Qwermes3.8 — NVFP4 GGUF (Qwen3.8-27B hybrid, fine-tuned)

NVFP4 GGUF of a fine-tuned Qwen3.8-27B (dense, 64-layer hybrid: 48 Gated DeltaNet linear-attention + 16 full-attention, Qwen3-VL-style wrapper stripped to text-only):

File	Quant	Size	Ollama tag	Verified
Qwermes3.8-nvfp4.gguf	NVFP4 (ModelOpt, MLP-only; attention/DeltaNet Q8_0)	20.0 GB	Qwermes3.8:nvfp4	✅ coherent, 262K ctx

Why this exists: this model family (Qwen3.5/3.8 hybrid GDN) has a nasty GPU-kernel gating trap on workstation Blackwell (sm_120). The GGUF works everywhere via llama.cpp/Ollama; NVFP4 additionally runs on Blackwell FP4 tensor cores. Details below — the benchmarks and the root-cause analysis are the useful part for the community.

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Benchmarks (RTX PRO 6000 Blackwell 96 GB, sm_120, CUDA 13, Ollama 0.33.1)

Fine-tune vs stock: Qwermes3.8 NVFP4 vs base Qwen3.8 (12 prompts × 2, num_ctx 8192, seed 42, thinking off)

Metric	Qwermes3.8 NVFP4 (20 GB)	Qwen3.8 base Q4_K_M (16.8 GB)
Decode avg	53.0 t/s	106.8 t/s
Prefill avg	527.6 t/s	414.3 t/s
Total tokens	1,694	4,820

The 2× decode gap is MTP, not the fine-tune. The stock model ships with a multi-token-prediction head (blk.64.nextn.*); this GGUF was converted --no-mtp. Same-quant control (both Q4_K_M): Qwermes 65.1 t/s vs base 84.1 t/s — the fine-tune costs ~29% decode speed (longer generations, same architecture).

⚠️ Scope: directional smoke test, not a scored eval. 12 hand-written prompts, single run, eyeball quality check. No rubric, no confidence intervals, no contamination check. Scored benchmarks (GSM8K/HumanEval/IFEval via lm-eval-harness) are in progress and will be published here when complete.

Perplexity (wikitext-2 test, full 1.29 MB, llama-perplexity, ngl 99, c 8192)

Quant	PPL
NVFP4	5.9276 ± 0.036

Why NVFP4 is (correctly) slower in vLLM on sm_120 — the GDN kernel gate

Qwen3.8-27B is hybrid: 48/64 layers are Gated DeltaNet (GDN) linear-attention. vLLM's fast GDN backends (FlashInfer/CuteDSL) gate to hardware with tcgen05/TMEM tensor-memory instructions: sm_90 and sm_10x (datacenter Blackwell, B200/GB200) with head_k_dim==128 + CUDA 13. sm_120 (RTX PRO 6000, RTX 5090, PRO 2000-series) is NOT on the list → Triton/FLA fallback:

• vLLM 0.27.1: Using Triton/FLA GDN prefill kernel — decode 42.2 t/s (vs Ollama 52.7), ~2.2s FIXED TTFT regardless of prompt length (measured: 1-token 2.24s, ~10k-token 4.97s).
• vLLM 0.22.0: same gate in source (verified). Also rejects unwrapped text-only configs (Qwen3_5TextConfig vs required VLM Qwen3_5Config).
• Marginal prefill is actually excellent (~5,500–11,000 tok/s measured) — the fixed 2.2s per-request cost dominates short prompts. vLLM is a batch/long-context play here, not an interactive one.
• Community confirmation: ai-muninn / DGX Spark (sm_121) — same wall; vLLM PR #44700 (GDN decode fix, +24% on B200) = zero movement on non-datacenter Blackwell. "The fast road isn't built here."

llama.cpp/Ollama's GDN kernels need no tcgen05 → full speed on sm_120. For interactive single-stream use on workstation Blackwell, Ollama + NVFP4 beats vLLM on every metric.

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NVFP4 build recipe (works — 5 failed attempts distilled)

Do NOT use llmcompressor for NVFP4 GGUFs on this arch. Its single-scale packing (scales) scrambles every dequant in llama.cpp. ModelOpt's two-level weight_scale/weight_scale_2 is what llama.cpp's _repack_nvfp4 expects.

