A sensitivity-aware mixed-precision quantization of Qwen3.5-27B, distilled from Claude Opus reasoning traces and quantized using a per-layer bit allocation strategy optimized for Apple Silicon (MLX).

Key Stats

Metric	Value
Parameters	27B dense
Disk size	~20 GB
Avg bits/weight	~6.5
Peak memory	~22 GB
Throughput (M3 Ultra)	27 tok/s (oMLX), 23 tok/s (mlx_lm)
Quality (Phipps eval v3)	4.45–4.47

Quantization Methodology — "Proposal C" (8/8/6/5)

This is NOT uniform quantization. Each component type gets a precision level matched to its sensitivity:

Bit Allocation

Component	Bits	Rationale
Embeddings (`embed_tokens`)	8	Token↔continuous space mapping — errors cascade
LM Head (`lm_head`)	8	Directly impacts output probabilities
GQA self-attention (q/k/v/o_proj)	8	Traditional softmax attention — the "precise reasoning" layers
DeltaNet linear attention inputs (qkv, z, b, a)	6	Linear recurrence is more robust to quantization noise
DeltaNet output projection	8	Last transform before residual stream
All MLP layers (gate/down/up_proj)	5	Bulk of parameters, tolerant due to element-wise nonlinearities

Architecture-Aware Design

Qwen3.5-27B uses a hybrid DeltaNet/GQA architecture with a 3:1 ratio:

Layers 0, 1, 2 → DeltaNet (linear attention)
Layer 3 → GQA (softmax attention)
Pattern repeats across all 64 layers (48 DeltaNet + 16 GQA)

Proposal C exploits this: GQA layers (which handle precise key-value attention) get full 8-bit, while DeltaNet layers (which use linear recurrence with gating) can tolerate 6-bit on inputs. MLP weights — the parameter-count majority — compress to 5-bit with minimal quality impact.

Why This Works

Protect information bottlenecks: Embeddings, LM head, and attention output projections are all 8-bit because errors there propagate directly into the residual stream
Match precision to mechanism: GQA softmax attention needs precision; DeltaNet's gating mechanism absorbs quantization noise
Compress where it's cheap: MLP layers have the most parameters but are the most quantization-tolerant

Results vs Uniform Quantization

Proposal C was validated against uniform 4-bit, 5-bit, and 8-bit baselines on the 80B MoE variant (Proposal A). Zero measurable quality loss vs the fp16 baseline at ~3x compression. The same methodology applied to the 27B dense model scores 4.45–4.47 on our eval harness — higher than the 80B MoE at 4.25 (which has only 3B active parameters per token vs 27B dense).

Usage

python

from mlx_lm import load, generate

model, tokenizer = load("Phipper/qwen3.5-27b-opus-distilled-proposal-c")
prompt = tokenizer.apply_chat_template(
    [{"role": "user", "content": "What is a leveraged buyout?"}],
    tokenize=False, add_generation_prompt=True
)
response = generate(model, tokenizer, prompt=prompt, max_tokens=512)
print(response)

Sampling Recommendations

Temperature: 0.7
Top-p: 0.8
Top-k: 20
Max tokens: 16384

Hardware Requirements

Apple Silicon with ≥24GB unified memory (M2 Pro/Max, M3 Pro/Max/Ultra, M4 Pro/Max)
MLX ≥0.31.1, mlx-lm ≥0.31.1

Credits

Created by Nate Baranski (@Phipper)
Base model: Qwen/Qwen3.5-27B (Apache 2.0)
Distillation: Opus reasoning traces
Quantization methodology & evaluation: Nate Baranski + Bessemer (Claude Code agent)