Gemma-4-31B-IT-Abliterated

An abliterated version of google/gemma-4-31B-it in BF16 precision. Abliteration removes refusal directions from model weights using the technique from Refusal in Language Models Is Mediated by a Single Direction (Arditi et al.), extended with SVD-based multi-direction subspace projection to capture refusal behavior encoded across multiple orthogonal directions.

Gemma 4 31B is a strong multimodal model with excellent reasoning, but Google's safety training is aggressive — it refuses a wide range of prompts including benign creative writing scenarios. This abliteration significantly reduces refusal rates while preserving the model's full capabilities, including vision.

Results

Refusal Rates

Tested on 100 harmful prompts across 3 modes (cold, system-prompted, retry) and 50 harmless prompts.

Mode	Baseline	Abliterated	Delta
Cold (no system prompt)	67%	32%	-35%
Prompted (creative writing system prompt)	47%	5%	-42%
Retry (prompted + retry on refusal)	40%	2%	-38%
Harmless	0%	0%	0%

Cold refusal remains higher than other abliterated models (Qwen3, Llama) — Google's safety training encodes refusal across non-linear mechanisms that are harder to fully remove with linear projection. With a system prompt, refusal drops to 5%, which is practical for most use cases.

MMLU (5-shot, generative)

Quick benchmark on 5 MMLU subjects via chat API. Not a full MMLU run — treat as a sanity check for capability preservation.

Subject	Baseline	Abliterated
Abstract Algebra	75.0%	75.0%
Anatomy	85.9%	88.1%
Astronomy	92.8%	92.8%
College Chemistry	57.0%	56.0%
College Physics	77.5%	74.5%
Overall (5 subjects)	79.5%	79.3%

The 0.2% difference is within noise. Abliteration preserved the model's reasoning capabilities.

Model Quality (Wikitext-2)

Evaluated on the full Wikitext-2 test set (291K tokens) to measure impact on general language modeling.

Perplexity

Model	Perplexity	Delta
Base (gemma-4-31B-it)	1.495	—
Abliterated	1.714	+14.7%

Both models achieve sub-2.0 perplexity on Wikitext-2, which is excellent. The +14.7% increase is modest and consistent with surgical weight modification — the model's general language capabilities remain strong.

KL Divergence (base || abliterated)

Per-token KL divergence over the output distribution, approximated via top-20 logprobs from vLLM.

Statistic	Value
Mean	0.354
Median	0.025
P95	1.371
P99	6.837

The median KL of 0.025 shows that on most tokens, the two models produce nearly identical distributions. The fat tail (P99 = 6.8) reflects tokens where abliteration had the most impact — likely positions where refusal-adjacent activations were strongest.

KV Cache Cosine Similarity

Layer-by-layer comparison of key and value cache activations on 50 Wikitext-2 samples (512 tokens each). This directly measures how much each layer's internal representations diverge between the base and abliterated models.

Layer Range	Key Similarity	Value Similarity	Notes
0–20	1.000000	1.000000	Unmodified layers — identical
21–30	0.9990–0.9999	0.9982–0.9999	Early ablated layers, minimal drift
31–40	0.9934–0.9984	0.9912–0.9972	Moderate drift
41–50	0.9913–0.9934	0.9817–0.9923	Peak drift zone (ablation peaks at 41, 58)
51–59	0.9891–0.9924	0.9830–0.9924	Highest value drift (layer 50: 0.982 values)
Overall	0.9966	0.9953

The most drifted layers by keys are 53, 51, 56, 55, 47. By values: 50, 52, 51, 57, 56. This aligns with the ablation configuration — layers 0–20 are bitwise identical, and drift onset at layer 21 exactly matches the first ablated layer.

How It Was Made

Measurement

Refusal directions were measured at full precision (BF16 weights, float32 compute) across all 60 layers using:

• 4,634 harmful prompts (augmented dataset covering violence, hate, cyber, fraud, drugs, self-harm, privacy, NSFW categories)
• 640 harmless prompts (standard harmless dataset)
• SVD decomposition (k=32) to extract orthogonal refusal directions per layer
• Projected orthogonalization to reduce collateral damage to non-refusal capabilities
• Welford's online algorithm in float32 for numerical stability

Ablation Configuration

• Layers ablated: 20 through 59 (40 of 60 layers)
• Directions: Top 8 SVD directions per layer (subspace projection)
• Measurement peaks: Layer 41 (secondary, quality 0.025) and Layer 58 (primary, quality 0.23)
• Scale factors: Variable per layer, up to 1.35 at peaks:
  • Layers 20-37: scale 0.35-0.45 (weak signal, gentle ablation)
  • Layers 38-51: scale 0.4-1.35 (bell curve around peak 41)
  • Layers 52-59: scale 0.6-1.35 (bell curve around peak 58)
• Weight targets: o_proj and down_proj in the language model only (vision encoder untouched)
• Technique: SVD subspace projection with norm preservation and projected orthogonalization
• Architecture note: Gemma 4 is multimodal with a separate vision encoder. Only model.language_model.layers.* weights are modified; model.vision_tower.* and model.embed_vision.* are copied verbatim.

Processing Details

Ablation was performed shard-by-shard on safetensors files, modifying weights in float32 precision then saving back to bfloat16. Full-precision measurement (no quantization during measurement) was critical — 4-bit quantized measurements produced noticeably weaker abliteration.

Usage

from transformers import AutoModelForImageTextToText, AutoProcessor

model_name = "null-space/gemma-4-31b-it-abliterated"

processor = AutoProcessor.from_pretrained(model_name)
model = AutoModelForImageTextToText.from_pretrained(
    model_name,
    torch_dtype="auto",
    device_map="auto",
)

messages = [
    {"role": "user", "content": "Your prompt here"}
]

inputs = processor.apply_chat_template(
    messages, tokenize=True, return_tensors="pt",
    return_dict=True, add_generation_prompt=True,
).to(model.device)

output = model.generate(**inputs, max_new_tokens=512)
print(processor.decode(output[0], skip_special_tokens=True))

Recommended Serving

vllm serve null-space/gemma-4-31b-it-abliterated \
    --tensor-parallel-size 2 \
    --max-model-len 8192

Fits comfortably on 2x GPUs with 48GB+ VRAM each at BF16.

Model Details

Property	Value
Base Model	google/gemma-4-31B-it
Architecture	Gemma4ForConditionalGeneration (multimodal)
Parameters	~32.7B
Hidden Size	5376
Attention Heads	32 Q / 16 KV (sliding), 4 KV (global)
Layers	60 (5 sliding + 1 full attention, repeating)
MLP Intermediate	21,504
Context Length	262,144 tokens
Vision	27-layer ViT encoder, 280 soft tokens per image
Precision	BF16
Model Size	~62 GB (2 shards)
Vocab Size	262,144

Ethical Notice

This model has had its refusal training partially removed. It will comply with many requests that the original model would refuse. You are solely responsible for how you use this model. It is intended for research into LLM alignment, safety evaluation, red-teaming, and creative writing applications.

Credits

• Base model: Google DeepMind
• Abliteration technique: Based on Refusal in Language Models Is Mediated by a Single Direction by Arditi et al., extended with SVD multi-direction subspace projection