47953 missed the v0.26.0 release branch by one day. It is first present in v0.26.1rc0. Patch 0002 is restored on the same evidence — its fix (vllm-project#44993, 0416dab) is likewise not an ancestor of v0.26.0, and its retirement rested on the same probe machinery. Also regenerates 0003 against v0.26.0 rather than a neighbouring tree: it was applying with fuzz and leaving a .orig file behind, which failed the apply/revert round-trip. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Signed-off-by: Mazyad Alabduljaleel <maz@level3.io>

Ancestry (witness one) verified 2026-08-11: b2b8f67 (0001/vllm-project#47953), 0416dab (0002/vllm-project#44993) and de6ec29 (0003/vllm-project#49302) are all ancestors of v0.27.1. Witness two is the release gate firing traffic through the patchless candidate before anything is promoted. - fork/patches: series and upstream.map emptied, patch files and notes deleted; README keeps the retirement record (0001's false retirement at v0.26.0 included) - pins bumped to v0.27.1 in the workflow (DEFAULT_BASE_TAG) and Dockerfile.audio (ARG BASE_TAG, what check-alignment reads) - bench: minus-arms and the 0003 waiver removed; R6/launcher/report tests now build synthetic minus profiles so the machinery stays tested for the day a patch returns - ledger: three new upstream CI-command bot workflows declared deleted - FORK.md: sync procedure corrected to merge the release TAG (tags are cut aside from main; check-alignment requires HEAD to sit on the pin), candidate/promote flow documented Co-authored-by: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Mazyad Alabduljaleel <maz@level3.io>

evantakahashi requested review from MatthewBonanni, benchislett and luccafong as code owners July 8, 2026 03:20

claude Bot reviewed Jul 8, 2026

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evantakahashi mentioned this pull request Jul 8, 2026

[Bug]: Gemma4 MTP fails to start after embedding sharing guard in PR #43957 #47794

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mergify Bot added speculative-decoding v1 bug Something isn't working labels Jul 8, 2026

benchislett approved these changes Jul 9, 2026

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benchislett added the verified Run pre-commit for new contributors without triggering other tests label Jul 9, 2026

Remove standalone test file per review

e1366d0

Signed-off-by: Evan Takahashi <evan10takahashi@gmail.com>

himorishige mentioned this pull request Jul 17, 2026

[Bug]: Gemma4 MTP speculative decoding crashes at engine init on 0.25.1 — "a and b must have same reduction dim" (regression from 0.21.0) #48848

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Mazyod mentioned this pull request Jul 18, 2026

[fork] Deterministic-tag patch model + migrate audio image build Mazyod/vllm#1

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mgoin added the ready ONLY add when PR is ready to merge/full CI is needed label Jul 20, 2026

mgoin approved these changes Jul 20, 2026

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Merge branch 'main' into fix-gemma4-mtp-embedding-sharing

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mgoin mentioned this pull request Jul 20, 2026

[CI] Add gemma-4-E4B-it-assistant to CI gsm8k for GemmaMTP #49243

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vllm-bot merged commit b2b8f67 into vllm-project:main Jul 21, 2026
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Mazyod mentioned this pull request Jul 25, 2026

[fork] Bump base tag to v0.26.0 and refresh the patch series Mazyod/vllm#6

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Chishenzheng mentioned this pull request Aug 28, 2026

[Bugfix][Spec Decode] Avoid temporary Qwen3.5 MTP vocab allocations #54201

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[Bugfix][Spec Decode] Restrict embedding-width share guard to EAGLE drafts - #47953

[Bugfix][Spec Decode] Restrict embedding-width share guard to EAGLE drafts#47953
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