Whats here

The draft head ships at Q8_0 and Q4_0, the same weights at two precisions. Q8_0 is the default and is what the speculative decoding tables below were measured with. Q4_0 saves 1.5 GB for tight VRAM budgets, and its acceptance rate has not been measured here.

The F16 vision projector uses the standard mmproj prefix for automatic discovery by compatible runtimes.

Overview

Files

Perplexity

Measured on this build, every quant in one session against the same f16 baseline, so the rows are comparable to each other.

Read the error bars before reading the ordering. Every row except IQ2_M sits inside a span of 0.026 against a standard error of roughly 0.25, so those quants are not separable from the f16 or from each other, and their ordering here is noise. Do not conclude that Q8_0 is worse than Q5_K_M. The only difference this measurement actually resolves is IQ2_M, which is about 2.8 standard errors above the baseline.

The noMTP-* twins are not listed because they measure identically to their fused counterparts. The MTP block is inert during a normal forward pass, which was confirmed here: fused and noMTP IQ2_M both return 7.8581, and fused and noMTP f16 both return 7.1557.

Corpus is Salesforce/wikitext, wikitext-2-raw-v1, test-00000-of-00001.parquet, text column joined with \n. This is the same file used for the importance matrix, described in full below.

llama-perplexity -m Qwen3.8-27B-Uncensored-IQ2_M.gguf \
  -f calibration.txt -ngl 99 --chunks 20

Baseline against the unmodified base model

Every row above is this model, so that table shows what quantization costs and nothing more. It does not show what the abliteration costs. That needs the unmodified base model run on the same harness, which is what follows.

Base files come from ggml-org/Qwen3.8-27B-GGUF. The bf16 GGUF of this model was converted locally from the published bf16 weights and is not shipped here, for the same reason the f16 is not.

The weight edit costs 0.0434 perplexity, about 0.7% relative. Measured as a paired difference over the same tokens rather than as two independent runs, the gap is 0.0437 +/- 0.0163, so it is a real cost rather than noise. The base Q8_0 row is the control: it lands 0.0007 from the base bf16, well below the gap being reported.

Distribution overlap was measured directly rather than inferred from the perplexity gap:

This is a different measurement from the first token KL under Measured behaviour, which is the figure Heretic optimizes against. This one runs over ordinary corpus text and covers every position, not just the first.

How the baseline was measured

One session, one machine, one binary, the same corpus file and the same settings for every row, with each model fully offloaded:

llama-perplexity -m Qwen3.8-27B-BF16.gguf \
  -f calibration.txt -c 2048 -b 2048 -ngl 99

These numbers do not line up with the table above. That table stops at 20 chunks and this one reads the whole file, so the two cover different amounts of text and cannot be compared directly. Rows within each table are comparable to each other. The whole file run carries a standard error of roughly 0.042 against 0.25 for the 20 chunk run, which is why a gap this small is worth reporting at all.

Perplexity detects gross quantization damage and nothing else. It does not measure reasoning, code, multilingual ability, or refusal behaviour. See Caveats that matter.

Importance matrix

Qwen3.8-27B-Uncensored-imatrix.dat is the importance matrix every quantization in this repo was built with. All twelve of the IQ2_M, IQ4_XS, Q4_K_M, Q5_K_M, Q6_K and Q8_0 files, fused and noMTP alike, record it in their own metadata. The standalone draft-Q8_0 head and the mmproj-Qwen3.8-27B-Uncensored-F16.gguf projector do not, because neither was built with one.

It is published so the files here can be reproduced, and so you can build quants that are not in this repo.

Two things to know before you use it:

• It is GGUF format despite the .dat extension (general.type = imatrix). llama.cpp builds predating GGUF imatrix support will not load it.
• It contains no entries for blk.64, the MTP block. llama-imatrix never activates the draft head during a normal forward pass, so no activations are collected for it. This matters below.

Provenance is checkable rather than asserted: requantizing the f16 to Q4_K_M with this file reproduces the published Qwen3.8-27B-Uncensored-Q4_K_M.gguf to byte-identical tensors across all 866 tensors.

Building other low bit quants yourself

The floor here is IQ2_M at 10.6 GB. If you want something smaller, or a type that is not published, the imatrix above lets you build it.

You will need the f16 GGUF, which is not published here because it is 54 GB. Build it from the bf16 weights, which are public:

hf download JonathanColetti/Qwen3.8-27B-Uncensored --local-dir Qwen3.8-27B-Uncensored
python convert_hf_to_gguf.py Qwen3.8-27B-Uncensored \
  --outfile Qwen3.8-27B-Uncensored-f16.gguf --outtype f16

Add --no-mtp to that command for the noMTP variant. The MTP shard is already grafted into the bf16 repo, so nothing needs restoring first.

