---
base_model: Qwen/Qwen3-VL-4B-Thinking
library_name: peft
pipeline_tag: image-text-to-text
tags:
  - base_model:adapter:Qwen/Qwen3-VL-4B-Thinking
  - lora
  - peft
  - transformers
  - spatial-reasoning
  - visual-question-answering
  - chain-of-thought
license: apache-2.0
datasets:
  - spatialchain/SpatialChain-Benchmark
language:
  - en
---

# Qwen3-VL-4B-Thinking — SpatialChain LoRA Adapter

A LoRA adapter for **Qwen3-VL-4B-Thinking** fine-tuned on the [SpatialChain-Benchmark](https://huggingface.co/datasets/spatialchain/SpatialChain-Benchmark) dataset. The model learns to produce **scene-graph-grounded chain-of-thought reasoning** for binary spatial visual questions, structured as:

```
<think>
[step-by-step spatial reasoning]
</think>
<answer>
yes / no
</answer>
```

---

## Model Details

| Field | Value |
|-------|-------|
| **Base model** | [Qwen/Qwen3-VL-4B-Thinking](https://huggingface.co/Qwen/Qwen3-VL-4B-Thinking) |
| **Adapter type** | LoRA (PEFT) |
| **Training data** | [SpatialChain-Benchmark](https://huggingface.co/datasets/spatialchain/SpatialChain-Benchmark) train split (28,350 examples) |
| **Task** | Binary spatial VQA with chain-of-thought |
| **Language** | English |
| **License** | Apache 2.0 |

---

## Quick Start

```python
from transformers import AutoProcessor, AutoModelForVision2Seq
from peft import PeftModel
from PIL import Image
import torch

base   = "Qwen/Qwen3-VL-4B-Thinking"
adapter = "spatialchain/Qwen3-VL-4B-Thinking-SpatialChain"

processor = AutoProcessor.from_pretrained(base, trust_remote_code=True)
model     = AutoModelForVision2Seq.from_pretrained(
    base, torch_dtype=torch.bfloat16, device_map="auto", trust_remote_code=True
)
model = PeftModel.from_pretrained(model, adapter)
model.eval()

image = Image.open("your_image.jpg").convert("RGB")

messages = [
    {
        "role": "system",
        "content": [{"type": "text", "text": (
            "Your task:\n"
            "1. Analyze the image carefully.\n"
            "2. Provide concise reasoning grounded in visible evidence from the image.\n"
            "3. End your response with 'Answer: <one short sentence>'."
        )}],
    },
    {
        "role": "user",
        "content": [
            {"type": "image", "image": image},
            {"type": "text",  "text": "Is there a fence to the left of the person?"},
        ],
    },
]

text = processor.apply_chat_template(
    messages, tokenize=False, add_generation_prompt=True
)
inputs = processor(text=text, images=[image], return_tensors="pt").to(model.device)

with torch.inference_mode():
    ids = model.generate(
        **inputs,
        max_new_tokens=512,
        do_sample=True,
        temperature=0.6,
        top_p=0.95,
        top_k=20,
    )

print(processor.tokenizer.decode(ids[0, inputs["input_ids"].shape[1]:], skip_special_tokens=True))
```

### With 4-bit quantization (lower VRAM)

```python
from transformers import BitsAndBytesConfig

bnb = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_quant_type="nf4",
    bnb_4bit_compute_dtype=torch.bfloat16,
    bnb_4bit_use_double_quant=True,
)
model = AutoModelForVision2Seq.from_pretrained(
    base, quantization_config=bnb, device_map="auto", trust_remote_code=True
)
model = PeftModel.from_pretrained(model, adapter)
```

---

## Training Details

### Dataset

[SpatialChain-Benchmark](https://huggingface.co/datasets/spatialchain/SpatialChain-Benchmark) — 28,350 training examples pairing spatially-oriented GQA questions with scene-graph-grounded reasoning chains. Questions cover 11 spatial relation types (`left_of`, `right_of`, `above`, `behind`, `near`, `inside`, …); chains were generated with Claude Haiku 4.5 (extended thinking) and retained only when the generated answer matched the GQA ground truth.