1. Unwrap the VLM wrapper first. The merged checkpoint has triple-nested keys (model.language_model.language_model.language_model.*). Every quantizer loads via AutoModelForCausalLM and reports every tensor UNEXPECTED → randomly initialized. This silent failure is the #1 garbage-output cause. Rewrite the safetensors headers to clean model.layers.* (hardlinks can't fix headers) and drop the vision tower.
2. ModelOpt 0.46.0 (studio venv, transformers 5.14.1): quantize() with NVFP4_MLP_ONLY_CFG (MLP→NVFP4 group 16, rest FP8) + calibration forward loop → compress() (packs real two-level scales; without it you get fake-quant BF16 at 50 GB) → save_pretrained().
3. Rename scale tensors for the GGUF converter: weight_quantizer._scale→weight_scale, _double_scale→weight_scale_2, input_quantizer._amax→input_scale (= amax/448, FP8 E4M3 max). Keep quantization_config in config.json or the converter never triggers NVFP4 repack.
4. Convert with llama.cpp b10679 convert_hf_to_gguf.py --outtype q8_0 --no-mtp → NVFP4 tensors repacked, leftover BF16 (attention/DeltaNet/vision) → Q8_0. Layout matches the known-good RadixArk Qwen3.8-27B-NVFP4 reference exactly.
5. Embed the chat template in the GGUF (tokenizer.chat_template — extract from a working GGUF of the same family or the source config). Without it, Ollama advertises only ['completion'] — no tools, no thinking — and rejects the enable_thinking param Hermes sends.

Dead ends (don't repeat)

• llmcompressor NVFP4 → garbage GGUF every time (single-scale packing).
• unsloth native export → CUDA OOM crash (parent + subprocess both hold 51 GB on a 96 GB card) → run quantize standalone.
• BF16 "control test" through plain transformers → garbage (VLM wrapper confuses AutoModelForCausalLM); only llama-cli/Ollama tests are valid.
• Every exit-139 segfault was a resident Ollama model eating VRAM — ollama stop before heavy HF loads.

Ollama registration (Windows)

Manual manifest in registry.ollama.ai/library/Qwermes3.8/<tag>/ — big local copies wedge ollama create. Blob filenames use sha256- (dash, Windows). Config blob: {"model_format":"gguf","model_family":"qwen35","file_type":1,"model_type":"27.3B"}. Params: {"temperature":0.7,"top_p":0.8,"num_ctx":262144,"stop":["<|im_start|>","<|im_end|>"]}.

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KV cache facts (DeltaNet is state-heavy!)

Measured: 117 KB/token → 128K ctx ≈ 14.7 GB, 262K ≈ 30 GB. Plan VRAM accordingly; a 16 GB card cannot hold 128K ctx fully in VRAM.

Thinking-mode quirk

On complex code prompts at small num_ctx, the model thinks to the context limit and emits zero content (8,120 tokens of pure thinking — fine-tune behavior, not a quant defect). Fix: "think": false in Ollama request bodies (top-level; Ollama ignores it under options).

Files

• Qwermes3.8-nvfp4.gguf — 20.0 GB, 192 NVFP4 tensors (64 layers × ffn gate/up/down, group 16), 306 Q8_0 (attention/SSM/vision), chat template embedded

Verify integrity after download:

sha256 71a78d965f36dcf8bb0669f0344f3bc7b7033b9e963ac6a8abe3e9a6a143378c  Qwermes3.8-nvfp4.gguf

Training data

Fine-tuned with QLoRA (r=32, α=64, LR 2e-4, cosine, 32K ctx, packing) on Hermes-3 multiturn data — agentlans/NousResearch-Hermes-3-Dataset-multiturn (13,259 rows, ~10M tokens, single pass ≈ 2 effective epochs at 32K packing). Base: Qwen/Qwen3.8-27B. The fine-tune targets assistant-style conversational behavior (Hermes-3 format); no additional proprietary data was used.

⚠️ Contamination note: the fine-tune data is conversational (Hermes-3 multiturn), not benchmark-derived — but no formal contamination audit was run. Treat scored results as upper bounds.

License

Apache-2.0 (base: Qwen/Qwen3.8-27B). Fine-tune weights derived from Hermes-ft project; no additional restrictions.