Qwen3.8-Flash-Next-NVFP4

Qwen3.8-Flash-Next — NVFP4 (experts-only, 4/6-optimal scales)

TL;DR — the plain-English version

What this is: Qwen3.8-Flash-Next — Qwen's fast 125B mixture-of-experts model (only ~6B parameters active per token, so it's quick) — shrunk from 360 GB to ~174 GB, with the quality-critical parts left completely untouched.

How: the model's "experts" are stored in NVFP4, a 4-bit format NVIDIA Blackwell GPUs run natively. Attention, the vision system, the speculative-decoding head, and the model's big 51 GB n-gram memory (PLE) stay at full precision.

Is the quality still good? Measured per-tensor error is ~8.7 % relative — good for 4-bit — using a scale-selection trick that cuts error ~16 % vs how most quants are made. As an extra safety check, our conversion was cross-verified against Qwen's own official FP8 release, expert by expert.

Can I run it? It's built for Blackwell GPUs via vLLM (the same dedicated image that serves the official FP8 release — this architecture runs on RTX PRO 6000-class cards today). Fair warning, stated plainly: we haven't completed our own end-to-end serving run of this file yet — it's verified numerically and structurally, and it uses the exact tensor layout the vLLM loader already consumes in production, but until we (or you!) serve it, treat it as very-likely-working rather than proven. This note will be updated.

VRAM tip: set VLLM_PLE_CPU_OFFLOAD=1 to keep the 51 GB n-gram memory in system RAM — GPU weight footprint drops to ~90 GB (two 96 GB cards territory).

New to quantization? Think high-quality JPEG vs RAW: much smaller, nearly indistinguishable — and this card publishes the exact measurements of what was lost.

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Weight-only NVFP4 (NVFP4A16) quantization of Qwen/Qwen3.8-Flash-Next — the 125B-A6B qwen4_exp hybrid MoE (Gated DeltaNet + Qwen Sparse Attention, PLE n-gram embedding, MTP, vision) — in standard compressed-tensors nvfp4-pack-quantized format. ~174 GB.

Quantized from the true BF16 release (360 GB, pinned f5d08274), not the FP8 export.

What makes this build different

• 4/6-optimal block scales: per 16-element block, the scale is chosen by exact post-rounding squared error between the classic amax/6 plan and ModelOpt's four-over-six plan — never worse than plain RTN by construction. On this model: −16.11 % total squared error vs plain RTN; 44.6 % of blocks chose M=4.
• Split-convention cross-check: the BF16 release stores experts stacked ([512, out, in], fused gate_up). This build unstacks them into the per-expert layout of Qwen's own official FP8 export — and cross-checks every sampled expert against that FP8 export (mean rel-err 0.0266 ≈ pure fp8 noise), ruling out gate/up-split errors by construction.

What is quantized, what is not

• NVFP4: routed-expert weights only (≈ 121B of the parameters).
• BF16 (untouched): the 51B PLE n-gram embedding, all attention (Qwen Sparse Attention + DeltaNet), shared experts, router gates, embeddings, lm_head, the full vision tower (byte-identical, verified per-tensor: 333 tensors), and the entire MTP tree (speculative decoding stays full-precision).

Serving notes

The qwen4_exp architecture requires the dedicated vLLM image (vllm/vllm-openai:qwen38-flash-next) until upstream support lands. Recipe-critical args: --max-num-seqs 256 (Mamba cache blocks; lower values error at startup), --no-enable-flashinfer-autotune, VLLM_ENGINE_READY_TIMEOUT_S=3600.

• The architecture serves on SM120 (RTX PRO 6000) today — we run the official FP8 release on 4×96 GB in production. Marlin is among the listed NVFP4 MoE backends for this arch.
• Honesty note: this NVFP4 checkpoint has been verified structurally and numerically (census, FP4-grid, FP8-export cross-check, vision byte-equality) but we have not yet served it end-to-end. The tensor layout mirrors the official FP8 export that the vLLM loader consumes in production, so loader-naming risk is low. This note will be updated when we complete a serving run.
• VRAM lever: VLLM_PLE_CPU_OFFLOAD=1 moves the 51B PLE embedding to host RAM (~90 GB on-GPU weights; needs ~+51 GB pod/host RAM).

Verification methodology

Quantization math bit-exact vs an llmcompressor NVFP4A16 oneshot and byte-identical vs NVIDIA ModelOpt's NVFP4QTensor export path at the pinned toolchain (torch 2.13.0 / compressed-tensors 0.18.0 / safetensors 0.8.0). Streaming shard-by-shard conversion (model never materialized). MANIFEST.sha256 covers every file. Built 2026-08-26.

Licensed under the Qwen Community License 1.0 (included), as is the base model.

Quantized by dabsLabs.