Each training example target:
```
<think>
Looking at the image, let me trace through this step-by-step:
(1) Locating the knife — I can see a knife on the left side of the plate.
(2) Finding the bread to the right of the knife — there is a large piece of bread ...
(3) Examining what is to the right of that bread — gray birds are standing on the plate.
(4) Looking for kittens — I do not see any kittens anywhere in the image.
</think>
<answer>
No, there is a bird to the right of the bread.
</answer>
```

### Hyperparameters

| Hyperparameter | Value |
|----------------|-------|
| Base model | Qwen3-VL-4B-Thinking |
| Quantization | 4-bit NF4 (BitsAndBytes) |
| LoRA rank (r) | 16 |
| LoRA alpha | 32 |
| LoRA dropout | 0.05 |
| RSLoRA | ✓ |
| Target modules | all-linear |
| Modules to save | `lm_head`, `embed_tokens` |
| Epochs | 2 |
| Per-device batch size | 4 |
| Gradient accumulation | 3 (effective batch = 12) |
| Learning rate | 3 × 10⁻⁵ |
| LR schedule | cosine |
| Warmup ratio | 0.05 |
| Max sequence length | 32,768 |
| Image max size | 640 px |
| Optimizer | AdamW fused |
| Hardware | 1 × A100 80 GB |
| Training framework | HuggingFace Transformers + PEFT |

---

## Evaluation

### SpatialChain test set (n = 899)

Evaluation uses two complementary axes. **Axis 1** measures VQA accuracy (exact match after normalisation). **Axis 2** uses a scene-graph-aware LLM judge scoring reasoning faithfulness and completeness independently of the final answer — see the [evaluation code](https://huggingface.co/datasets/spatialchain/SpatialChain-Benchmark) for the full judge protocol.

| Metric | Base (4B) | **This model (4B FT)** |
|--------|-----------|------------------------|
| VQA Accuracy | 78.44% | **82.23%** |
| Macro F1 | 82.01% | **86.67%** |
| Yes-accuracy | 77.74% | 91.34% |
| No-accuracy | 79.64% | 66.57% |
| ROUGE-1 vs. reference chain | 0.403 | **0.657** |
| Token F1 vs. reference chain | 0.392 | **0.646** |
| Reasoning faithfulness (judge) | 0.585 | **0.631** |
| Reasoning completeness (judge) | 0.658 | **0.708** |
| Pass rate | 77.6% | **80.2%** |
| Shortcut rate ↓ | 26.4% | **19.4%** |

**Shortcut rate** = fraction of *correct* answers where the judge scores reasoning faithfulness < 0.5. Lower is better.

### External benchmarks

SFT on SpatialChain improves in-domain performance but introduces a **stylistic specialisation effect** on out-of-distribution benchmarks — the model adopts the SpatialChain chain format even when the input distribution differs. Replay-augmented training is recommended to mitigate this.

| Benchmark | Base | Fine-tuned | Δ |
|-----------|------|------------|---|
| SpatialChain test | 78.4% | **82.2%** | +3.8 pp |
| [FlagEval/ERQA](https://huggingface.co/datasets/FlagEval/ERQA) | 45.3% | 38.0% | −7.3 pp |
| [FlagEval/EmbSpatial-Bench](https://huggingface.co/datasets/FlagEval/EmbSpatial-Bench) | 79.1% | 75.7% | −3.4 pp |

---

## Intended Use

- **Spatial VQA** — binary yes/no questions about object positions and relations in images
- **Reasoning audit** — producing interpretable spatial chains that can be verified against scene structure
- **Research** — studying the relationship between chain-of-thought quality and answer correctness in VLMs

## Out-of-Scope Use

- Tasks requiring metric depth or 3D reasoning (scene graphs are symbolic, not metric)
- Open-ended image captioning or generation
- Non-English inputs

## Bias and Limitations

- **Yes-bias** — the fine-tuned model exhibits a larger yes/no accuracy gap (+24.8 pp) than the base model (+1.9 pp), consistent with the 58% yes-rate in training data. Evaluation should report Yes-acc and No-acc separately.
- **Stylistic specialisation** — the model adopts a fixed reasoning format ("Looking at the image, let me trace through this step-by-step…") on all inputs, which may degrade performance on benchmarks with different prompt styles.
- **GQA domain** — training images are sourced from GQA (Visual Genome); performance on non-natural-image domains is unknown.
- **Projective bias** — 62.7% of training examples involve `left_of` / `right_of` relations; depth-ordered relations (`close`, `far`) are underrepresented.

---

## Citation

```bibtex
@article{spatialchain2026,
  title   = {SpatialChain: A Benchmark for Auditing Spatial Reasoning Faithfulness in VLMs},
  author  = {Anonymous},
  journal = {Under review at NeurIPS 2026},
  year    = {2026}
}
```

---

## Environmental Impact

Training ran for approximately **5 hours** on a single **A100 80 GB** GPU (cloud instance). Carbon emissions can be estimated with the [ML Impact Calculator](https://mlco2.github.io/impact#compute).