Qwen3.8-27B-Text-NVFP4-MTP — an experiment

An NVFP4-quantized, text-only build of Qwen/Qwen3.8-27B: vision tower removed, MTP head grafted back in bf16 so vLLM speculative decoding works, exported in modelopt format for the native NVFP4 path on NVIDIA Blackwell / SM120.

TL;DR

This repo exists to answer one question: on a 32 GB Blackwell card (RTX 5090), does physically stripping the vision tower give you more context or more speed? It doesn't — it only removes the 921 MB vision tower from the download (nothing at runtime), because vLLM already skips that tower at load when you pass --language-model-only. Note this build also grafts back an 849 MB bf16 MTP head, so the net file is only ~0.95 GB smaller than a build that carries both vision and MTP (e.g. the gittensor build below).

Recommendation: on 32 GB, run the full multimodal build instead and toggle MTP for what you need — MTP off → bigger KV cache (~199K context, vision on), MTP on → less context (~123K) but ~1.6× the speed. Pure-text on the multimodal build (--language-model-only) is identical to this stripped build, so there's rarely a reason to prefer this one. It's published for reproducibility and for the ~1 GB smaller download.

The experiment

• Base / method: Qwen/Qwen3.8-27B → nvidia-modelopt 0.43.0 NVFP4 (NVFP4_DEFAULT_CFG, group_size 16), keeping lm_head, *linear_attn.conv1d*, mtp.*, and model.visual.* in bf16; then the vision tensors are physically deleted and the 15 bf16 mtp.* tensors grafted back so speculative decoding engages.
• Hardware / engine: one RTX 5090 (32 GB), vLLM 0.27.2 (needs the ≥0.27.2 gated-delta-net fix; older builds crash MTP at long context), --gpu-memory-utilization 0.93, fp8 KV cache, --max-num-seqs 1.
• Important condition — this was a shared card, not a dedicated one. The desktop was live on the same 5090 (~2 GB of VRAM held by the OS compositor) during every run, and vLLM's cuda-graph profiler over-reserves another ~0.4 GB unless disabled. So the numbers below are conservative, shared-card results, taken with VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS=0 set to reclaim that 0.4 GB. A card fully dedicated to inference (desktop moved to an integrated GPU, ~2 GB freed) does strictly better — more KV cache, higher context ceilings.
• What we compared: this text-only build vs. a full multimodal NVFP4 build of the same model, across vision-on/off and MTP-on/off, measuring the KV-cache ceiling and steady-state decode tok/s.

Results

Serve mode (multimodal build unless noted)	Vision	Max context¹	Decode tok/s
vision on, MTP off (drop --speculative-config)	✅	~199K	~77
vision on, MTP on	✅	~123K	~125–135²
--language-model-only, MTP on (text)	—	~200K	~130–140
this text-only build, --language-model-only, MTP	—	~200K	~130–140

¹ Every ~200K/199K figure is conditional on VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS=0 (see below); without it you top out at ~185–188K. ² An image in the prompt does not slow decode — the vision encoder runs once during prefill (measured ~133 tok/s decoding with an image present). Time-to-first-token grows with image resolution; steady decode doesn't.

What each row taught us:

1. Stripping vision ≠ more context or speed. Text-only build and multimodal build, both --language-model-only
   • MTP, are identical: KV 201,428 tokens, ~130–140 tok/s each. Deleting the tower only shrinks the download.
2. The 200K lever is a vLLM env var, not the model. vLLM 0.27.x's cuda-graph profiler over-reserves ~0.4 GB (estimates 0.40 GB, actually uses 0.04 GB). VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS=0 hands it back (KV 6.95 → 7.35 GB) → full 200K fits at util 0.93. Applies to any NVFP4 27B on 32 GB. (Context: a normal desktop compositor already holds ~2 GB of the 5090; dedicate the card — desktop on an iGPU — and you can push past 200K.)
3. MTP, not vision, is what eats KV cache. The vision tower costs ~0.17 GB of weights (≈nothing). Enabling MTP is what roughly halves usable context (199K → 123K with vision on).
4. Vision works and is nearly free. The multimodal build correctly read shapes + text from a test image.
5. On 32 GB you can't have vision + MTP-speed + 200K at once — pick two. The scarce resource is KV cache.

Conclusion

Prefer the multimodal build on 32 GB: it's a strict superset (does everything this build does, plus vision) and you choose context-vs-speed by toggling MTP. Reach for this text-only build only if you never send images and want the ~0.95 GB smaller download or a guaranteed text-only artifact.

Two conditions behind every number here: they were measured on a shared card (the desktop held 2 GB of the 5090) with VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS=0 set to reclaim vLLM's ~0.4 GB profiler over-reservation. Without that env var you lose ~12–15K of context (185–188K instead of 200K); dedicate the card (desktop on an iGPU, ~2 GB freed) and every context ceiling above rises further.

Serving this build (vLLM ≥ 0.27.2, Blackwell / SM120)

# The env var is REQUIRED to reach 200K on a 32 GB card. Without it you get ~185–188K.
VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS=0 \
vllm serve dbrasdasilva/Qwen3.8-27B-Text-NVFP4-MTP \
    --trust-remote-code \
    --quantization modelopt \
    --language-model-only \
    --kv-cache-dtype fp8 \
    --max-model-len 200000 \
    --max-num-seqs 1 \
    --speculative-config '{"method":"mtp","num_speculative_tokens":3}'

--language-model-only is required for this build. The vision tower and its preprocessor_config.json were removed, but the arch class is still Qwen3_5ForConditionalGeneration — without the flag vLLM tries to build an image processor that no longer exists and aborts with Can't load image processor for '…'.

Model details & credit

• Quant: nvidia-modelopt 0.43.0 NVFP4, group_size 16, KV-cache quant off; kept in bf16: lm_head, model.visual.* (then deleted), *linear_attn.conv1d* (Mamba-style SSM convs), mtp.* (grafted back). Calibration: 20 samples from neuralmagic/calibration (LLM split), max_seq_len 8192.
• Vision removal (how): after the ModelOpt NVFP4 export, the model.visual.* tensors are dropped from every .safetensors shard and from model.safetensors.index.json, and vision_config + the image/video/vision-start token ids are removed from config.json (and preprocessor_config.json deleted), using the safetensors library. The approach follows lna-lab's GGUF-to-NVFP4-SM120 recipe (credited below).
• Working MTP matters: naive AutoModelForCausalLM export drops the MTP head → 0% draft acceptance; the ignore pattern must be mtp* (not model.mtp.*, which never matches). With it, ~72% acceptance measured.
• Size accounting (measured): core weights ~18.79 GB; the removed vision tower is 921 MB (333 bf16 tensors); the grafted MTP head is 849 MB (bf16). Because those two nearly cancel, this build is only ~0.07 GB smaller than a multimodal build without MTP, but ~0.95 GB smaller than one with both vision and MTP (the gittensor build: 20.59 GB vs this 19.64 GB). The download saving is entirely the vision tower.

This is a derivative work — all capability belongs to its authors; this repo only re-quantizes and strips:

1. Qwen team (Alibaba) — Qwen/Qwen3.8-27B (Apache-2.0).
2. osoleve — the original text-only NVFP4 + MTP-graft recipe this descends from, osoleve/Qwen3.5-27B-Text-NVFP4-MTP.
3. lna-lab — the reproducible pipeline, lna-lab/GGUF-to-NVFP4-SM120.
4. sakamakismile — the Qwen3.6 build this mirrors, sakamakismile/Qwen3.6-27B-Text-NVFP4-MTP.