The MTP block must be pinned. Because the imatrix has no blk.64 entries, and because IQ3_XXS, IQ2_XXS, IQ2_S and IQ2_M require per-tensor importance data, a fused low-bit build without a pin does not merely degrade the draft head. llama-quantize refuses to run at all. Pinning blk.64 to q8_0 sidesteps the requirement and keeps the draft head intact:

llama-quantize \
  --imatrix Qwen3.8-27B-Uncensored-imatrix.dat \
  --tensor-type 'blk\.64\.=q8_0' \
  --token-embedding-type q4_K \
  Qwen3.8-27B-Uncensored-f16.gguf Qwen3.8-27B-Uncensored-IQ2_XXS.gguf IQ2_XXS

--token-embedding-type q4_K is the largest size lever on this model. llama.cpp force-bumps token_embd to Q5_K on every IQ2/IQ1 ftype, and at 248320 vocab that is roughly 8 to 10% of parameters. Do not go below q4_K. Omit the --tensor-type pin for noMTP-* builds, since there is no block to pin.

Verify afterwards that the block survived, rather than assuming it did:

python quantize.py inspect Qwen3.8-27B-Uncensored-IQ2_XXS.gguf   # expect 65/65, has_mtp: true

2 bit warning. IQ2_M is the most degraded file in this repo, and anything you build below it will be worse. Expect the loss to land hardest on the thing this model is used for. Behaviour near the old refusal boundary is already its least stable property (see Caveats that matter), and 2 bit compounds exactly that. Perplexity will tell you the model is not broken. It will not tell you the refusal boundary still behaves the way it does at Q6_K, and nothing in this repo measures that at 2 bit.

Third party quants

A third party publishes a range of mixed-precision variants derived from this model:

• zerodigest/Qwen3.8-27B-Uncensored-YMQ-MTP-GGUF

Not produced by or affiliated with this repo. It is not built by me, I have not verified its files or its published numbers, and any metrics quoted there were not measured on the harness used here, so they are not comparable to the perplexity figures in this card. Linked because people ask for sizes I do not ship, not as an endorsement.

Usage

ComfyUI

Tested versions:

• ComfyUI commit 0a33ed6c28f926d14536235771c222f9e6d1026b
• ComfyUI QwenVL commit e79582111e5835787574bcc17adc4bf4cf3a07f4
• llama cpp python version 0.3.48 at commit c57b174711166b889f06a35a30af95a75db24705

# Install ComfyUI and the QwenVL nodes
git clone https://github.com/comfyanonymous/ComfyUI.git
git -C ComfyUI checkout 0a33ed6c28f926d14536235771c222f9e6d1026b
git clone https://github.com/1038lab/ComfyUI-QwenVL.git ComfyUI/custom_nodes/ComfyUI-QwenVL
git -C ComfyUI/custom_nodes/ComfyUI-QwenVL checkout e79582111e5835787574bcc17adc4bf4cf3a07f4

# Install dependencies
uv pip install -r ComfyUI/requirements.txt
uv pip install -r ComfyUI/custom_nodes/ComfyUI-QwenVL/requirements.txt
CMAKE_ARGS="-DGGML_CUDA=on -DLLAMA_CPP_PYTHON_VISION=on" \
  uv pip install \
  "llama-cpp-python @ git+https://github.com/JamePeng/llama-cpp-python.git@c57b174711166b889f06a35a30af95a75db24705"

# Download the language model and F16 vision projector
hf download JonathanColetti/Qwen3.8-27B-Uncensored-GGUF \
  Qwen3.8-27B-Uncensored-Q4_K_M.gguf \
  mmproj-Qwen3.8-27B-Uncensored-F16.gguf \
  --local-dir \
  ComfyUI/models/LLM/GGUF/JonathanColetti/Qwen3.8-27B-Uncensored-GGUF

# Start ComfyUI
cd ComfyUI
python main.py --listen 127.0.0.1 --port 8188

Add the model to ComfyUI/custom_nodes/ComfyUI-QwenVL/custom_models.json:

{
  "hf_models": {},
  "gguf_models": {
    "Qwen3.8 27B Uncensored Q4 K M": {
      "author": "JonathanColetti",
      "repo_name": "Qwen3.8-27B-Uncensored-GGUF",
      "repo_id": "JonathanColetti/Qwen3.8-27B-Uncensored-GGUF",
      "model_files": [
        "Qwen3.8-27B-Uncensored-Q4_K_M.gguf"
      ]
    }
  }
}

The projector is discovered automatically because its filename starts with mmproj.

llama cpp

llama-server -m Qwen3.8-27B-Uncensored-Q4_K_M.gguf \
  --spec-type draft-mtp --spec-draft-n-max 2 \
  -ngl 99 -c 8192

Target plus explicit draft:

llama-server -m Qwen3.8-27B-Uncensored-noMTP-Q4_K_M.gguf \
  --spec-type draft-mtp \
  --model-draft Qwen3.8-27B-Uncensored-draft-Q8_0.gguf \
  -ngl 99 -c 8192

--spec-draft-n-max defaults to 3. Throughput depends on your hardware, so sweep it — measurements across draft lengths are in qwen3.8-spec-decode-bench.

