---
license: apache-2.0
language:
- en
pipeline_tag: text-generation
tags:
- causal-lm
- gpt
- small-language-model
- arithmetic
- custom-tokenizer
- custom-code
- safetensors
- lm-evaluation-harness
datasets:
- openbmb/Ultra-FineWeb
- HuggingFaceFW/fineweb-edu
- HuggingFaceTB/finemath
- HuggingFaceTB/smollm-corpus
---

![bg](bg.png)

# Atom2.7m

Atom2.7m is a small decoder-only causal language model trained with a general byte-level BPE tokenizer plus arithmetic-specific digit features. The model has 2,738,880 parameters and uses custom code for both the model and the tokenizer path.

The main result is on [ArithMark 2.0](https://huggingface.co/datasets/AxiomicLabs/ArithMark-2.0), a 2,500-example integer-arithmetic continuation benchmark. Atom2.7m scores 69.24% accuracy. This places it above the nearby published range of SmolLM2-1.7B at 66.12% and Qwen2.5-0.5B at 63.04%, while using only 2.74M parameters.

The result shows the leverage of domain-specific design. With arithmetic-aware tokenization and digit features, Atom2.7m reaches the same ArithMark score band as models hundreds of times larger.

## Model Details

- Architecture: decoder-only GPT
- Parameters: 2,738,880
- Layers: 5
- Hidden size: 192
- Attention heads: 4
- KV heads: 2
- Attention: grouped-query causal self-attention with RoPE and XSA projection
- Context length: 512
- Vocabulary size: 4,096
- Token embeddings: tied input/output embeddings
- Arithmetic feature embeddings:
  - `place_vocab_size`: 66
  - `role_vocab_size`: 12

## Tokenizer

Use this model with `trust_remote_code=True`. The submission includes an `AtomTokenizer` remote-code wrapper in `tokenization_atom.py` so standard Hugging Face callers can use `AutoTokenizer.from_pretrained(...)`.

The tokenizer keeps byte-level BPE for ordinary text, but treats arithmetic sensitive spans specially:

- digits `0`-`9` are atomic and never BPE-merged
- digit spans are emitted least-significant-digit first
- `+ - * / = ( )` are isolated atomic tokens
- whitespace is isolated from text
- arithmetic feature IDs are derived by the model from token IDs at inference time

Training and custom tooling may still pass aligned `place_ids` and `role_ids`, but generic inference and evaluation only need `input_ids` and `attention_mask`.

## Usage

```python
import torch
from transformers import AutoModelForCausalLM, AutoTokenizer

model_dir = "."

model = AutoModelForCausalLM.from_pretrained(
    model_dir,
    trust_remote_code=True,
).eval()
tokenizer = AutoTokenizer.from_pretrained(
    model_dir,
    trust_remote_code=True,
)

text = "12 + 34 ="
inputs = tokenizer(text, return_tensors="pt", add_special_tokens=False)

with torch.no_grad():
    outputs = model(**inputs)
```

## Evaluation

### ArithMark 2.0

Use the included benchmark script:

```bash
python benchmark_fusion_arithmark.py \
  --checkpoint . \
  --data-path arithmark_2.0.jsonl \
  --batch-size 64 \
  --device cuda \
  --output benchmark_results/fusion_arithmark_2.0_results.json
```

### lm-evaluation-harness

For lm-evaluation-harness tasks, use the standard `hf` model with remote code enabled:

```bash
lm_eval \
  --model hf \
  --model_args pretrained=.,trust_remote_code=True,dtype=bfloat16,max_length=548 \
  --tasks hellaswag,arc_easy,arc_challenge,piqa \
  --device cuda:0 \
  --batch_size auto:1 \
  --output_path benchmark_results/lm_eval
```

`max_length=548` is passed to the lm-evaluation-harness wrapper so long
multiple-choice continuations do not trip the harness assertion that a
continuation must fit inside the model window. The tokenizer also advertises
`model_max_length=548`, matching the longest sequence observed in this eval run.
The checkpoint was trained with a 512-token context, but the RoPE
implementation can score this slightly longer harness window; reduce batch size
or set `max_length` to the longest sequence found if a task variant contains
longer continuations.

## Results

| Benchmark | Metric | Value |
| --- | --- | ---: |
| ArithMark 2.0 | acc | 0.6924 |
| arc_challenge | acc_norm | 0.2099 |
| arc_easy | acc_norm | 0.3161 |
| hellaswag | acc_norm | 0.2701 |
| piqa | acc_norm | 0.5299 |

## Training Data

The pretraining mixture targeted about 3.5B tokens:

- Ultra-FineWeb: 900M
- FineWeb-Edu: 900M
- FineMath: 450M
- Cosmopedia-v2: 337.5M
- UltraData-Math-L2-preview: 337.5M
- Ultra-FineWeb-L3-en-QA-Synthetic: 225M
- Synthetic-Arithmetic: 350M

Synthetic-Arithmetic is canonical integer equation data. The training curriculum is included as `pretraining_curriculum.json`.

## Limitations

- This is a very small model and should be treated as an experimental research artifact.
- Use `trust_remote_code=True` so `AutoTokenizer` applies the digit-span transform.
- Numeric text is represented least-significant-digit first internally.
- Role annotations intentionally target strict integer equations, not broad math prose, decimals, rationals, or QA formats.

## Files

- `model.safetensors`: model weights
- `config.json`, `config.py`, `configuration_gpt.py`, `model.py`: custom model code
- `tokenizer.json`, `tokenization_atom.py`: tokenizer files and remote-code wrapper
- `benchmark_fusion_arithmark.py`: ArithMark evaluation
- `arithmark_2.0.jsonl`: local ArithMark 2.0 data for the standalone benchmark script
- `pretraining_curriculum.json`: training curriculum

## References / Design Influences

- [Attention Is All You Need](https://arxiv.org/abs/1706.03762) - additive positional information in Transformer inputs
- [Exclusive Self Attention](https://arxiv.org/abs/2603.09078) - related attention work on reducing self-position dominance in sequence modeling
- [Position Coupling: Improving Length Generalization of Arithmetic Transformers Using Task Structure](https://arxiv.org/abs/2405.20671) - coupling digit positions by arithmetic significance
- [Transformers Can Do Arithmetic with the Right Embeddings](https://arxiv.org/abs/2405.17399) - digit-position embeddings for arithmetic