NVFP4 (W4A4) quantization of bottlecapai/ThinkingCap-Qwen3.6-27B — BottleCap AI's token-efficient reasoning fine-tune of Qwen3.6-27B. Produced with llm-compressor → compressed-tensors, with the native MTP speculative-decode head preserved (bf16) and the Qwen3-VL vision tower preserved (bf16).

Why this pairing is nice. ThinkingCap keeps Qwen3.6-27B's answer quality while cutting <think> length by ~46 % on average (out-of-domain macro), and NVFP4 + the MTP draft cut the cost of every token. Fewer thinking tokens × faster tokens = a markedly snappier local reasoner — with no measurable accuracy loss on the parent's benchmarks (macro 81.5 → 80.7; LiveCodeBench 80.7 → 84.3; truncation 2.9 % → 0.4 %, per the base model card).

20.6 GB on disk (down from ~55.6 GB bf16). Serves on stock vLLM 0.21+ — no --quantization flag needed (auto-detected).

Architecture

Qwen3_5ForConditionalGeneration (model_type qwen3_5), dense 27.8 B:

Hybrid attention — 48 Gated-DeltaNet (linear) + 16 full-attention layers (64 total, full_attention_interval=4), hidden 5120, 262 K native context.
Vision — Qwen3-VL ViT (1152→5120), kept bf16; serve text-only with --limit-mm-per-prompt.
Native MTP (mtp_num_hidden_layers=1), kept bf16 → drives vLLM speculative decoding.
Thinking-by-default reasoning model (<think>…</think>, use --reasoning-parser qwen3) — but a token-efficient one.

Quantization recipe

QuantizationModifier(targets="Linear", scheme="NVFP4",  # W4A4, group_size 16
  ignore=["lm_head", "re:.*visual.*", "re:.*conv1d.*", "re:.*mtp.*"])

Vision tower, DeltaNet causal conv1d, lm_head, and the entire MTP head stay bf16; everything else is NVFP4 W4A4. 32 calibration samples (neuralmagic/calibration), seq 8192, pure-CPU load (sequential-pipeline onload).
ThinkingCap ships the MTP head as a separate model-base-aux.safetensors (15 bf16 tensors). Those are grafted into the NVFP4 output (model-mtp-bf16.safetensors) and spliced into the safetensors index.
Note for re-bakers: the grafted MTP modules must also be added to quantization_config.ignore, otherwise vLLM matches mtp.*_proj against targets=["Linear"], expects NVFP4 scales that do not exist, and loads the Qwen3_5MTP draft as garbage → 0 % spec-decode acceptance. This bake adds them automatically.

Serving (vLLM ≥ 0.21)

bash

vllm serve sakamakismile/ThinkingCap-Qwen3.6-27B-NVFP4 \
  --tensor-parallel-size 4 --max-model-len 131072 \
  --max-num-seqs 16 --gpu-memory-utilization 0.90 --kv-cache-dtype fp8 \
  --reasoning-parser qwen3 --limit-mm-per-prompt '{"image":0,"video":0}' \
  --speculative-config '{"method":"qwen3_5_mtp","num_speculative_tokens":3}'

On NVLink-less boxes add NCCL_P2P_DISABLE=1 + --disable-custom-all-reduce (and NCCL_CUMEM_ENABLE=0 if TP=8 CUDA-graph capture hangs). Drop --speculative-config for plain decode. The hybrid model's KV is light (only the 16 full-attention layers cache), so full 128 K context fits even at TP=2.

Reasoning model → set max_tokens ≥ 4096 (prefer 8192+). Even though ThinkingCap thinks less, at a tiny budget it can still spend it all inside <think> and return empty content.
Do not produce a W4A16 / NVFP4A16 variant — it fails to serve on vLLM (gptq_marlin_repack: size_n=24 not divisible by tile_n_size=64; the 24 attention-heads / DeltaNet odd dims violate Marlin's tile constraint). W4A4 avoids Marlin (NVFP4 cutlass/FlashInfer path).
Sampling: the base recommends temperature=1.0, top_p=0.95, top_k=20.

License & attribution

Apache-2.0, inherited from the base models. Token-efficiency fine-tune by BottleCap AI; base Qwen3.6-27B by the Qwen Team. NVFP4 quantization by sakamakismile (Lna-Lab), reusing the validated qwen3_5 dense+MTP recipe shared with sakamakismile/Qwen3.6-27B-MTP-pi-tune-NVFP4.