Verification

Each artifact was checked post-quantization for MTP tensor survival rather than inferred from the conversion flag:

python quantize.py inspect Qwen3.8-27B-Uncensored-Q4_K_M.gguf

This reports metadata keys, declared block_count, and blocks actually present. A fused file whose present-block count does not exceed its declared count did not retain the MTP block.

Measured behaviour

Benchmarked against the unmodified base model on identical settings. The delta is the figure that matters: it isolates what the weight edit cost.

zero shot via lm-evaluation-harness, bf16, both models scored in the same session. Every delta is within or close to the reported standard error (MMLU +/- 0.30, ARC +/- 1.44, HellaSwag +/- 0.38, Winogrande +/- 1.21), so none is clearly separable from run-to-run noise.

These are 0-shot and are not comparable to Qwen's published scores, which use few-shot prompting. They are directly comparable to each other, which is the point. Note also that ARC-Challenge is low for a model at this MMLU. The base scores 58.9 under the same settings, so that is format sensitivity in a reasoning-tuned model, not abliteration damage.

What the benchmarks do not cover: no generative evaluation (GSM8K, HumanEval), no math or code, no multilingual, and the harness loads the text stack only. Nothing here measures the vision tower or MTP speculative decoding.

Search: 200 Heretic trials, 23 non-dominated points. The published model is the marked row.

How to read these

Refusal rate is the count of refusals over 100 heldout prompts from mlabonne/harmful_behaviors (test split) - explicitly harmful requests, not benign ones. So this number is not an over-refusal rate: it does not tell you how often the model declines legitimate work. It tells you how much of the original safety behaviour on harmful requests remains.

KL divergence is measured against the unmodified base model over first-token distributions, and is the optimizers proxy for "how much did we damage the model". Lower is closer to base. It is a proxy, not a capability measurement. A low KL does not certify that reasoning or coding ability survived, and nothing here does certify that.

The two trade off against each other. Heretic searches a Pareto front between them; the published point is one choice on that front, not a global optimum.

Caveats that matter

• Refusals were measured in non-thinking mode. This models chat template opens a <think> block, so the evaluation closes it explicitly to score answers rather than reasoning traces. With thinking enabled the refusal rate may differ, in either direction.
• The measurement is 100 prompts from one dataset. It generalizes to that distribution of harmful requests and no further. Refusal behaviour on other topics is uncharacterized.
• Perplexity is wikitext-2 only (see the Files table). It detects gross quantization damage. It does not detect capability loss on reasoning, code, or multilingual work.
• Quantization compounds everything above. The measurements were taken on the bf16 merge; the files you download are quantized.

Requirements

MTP speculative decoding landed in llama.cpp PR #22673. Builds older than that will load these files and silently ignore the MTP tensors.

Limitations

• Refusals are reduced, not eliminated, and not redirected. This model attempts many requests the original declines, but a meaningful fraction still get refused — see Measured behaviour.
• Behaviour near the old refusal boundary is less stable than the base model.
• Lower quants compound that, and IQ2_M compounds it most. Evaluate behaviour on Q6_K or Q8_0, not on IQ2_M or IQ4_XS. Nothing here measures the refusal boundary at 2-bit.
• Capability benchmarks show a 0.5-point mean drop vs base across MMLU, ARC-Challenge, HellaSwag and Winogrande. See Measured behaviour. No generative, math, code, or multilingual evaluation was run.

License

Apache 2.0, inherited from Qwen/Qwen3.8-27B. The base model's license and acceptable use policy still apply to your use of this derivative.

Speculative decoding, measured on this model

IQ2_M

Measured on this build: NVIDIA H200 NVL, 256 generated tokens, median of 3 repetitions, n_max swept 1 to 3. The table above dates from the original release and its hardware is not recorded, so compare the ratios rather than the absolute rates.

The MTP head survives 2-bit quantization because it is pinned to q8_0 rather than quantized with the rest of the stack, so speculative decoding still pays here.

Pairing noMTP-IQ2_M with the published draft-Q8_0 reaches 97.7 tok/s on the prose prompt at n_max 2, or 1.30x, which beats the fused file on that prompt. The reason is that the fused MTP head shares the low-bit token_embd (q4_K) and output (Q5_K) tensors with the main model, while the standalone draft carries its own Q8_0 copies. The split setup needs 13.3 GB of weights against 10.6 GB for the fused file, so it is the better option only if you have the VRAM to spare.