cyburn/Qwen3.6-35B-A3B-Claude-4.7-Opus-Reasoning-Distilled-PrismaQuant-4.75bit-vllm

Qwen3.6-35B-A3B — Claude 4.7 Opus Reasoning Distilled — PrismaQuant 4.75 bpp

Mixed-precision quantization of Qwen3.6-35B-A3B-Claude-4.7-Opus-Reasoning-Distilled produced by PrismaQuant — a per-Linear sensitivity-driven allocator that chooses each Linear module's format individually under a total-bit budget.

The source model is a reasoning distillation of Qwen3.6-35B-A3B trained on Claude 4.7 Opus chain-of-thought traces. It retains the full multimodal (vision + text) and MTP speculative-decoding head of the base architecture, with added reasoning capability. Enable thinking mode at inference time (see Serving below).

Why "every layer refracts into a different format": a naive uniform NVFP4 either leaves disk on the table (keeping everything BF16 "to be safe") or loses quality (quantizing sensitive layers to 4-bit). PrismaQuant measures the actual Fisher-weighted MSE for every (Linear, format) pair and runs a multi-choice knapsack under a total-bit budget, so every bit lives where it buys the most likelihood.

---

At a glance

Metric	BF16 source	This artifact
Size on disk	~70 GB	~22 GB
Average bits per param	16	4.75
Reasoning (Claude 4.7 Opus distillation)	✓	✓
Multimodal (vision + text)	✓	✓
MTP speculative decoding heads	✓	✓
Loads in vLLM (stock compressed-tensors)	✓	✓
Runtime backend	any	vLLM only

---

Precision mix

This checkpoint uses three precisions, selected per-Linear by the allocator from measured sensitivity — not chosen uniformly. The visual encoder is kept entirely at BF16 (--visual-format=BF16), preserving full visual fidelity at the cost of a small disk overhead vs a quantized visual tower.

Allocator result (text body, 401 Linears):

Format	Bits	Count (body alloc)
NVFP4	4.50	121 Linears
MXFP8_E4M3	8.25	48 Linears
BF16	16.00	109 Linears

Export recipe (all modules, 511 entries including MTP + visual passthrough):

Format	W	A	Use	Count
NVFP4	4-bit (FP4, group_size=16 with per-group FP8 scale + per-tensor global)	4-bit (dynamic)	Bulk MoE per-expert + medium-sensitivity dense Linears	154 entries (71 dense + 80 per-expert + MTP)
MXFP8	8-bit (E4M3, group_size=32 with per-group E8M0 scale)	8-bit (dynamic)	High-sensitivity dense Linears the allocator won't risk at 4-bit	76 Linears
BF16	16-bit	16-bit	Router, norms, biases, embed / lm_head, pos_embed, full visual encoder (110 Linears)	281 entries

The allocator couples gate_up_proj / down_proj siblings so they share one per-tensor global scale (vLLM's FusedMoE requires this), and fused attention siblings (q_proj/k_proj/v_proj) share one per-tensor global scale so the packed qkv_proj loads without the "accuracy mismatch" warning. 112 fused-sibling groups were resolved during allocation.

61 Linears were ineligible for MXFP8_E4M3 due to kernel shape constraints and were restricted to the NVFP4 / BF16 choice.

Activation-aware passes applied during export

On every NVFP4 weight the exporter runs, in order:

1. GPTQ-OBS one-shot rounding — block-wise error propagation along the group-quant structure using the calibration Hessian. Closed-form, not iterative. Handles cross-column activation coupling.
2. Closed-form per-group scale sweep — for each 16-weight NVFP4 group, enumerate grid=32 candidate scales spanning [0.5·s₀, 1.5·s₀], round each weight to its nearest codebook neighbor at every candidate scale, pick the (scale, rounding-set) configuration minimizing activation-weighted per-group MSE sum_j a_j² · (w_orig,j - w_q,j)². Improve-or-keep gate against the post-GPTQ weight.

Measured per-Linear output-MSE vs RTN baseline (geomean):

Pipeline variant	out_mse ratio vs RTN
RTN (no passes)	1.00
GPTQ only	0.41
GPTQ + act_round polish (prior pipeline)	0.99 (act_round undid GPTQ)
scale_sweep only	0.33
GPTQ + scale_sweep (this artifact)	0.33

AWQ's γ-fold is not applied. On NVFP4's 16-channel groups, AWQ's per-channel rescaling pushes mixed-scale values into the same group and inflates per-group quant noise rather than reducing it.

---

Pareto curve (body allocation)

The allocator sweeps a range of bit targets and reports predicted Δloss (Fisher-weighted MSE proxy). Target 4.75 bpp was chosen; the allocator suggested 4.5 bpp as the knee.

