---
library_name: transformers
license: apache-2.0
base_model: Qwen/Qwen3.5-27B
tags:
  - rotorquant
  - 2bit
  - kv-cache
  - quantized
  - qwen3.5
  - thinking
language:
  - en
pipeline_tag: text-generation
---

# Qwen3.5-27B-RotorQuant-2bit

**2-bit KV cache compression** for [Qwen/Qwen3.5-27B](https://huggingface.co/Qwen/Qwen3.5-27B) using [RotorQuant](https://github.com/scrya-com/rotorquant).

> This is a **KV-cache-only** repository. It contains no model weight files — only the configuration and model card for applying RotorQuant 2-bit KV cache quantization at runtime on the original Qwen3.5-27B weights.

## Overview

Qwen3.5-27B is a 27B-parameter hybrid transformer with 262K native context and built-in thinking mode (the model generates internal reasoning tokens before answering). Thinking mode makes KV cache compression especially valuable, since the reasoning chain can consume substantial cache memory.

RotorQuant applies rotation-based isotropic quantization to the KV cache, achieving better quality and speed than standard quantization approaches at the same bit width.

### RotorQuant Advantages

| Metric | RotorQuant 2-bit | Standard 2-bit |
|---|---|---|
| Prefill speed | **5.3x faster** | Baseline |
| Decode speed | **28% faster** | Baseline |
| Perplexity | **6.91** | 7.07 |

RotorQuant achieves lower perplexity (better quality) while also being faster — a rare combination at aggressive quantization levels.

## Specifications

| Property | Value |
|---|---|
| Base model | Qwen/Qwen3.5-27B |
| Parameters | 27B |
| Architecture | Hybrid Transformer |
| Native context | 262,144 tokens |
| Thinking mode | Yes |
| KV cache method | RotorQuant 2-bit (IsoQuant) |
| KV cache compression | ~10x vs FP16 |
| Weights | Original (FP16/BF16, loaded separately) |

## Memory Estimates

| Component | Estimate |
|---|---|
| Model weights (BF16) | ~54 GB |
| KV cache at 128K context (2-bit RotorQuant) | ~1.3 GB |
| KV cache at 128K context (FP16, baseline) | ~12.8 GB |

## Quickstart

```python
from transformers import AutoModelForCausalLM, AutoTokenizer
from turboquant import IsoQuantCache

model_id = "Qwen/Qwen3.5-27B"

tokenizer = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(model_id, torch_dtype="auto", device_map="auto")

# Apply 2-bit RotorQuant KV cache compression
cache = IsoQuantCache(bits=2)

messages = [{"role": "user", "content": "Explain the Riemann hypothesis in simple terms."}]
inputs = tokenizer.apply_chat_template(messages, return_tensors="pt").to(model.device)

outputs = model.generate(
    inputs,
    max_new_tokens=2048,
    past_key_values=cache,
)
print(tokenizer.decode(outputs[0], skip_special_tokens=True))
```

## Quality Notes

- **2-bit is aggressive quantization**, but RotorQuant's rotation-based approach preserves more quality than standard methods (perplexity 6.91 vs 7.07).
- Best suited for memory-constrained scenarios where fitting long-context inference on limited hardware is essential.
- For higher quality with moderate compression, consider 4-bit KV cache variants.
- Thinking mode reasoning quality may be more sensitive to cache quantization since the model relies on cached reasoning tokens for its final answer.

## References

- [RotorQuant](https://github.com/scrya-com/rotorquant) — Rotation-based isotropic KV cache quantization
- [Qwen3.5-27B base model](https://huggingface.co/Qwen/Qwen3.5-27B)

## See Also

- [majentik/Qwen3.5-27B-TurboQuant-2bit](https://huggingface.co/majentik/Qwen3.5-27B-TurboQuant-2bit) — TurboQuant 2-bit KV cache variant
- [majentik/Qwen3.5-27B-TurboQuant-MLX-2bit](https://huggingface.co/majentik/Qwen3.5-27B-TurboQuant-MLX-2bit) — MLX 2-bit weights + TurboQuant KV cache
- [majentik/Qwen3.5-27B-RotorQuant-MLX-2bit](https://huggingface.co/majentik/Qwen3.5-27B-RotorQuant-MLX-2bit) — MLX 2-bit weights + RotorQuant KV cache