Lorbus/Qwen3.6-27B-int4-AutoRound

Qwen3.6-27B INT4 AutoRound

A W4A16 (INT4 weight, FP16 activation) quantization of Qwen/Qwen3.6-27B, produced with Intel's AutoRound.

TL;DR

• Base: Qwen3.6-27B (27B dense VLM, Apr 21 2026)
• Quant: INT4 W4A16, group_size 128, symmetric
• Tool: auto-round (default recipe, 200 iters, torch.compile)
• Size: 18 GB (down from ~54 GB BF16) — 3x reduction
• MTP preserved: The native Multi-Token Prediction head is kept in BF16, enabling native speculative decoding in vLLM (≈90% draft acceptance in our tests, ~2x throughput)
• Accuracy: Default AutoRound recipe preserves quality well; layer-norm weights, router layers, RMSNorm, linear_attn.in_proj_a/b, and MTP's fusion fc are kept unquantized (they're small and benefit from full precision)

Quick inference with vLLM (with MTP speculative decoding)

Requires vLLM that supports Qwen3_5 MTP (most recent nightlies — tested with eugr/spark-vllm-docker fork 0.19.1rc1.dev39+g7055d32a7):

vllm serve Lorbus/Qwen3.6-27B-int4-AutoRound \
  --dtype half \
  --max-model-len 262144 \
  --gpu-memory-utilization 0.85 \
  --kv-cache-dtype tq-t4nc \
  --max-num-seqs 3 \
  --reasoning-parser qwen3 \
  --enable-auto-tool-choice \
  --tool-call-parser qwen3_xml \
  --port 8888 --host 0.0.0.0 \
  --trust-remote-code \
  --compilation-config.cudagraph_mode none \
  --speculative-config '{"method": "mtp", "num_speculative_tokens": 1}'

Notes:

• --kv-cache-dtype tq-t4nc (TurboQuant 4-bit) halves KV memory vs fp8. Use --kv-cache-dtype fp8 for mainline vLLM without the TurboQuant fork.
• --compilation-config.cudagraph_mode none is currently needed on Blackwell consumer (SM120/SM121) GPUs — CUDA graph capture hits a cudaErrorStreamCaptureInvalidated on the MTP module in some vLLM nightlies.
• --speculative-config enables the model's native MTP head as a built-in drafter.

OpenAI-compatible request

from openai import OpenAI
client = OpenAI(base_url="http://localhost:8888/v1", api_key="EMPTY")
r = client.chat.completions.create(
    model="Lorbus/Qwen3.6-27B-int4-AutoRound",
    messages=[{"role": "user", "content": "Write a quicksort in Python."}],
    max_tokens=512,
)
print(r.choices[0].message.content)

Transformers (no spec decoding)

from transformers import AutoModelForCausalLM, AutoTokenizer
m = AutoModelForCausalLM.from_pretrained(
    "Lorbus/Qwen3.6-27B-int4-AutoRound",
    trust_remote_code=True,
    device_map="auto",
)
tok = AutoTokenizer.from_pretrained("Lorbus/Qwen3.6-27B-int4-AutoRound")
msg = [{"role": "user", "content": "Explain quantum computing briefly."}]
ids = tok.apply_chat_template(msg, add_generation_prompt=True, return_tensors="pt").to(m.device)
print(tok.decode(m.generate(ids, max_new_tokens=256)[0]))

Quantization details

Field	Value
Base	Qwen/Qwen3.6-27B
Method	AutoRound (intel/auto-round), default recipe
Scheme	W4A16 (4-bit weights, FP16 activations)
Bits	4
Group size	128
Symmetric	yes
Packing format	auto_round:auto_gptq
Unquantized layers	linear_attn.in_proj_a/b, mtp.fc, all LayerNorms and RMSNorms, router gates
Calibration samples	128 (default)
Iterations	200
torch.compile	enabled
GPU used for quant	1× RTX 5090 (32 GB, SM120), low_gpu_mem_usage=True
Quant wall time	~1h 40min

Unquantized layers — why

• linear_attn.in_proj_a/b: these are low-rank projections in Qwen3.6's Gated DeltaNet. Their shapes are not divisible by 32 (group_size), so AutoRound skips them. They account for a tiny fraction of parameters.
• mtp.fc: the Multi-Token Prediction fusion layer. AutoRound initially quantized it to GPTQ-packed INT4, but vLLM's Qwen3_5MTP loader expects an unquantized fc.weight. We dequantized it to BF16 so MTP works natively. If you use this quant without MTP, the fc weight is still there and harmless.
• Norms, routers: precision-sensitive and very small.

MTP fix — what's different from a vanilla AutoRound run

A plain auto-round run on a Qwen3.5/3.6 model packs mtp.fc as INT4. In that form, vLLM skips loading the layer entirely (param name mismatch between fc.qweight and the expected fc.weight), which makes MTP speculative decoding produce 0% acceptance.

This release dequantizes mtp.fc back to BF16 after AutoRound finishes. The layer is only 100 MB (5120 × 10240 × 2 bytes) so the file size impact is negligible. Result: MTP works out of the box and reaches **80-90% draft acceptance** on typical prompts.

