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…erministic MoE finalize FlashInfer's cutlass_fused_moe reduces the top-k expert outputs in a fused epilogue that uses atomics and is documented as nondeterministic; vLLM never passed `use_fused_finalize`, so there was no way to get reproducible logits from the FlashInfer CUTLASS MoE path. On Qwen3.8-Flash-Next (NVFP4) on a GB10, identical requests at temperature 0 returned different logits, traced by per-module hashing to `mlp.experts`; passing `use_fused_finalize=False` makes all tokens bit-identical across requests (verified in three serving shapes). Default unchanged (fused). Fixes vllm-project#54945 Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_011SuBgdp87NbfLbiigmzn1z Signed-off-by: Jürgen Schmied <juergenschmied70@gmail.com>
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Purpose
Fixes #54945.
flashinfer.fused_moe.cutlass_fused_moehas ause_fused_finalizeargument whose default (fused epilogue, atomics) is documented as nondeterministic. vLLM'sflashinfer_cutlass_moe.pynever passed it, so identical requests at temperature 0 could return different logits on this backend with no switch to turn it off. This addsVLLM_FLASHINFER_MOE_FUSED_FINALIZE(default1, behaviour unchanged) and passes it through.Test Plan
temperature=0,max_tokens=4,top_logprobs=20, hashing every token's top-20 (script in [Bug] FlashInfer CUTLASS NVFP4 MoE gives different logits for identical requests (fused finalize);use_fused_finalize=Falseis bit-stable #54945).Test Result
VLLM_FLASHINFER_MOE_FUSED_FINALIZE=0Decode cost of the unfused finalize: +3.6 % in one 8-turn agent-loop measurement (43.92 → 45.50 ms/tok; run-to-run band for this config 42.8–47.7), i.e. indicative.
ruff check/ruff format --checkclean.Note for FlashInfer ≤ 0.6.17: with a populated
VLLM_FLASHINFER_AUTOTUNE_CACHE_DIR, switching the finalize fails at init withInvalid gemm2 profile idbecause the autotune cache key did not distinguish the two runners; fixed in FlashInfer from v0.6.18rc2 (MoERunner.get_cache_key_extras). Use a fresh cache dir on older versions.Includes AI-assisted code (Claude Code); the change and the repro were reviewed and run by me.
Generated with [Claude Code]