[Bug]: NVFP4 MoE monolithic backend (trtllm_fp4_block_scale_moe) produces semantically corrupt output for non-gated ReLU² MoE (Nemotron-3-Nano) on SM100; flashinfer_cutlass is correct on the same GPU

[ldkhang1201]

Summary

Serving nvidia/NVIDIA-Nemotron-3-Nano-30B-A3B-NVFP4 (NemotronH hybrid Mamba-Transformer MoE, ModelOpt NVFP4, FP8 KV cache) with default backend selection on SM100-family GPUs (tested: B200) produces grammatically fluent but semantically corrupted output — 100% deterministic under greedy decoding. The same checkpoint on the same vLLM version is correct on SM120 (RTX PRO 4500 Blackwell).

An A/B/A experiment on the same B200 isolates the fault to the auto-selected FLASHINFER_TRTLLM monolithic NVFP4 MoE backend: adding --moe-backend=flashinfer_cutlass fully fixes the output on the same GPU with the same checkpoint; reverting to the default reproduces the broken logprobs bit-identically.

Environment

• vLLM v0.27.0 (the relevant dispatch code is byte-identical in v0.26.0; the flashinfer pin changed 0.6.14 → 0.6.16.post3 between the tags)
• flashinfer-python 0.6.16.post3 (per requirements/cuda.txt)
• Broken: B200 (compute capability 10.0). Clean: RTX PRO 4500 Blackwell (12.0)
• Model: nvidia/NVIDIA-Nemotron-3-Nano-30B-A3B-NVFP4 — non-gated ReLU² MoE, 128 experts, top-6, sigmoid+bias grouped top-k with n_group=1, hidden_size=2688, kv_cache_quant_algo: FP8, group_size: 16
• TP=1, PP=1, otherwise default flags

Reproduction

vllm serve nvidia/NVIDIA-Nemotron-3-Nano-30B-A3B-NVFP4 --tensor-parallel-size 1

curl -s localhost:8000/v1/completions -H 'Content-Type: application/json' \
  -d '{"model":"nvidia/NVIDIA-Nemotron-3-Nano-30B-A3B-NVFP4","prompt":"The capital of France is","max_tokens":16,"temperature":0}'

Expected: Paris… — Actual on B200 with the default backend: **"The Last of Us"** (2013) – a

Other 100%-deterministic failures at temperature 0: Question: What is 2+2? Answer: never contains 4; the model cannot continue 1, 2, 3, 4,.

Evidence

Top-5 logprobs for The capital of France is (greedy, 1 token), same B200, same checkpoint:

MoE backend	top-5 logprobs
default (FLASHINFER_TRTLLM monolithic)	' **' -2.0724, '.' -2.3224, ' ' -2.5099, ',' -2.7599, '**' -2.8849 — no content token in the top-5
--moe-backend=flashinfer_cutlass	' Paris' -0.0278 — healthy

Two forensic details:

1. A/B/A determinism: switching back to the default reproduces the broken logprobs bit-identically to 10 decimal places across restarts — this is a deterministic contract/kernel defect, not a race.
2. The broken logits sit on a 1/16 lattice: the gaps between the top-5 raw logits are exactly 4/16, 3/16, 4/16, 2/16. Healthy bf16 logits don't do that; it suggests the MoE contribution to the residual stream is coarsely mis-scaled/degenerate rather than "wrong but rich" — pointing at scale application rather than e.g. wrong expert selection.

During the healthy flashinfer_cutlass run, every other SM100-default path was still active (trtllm-gen FP8-Q attention, CuTe-DSL dense NVFP4 GEMMs), so those are exonerated; the defect is fully contained in what --moe-backend swaps.

Analysis (v0.27.0 tree)

Selection chain: ModelOptNvFp4FusedMoE → select_nvfp4_moe_backend (vllm/model_executor/layers/fused_moe/oracle/nvfp4.py) tries FLASHINFER_TRTLLM first. TrtLlmNvFp4ExpertsMonolithic._supports_current_device() requires is_device_capability_family(100) — which is why SM120 never runs this path (matching the clean/broken hardware split) — and _supports_no_act_and_mul() returns True with the comment "(i.e. Nemotron-Nano)", making this model the primary user of the non-gated variant.

The monolithic bundle differs from the modular flashinfer_cutlass path in several coupled ways; the defect must live in one of these:

1. Hidden-size padding contract: expert weights zero-padded 2688 → 2816 (round_up(…, 256)), activations padded at runtime and the output sliced back; after nvfp4_block_scale_interleave + epilogue tile shuffle, each row has 168 real + 8 padded scale groups. A scale offset/stride mismatch here best fits the 1/16-lattice logits.
2. Non-gated (ReLU²) scale plumbing: g1_scale_c = a2_gscale.clone(), output1_scale_scalar = 1/a2_scale, output1_scale_gate_scalar = w13_scale_2 * a13_scale — gate-slot scalars reused for a MoE that has no gate half; the contract with the flashinfer kernel is implicit.
3. In-kernel routing: flashinfer.fused_moe.trtllm_fp4_block_scale_moe computes DeepSeekV3-style routing internally (degenerate n_group=1, top-6 of 128, fp32 sigmoid+bias) vs vLLM's own ops.grouped_topk on the modular path.
4. Activation scale layout: linear (non-swizzled) — unique to this backend.
5. routing_replay_out kwarg (new call-site argument in v0.27.0) ties behavior to the installed flashinfer signature.

Suggested immediate mitigation

Until the monolithic non-gated path passes a numeric parity test, TrtLlmNvFp4ExpertsMonolithic._supports_no_act_and_mul() should return False so auto-selection falls back to FLASHINFER_CUTLASS (verified correct for this model on both SM100 and SM120). Longer-term: a CI parity test (monolithic vs modular vs QDQ emulation) on a small non-gated ReLU² MoE with hidden size not divisible by 256 would catch this entire defect class.

User workaround until then: --moe-backend=flashinfer_cutlass.