Target bpp	NVFP4	MXFP8_E4M3	BF16	Δloss (pred)
4.50	246	1	31	1.148e+01
4.60	183	24	71	1.040e+01
4.70	140	40	98	1.009e+01
4.75	121	48	109	9.969e+00
4.85	94	39	145	9.795e+00
5.00	102	42	134	9.415e+00
5.25	99	48	131	8.646e+00
5.50	100	46	132	7.885e+00
6.00	87	38	153	7.408e+00
7.00	69	35	174	5.683e+00
8.25	54	30	194	3.524e+00

---

Which layers are quantized

Text body (DeltaNet linear-attention + dense MoE, 40 layers)

• Full attention Linears (q_proj, k_proj, v_proj, o_proj): mixed NVFP4 / MXFP8 / BF16 per-Linear by sensitivity
• DeltaNet linear-attention Linears (in_proj_qkv, in_proj_z, in_proj_a, in_proj_b, out_proj): same — note in_proj_a / in_proj_b variants were ineligible for MXFP8 due to shape constraints and resolved to NVFP4 or BF16
• MoE experts (gate_up_proj, down_proj): per-expert NVFP4 (packed_moe_per_expert) with joint per-tensor scale across the gate_up pair so vLLM FusedMoE loads them (80 per-expert entries)
• Shared expert MLP: same per-Linear policy
• Router (mlp.gate): always BF16 (tiny, sensitive)

Multi-token-prediction (MTP) head

Speculative-decoding head (1 layer) + its own MoE block quantized under the same per-Linear policy (3 089 MTP tensors merged at export). --speculative-config method=mtp drafts at the same precision as the body.

Visual encoder (110 Linears — BF16)

All 110 visual Linears are kept at BF16 via --visual-format=BF16. This is a deliberate conservative choice — the visual tower is left at full precision rather than running sensitivity-driven allocation on it. Disk overhead vs a quantized visual tower is small (< 2 GB) and visual quality is fully preserved.

Passthrough (unquantized)

• lm_head — kept at BF16. vLLM's ParallelLMHead only accepts a single weight parameter; the compressed-tensors runtime rejects a compressed lm_head with KeyError: lm_head.input_global_scale. This is a vLLM runtime limitation, not a PrismaQuant design decision. The allocator would have chosen NVFP4 here.
• RMSNorm weights (all layers + MTP + visual)
• All biases
• embed_tokens
• model.visual.pos_embed (Parameter/Embedding, not an nn.Linear)

---

Serving (vLLM only)

This artifact is only runnable via vLLM's stock compressed-tensors support. vLLM 0.11+ is required.

vllm serve cyburn/Qwen3.6-35B-A3B-Claude-4.7-Opus-Reasoning-Distilled-PrismaQuant-4.75bit-vllm \
    --trust-remote-code \
    --max-model-len 32768 \
    --gpu-memory-utilization 0.90 \
    --speculative-config '{"method":"mtp","num_speculative_tokens":3}'

Thinking / reasoning mode

This model was distilled from Claude 4.7 Opus chain-of-thought traces. Enable reasoning at inference time via the chat template:

messages = [{"role": "user", "content": "..."}]
text = tokenizer.apply_chat_template(
    messages,
    tokenize=False,
    add_generation_prompt=True,
    enable_thinking=True,   # enables <think>...</think> CoT blocks
)

Or pass /think at the start of your prompt in interactive use. The model works without thinking mode enabled — it will respond directly — but reasoning quality is higher with it on.

• FlashInfer NVFP4 attention is picked up automatically; set VLLM_USE_FLASHINFER_NVFP4=1 to make the preference explicit.
• MTP speculative decoding at n=3 is the measured optimum for this family (n=2 leaves ~10 % tok/s on the table, n=4 regresses).
• Visual inputs work via vLLM's standard image-text-to-text chat API — no special flags.

---

Reproducing this artifact

Full pipeline is in the PrismaQuant repo:

1. Sensitivity probe — streaming per-shard empirical-Fisher trace (diagonal) across body + MTP Linears.
2. Per-(Linear, format) cost measurement — for each Linear and each candidate format, the per-group RTN error weighted by cached input activations.
3. Multi-choice knapsack allocator — picks one format per Linear minimizing total predicted Δloss under the bit budget. Target 4.75 bpp.
4. Export — streams each body / MTP shard, applies GPTQ + activation-weighted scale sweep to its NVFP4 entries, writes the compressed-tensors format. Visual encoder stamped uniformly at BF16 via --visual-format=BF16.

---

Known issues / limitations

• vLLM only at serve time. No transformers-runtime path for this precision mix today.
• lm_head stays BF16 because vLLM's ParallelLMHead does not register the NVFP4/MXFP8 compressed-tensors schemes. Costs a small amount of disk vs full NVFP4.
• Visual encoder is BF16. If disk size is the primary constraint and visual quality is not critical, a variant with sensitivity-driven visual quantization (as in rdtand/Qwen3.6-35B-A3B-PrismaQuant-4.75bit-vllm) could reduce the visual tower to ~22 % of its BF16 size.
• MTP n=4 regresses on this family. Stick to n=3 unless you verify against the draft-head acceptance-rate trace.

---

Links

• PrismaQuant: github.com/RobTand/prismaquant
• Base model: Qwen/Qwen3.6-35B-A3B
• Non-distilled sibling: rdtand/Qwen3.6-35B-A3B-PrismaQuant-4.75bit-vllm

---

Citation

@software{prismaquant2026,
  title        = {PrismaQuant: per-Linear sensitivity-driven mixed-precision
                  quantization for LLMs},
  author       = {Tand, Rob},
  year         = 2026,
  url          = {https://github.com/RobTand/prismaquant},
}