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UofTCSSLab/
Maia3-5M

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Maia3-5M

Part of the Maia3 family of transformer models for human chess move prediction. This is the 5M-parameter variant.

For full details — architecture details, training recipe, full evaluation, and ablations — see our paper Chessformer: A Unified Architecture for Chess Modeling (ICLR 2026).

Model summary

Family: Maia3 — human move prediction models built on the Chessformer architecture
Architecture: encoder-only transformer with board squares as tokens, augmented by Geometric Attention Bias (GAB) — a dynamic positional encoding that adapts to the geometry of chess — and an attention-based source-destination policy head
Parameters: 5M
Task: predicting the move a human player of a given skill level would make from a given position
Training data: Lichess human games, January 2023 – July 2025
License: CC BY 4.0 (paper); see repo for code/weights license

Intended use

Maia3 models predict human chess moves conditioned on player rating. Typical uses include:

Research on human chess modeling and human–AI alignment
Move-suggestion and analysis tools that emulate play at a chosen rating
Mechanistic interpretability research — the square-token design makes attention patterns and activations directly attributable to board squares, and the repo ships a cross-layer transcoder (transcoder.py) for studying internal features

Not intended for maximum playing strength. For strong engine play built on the same architecture, see the Chessformer integration into Leela Chess Zero described in the paper.

How to use

Maia3-5M is a PyTorch checkpoint trained with the code at CSSLab/maia3. Clone that repo, set up the conda environment, and load the checkpoint following the instructions in its README.

Architecture hyperparameters for this variant are defined in ablate_size.sh in the training repo.

Training

Data: Lichess monthly game dumps, January 2023 – July 2025
Validation: Allie-style annotated test set (data/evals/2022-test-annotated.jsonl)
Code: CSSLab/maia3
Config: size ablation row corresponding to 5M parameters in ablate_size.sh

Evaluation

The Maia3 family reaches 57.1% move-matching accuracy on human moves, significantly surpassing the previous state of the art with fewer than a quarter of the parameters. Per-size accuracy curves, scaling analysis, and skill-conditioned breakdowns are reported in the paper.

Citation

bibtex

@inproceedings{monroe2026chessformer,
  title={Chessformer: A Unified Architecture for Chess Modeling},
  author={Daniel Monroe and George Eilender and Philip Chalmers and Zhenwei Tang and Ashton Anderson},
  booktitle={The Fourteenth International Conference on Learning Representations},
  year={2026},
  url={https://openreview.net/forum?id=2ltBRzEHyd}
}

UofTCSSLab/Maia3-5M