Performance

Benchmarked on 1× RTX 5090 (32 GB) with vLLM + TurboQuant 4-bit KV cache + MTP:

Prompt type	max_tokens	Throughput
"Write a haiku"	128	58 tok/s
"Explain quantum computing in 3 paragraphs"	256	60 tok/s
"Write 8 paragraphs about deep learning history"	1024	60 tok/s
"What is 127*83? Show reasoning"	256	61 tok/s

With MTP off (--speculative-config removed): ~32 tok/s. The 2x speedup comes from MTP speculative decoding with ~85% acceptance.

Known limitations

• The model is a vision-language model — you can feed images in OpenAI-compat messages with an image_url content part. Image quantization was not the focus here; MoonViT encoder weights are kept at their original precision (BF16/FP16 as in the base model).
• bits: 4 at group_size: 128 prioritizes throughput/memory over maximal accuracy. For accuracy-critical work, try the auto-round-best recipe (1000 iters, ~5-10x slower) or a higher bit width.
• Not tested extensively beyond 128K context. Qwen3.6's partial_rotary_factor RoPE scaling is preserved, so 262K should work.

Reproduction

pip install auto-round-nightly

auto-round \
  --model Qwen/Qwen3.6-27B \
  --scheme W4A16 \
  --format auto_round \
  --output_dir Qwen3.6-27B-int4-AutoRound \
  --enable_torch_compile \
  --low_gpu_mem_usage \
  --device_map 0

Then dequantize mtp.fc for MTP compatibility — see the dequant_mtp_fc.py script below:

mtp.fc dequant script (click to expand)

#!/usr/bin/env python3
"""Dequantize mtp.fc from GPTQ INT4 back to bf16 so vLLM's MTP loader picks it up."""
import json, shutil
from pathlib import Path
import torch
from safetensors import safe_open
from safetensors.torch import save_file

BASE = Path("Qwen3.6-27B-int4-AutoRound")
EXTRA = BASE / "model_extra_tensors.safetensors"
INDEX = BASE / "model.safetensors.index.json"

tensors = {}
with safe_open(EXTRA, framework="pt") as f:
    meta = f.metadata() or {}
    for k in f.keys():
        tensors[k] = f.get_tensor(k)

qw = tensors["mtp.fc.qweight"]    # [1280, 5120] int32
qz = tensors["mtp.fc.qzeros"]     # [80, 640] int32
sc = tensors["mtp.fc.scales"]     # [80, 5120] fp16

in_features = qw.shape[0] * 8     # 10240
out_features = qw.shape[1]        # 5120
group_size = 128
num_groups = in_features // group_size   # 80

def unpack_int32_4bit(packed, axis, factor=8):
    dev = packed.device
    shifts = torch.arange(0, 32, 4, device=dev, dtype=torch.int32)
    expanded = (packed.unsqueeze(axis + 1) >> shifts.view([8 if i == axis + 1 else 1 for i in range(packed.ndim + 1)])) & 0xF
    new_shape = list(packed.shape); new_shape[axis] *= factor
    return expanded.reshape(new_shape).to(torch.int8)

w_int = unpack_int32_4bit(qw, axis=0)   # [10240, 5120]
z_int = unpack_int32_4bit(qz, axis=1)   # [80, 5120]

w_grouped = w_int.view(num_groups, group_size, out_features).to(torch.float32)
w_fp32 = (w_grouped - z_int.unsqueeze(1).to(torch.float32)) * sc.unsqueeze(1).to(torch.float32)
w_final = w_fp32.view(in_features, out_features).t().contiguous().to(torch.bfloat16)   # [5120, 10240]

# Replace
for k in ("mtp.fc.qweight", "mtp.fc.qzeros", "mtp.fc.scales"):
    del tensors[k]
tensors["mtp.fc.weight"] = w_final

save_file(tensors, str(EXTRA), metadata=meta)

# Update index
idx = json.loads(INDEX.read_text())
for k in ("mtp.fc.qweight", "mtp.fc.qzeros", "mtp.fc.scales"):
    idx["weight_map"].pop(k, None)
idx["weight_map"]["mtp.fc.weight"] = EXTRA.name
# Recompute total_size
from collections import defaultdict
shard_sizes = defaultdict(int)
for sf in set(idx["weight_map"].values()):
    with safe_open(BASE / sf, framework="pt") as f:
        for k in f.keys():
            t = f.get_tensor(k)
            shard_sizes[sf] += t.numel() * t.element_size()
idx["metadata"]["total_size"] = sum(shard_sizes.values())
INDEX.write_text(json.dumps(idx, indent=2))

Acknowledgements

• Alibaba / Qwen team for the base Qwen3.6-27B model
• Intel AutoRound team for the quantization framework
• @eugr for the spark-vllm-docker fork and TurboQuant KV cache work
• vLLM project for the inference engine and Qwen3_5 MTP support

License

Apache 2.0 — same as Qwen3.6-27B base.

Citation

If you use this quant, please cite the original Qwen3.6 release (see base model card) and the AutoRound paper:

@article{cheng2023autoround,
  title   = {Optimize Weight Rounding via Signed Gradient Descent for the Quantization of LLMs},
  author  = {Cheng, Wenhua and Zhang, Weiwei and Shen, Haihao and Cai, Yiyang and He, Xin and Lv, Kaokao and Liu, Yi},
  journal = {arXiv preprint arXiv:2309.05516},
  year    = {2023}
}