chessformer_lens

A toolkit and app for mechanistic interpretability of transformer based chess models.

pip install chessformer_lens

Try the app in your browser, no install needed:

Paper: Increasing Skill Level Recruits Deeper Attention Layers in a Frozen Chess Transformer (Litman, 2026). Every figure and most of the analysis in this paper was made with this library.

Pip install:

python3 -m venv .venv && source .venv/bin/activate
pip install chessformer_lens
# for Maia-3 models, first: pip install https://github.com/CSSLab/maia3/archive/1e13597c42d4858b7cfd7cfdae01e297263364b2.zip
# Leela's BT4 net downloads itself on first use

or Clone if you want to edit the code:

git clone https://github.com/chessformer-lens/chessformer_lens
cd chessformer_lens
pip install -r requirements.txt

Demo Notebook:

There is wonderful prior chess-interp work, for instance: McGrath et al. on AlphaZero concepts, Jenner et al. on lookahead in Leela, Karvonen on chess-GPT, but there is no general infrastructure for rigorous interpretability work or interactive visualization for these models.

Chessformer_lens is built to do both, especially inspired by Neel Nanda's transformer_lens library.

Chess is unusually suited for AI interpretability because it requires complex and structured reasoning in a compact domain, and it has a hierarchy of human concepts (squares -> threats -> tactics -> strategy) which can provide deep insight into how AI carves features.

Transformer based chess models ("chessformer") are a good lens for studying LLMs because outputs have a clear ground source for the evaluation of a move and so avoid the vagueness of language, because attention functions with clearly interpretable roles on a chess board, and because informative chessformer results are straightforward to scale and investigate in LLMs.

This repo's core is one engine with three frontends: engine.py is the interp core (model + hooks + logit lens + head ablation + list of values through depth). It does all of the analysis for:

• app.py
• interp_plot.py
• interp_widget.py

Users are encouraged to read the user guides for each of these modules which can be found at the top of the respective scripts. Image from paper produced with chessformer_lens

Leela Chess Zero

This is an extremely strong engine that has many many more layers and parameters but the same square-token design as others like Maia-3.

Weights: bt4 (the base BT4 net) or bt4-332 (its policytune-332 fine-tune) download from lczero.org on first use into ~/.cache/chessformer_lens. A path to any lc0 .pb.gz works too.

Load. One alias table covers both families; neither is a default.

from chessformer_lens import LeelaEngine, load_engine
eng = LeelaEngine("bt4")

Quickstart in notebook:

Recall that FEN is the modern notation for a chess position

!pip install -q https://github.com/CSSLab/maia3/archive/1e13597c42d4858b7cfd7cfdae01e297263364b2.zip
!pip install -q "chessformer_lens>=2.0"

import chess
from chessformer_lens import MaiaEngine
import chessformer_lens.interp_widget as iw

eng = MaiaEngine()          
# Opera Game FEN right at legendary queen sacrifice 
fen = "4kb1r/p2n1ppp/4q3/4p1B1/4P3/1Q6/PPP2PPP/2KR4 w k - 0 16" 
# [or insert another FEN string]

try:
    board = chess.Board(fen)
except ValueError as e:
    print(f"Invalid FEN: {e}")   

#Interact with the attention widget
#click between different layers and heads in the widget
iw.attention_widget(eng, board, 1500,layer=4,head=3) 

The app

To me, the app is the pièce de résistance and usage should be rather intuitive, nonetheless, a detailed guide can be found in app_guide.md.

Play a transformer-based chess bot ("chessformer") and watch its move policy, its attention (both semantic QKᵀ and unique geometric GAB), and — for any move you click — its logit through depth, the heads that carry it, and the neurons that carry it. The model loads on a background thread so the window opens instantly.

#Various ways to launch, note that higher parameter models have 16 and 32 heads per layer, and 512 and 1024 dim of the model (d_model):
chessformer_lens                    # no model named: opens a model picker
chessformer_lens --list             # print every engine and its aliases
chessformer_lens 23m               
chessformer_lens --model maia3-79m 
CHESSFORMER_MODEL=5m chessformer_lens
chessformer_lens bt4                # Leela Chess Zero BT4 — see above   
chessformer_lens --web              # the same app in your web browser

Code layout: all found within the chessformer_lens package

• __init__.py — the package surface: MaiaEngine, LeelaEngine, load_engine, build_cfg, pick_device, Conditioning, ModelAdapter, register_adapter, available_models, attention_widget, gab_widget, __version__.
• engine.py — the interp core (model + hooks + logit lens + head ablation + list of values through depth): ChessformerEngine holds every method and works on any model a ModelAdapter describes; MaiaEngine and LeelaEngine are thin wrappers. No UI deps; imports cleanly in a notebook.
• core.py — the model-agnostic layer: ModelAdapter, the adapter registry, and the types the engine speaks in (Conditioning, TokenLayout, ValueEstimate, …).
• adapters/ — one adapter per model family: lc0.py (Leela BT4, no maia3 needed), maia3.py (Maia-3), smolgen.py (the smolgen/GAB attention both use).
• interp_plot.py — matplotlib figures that the app uses
• interp_widget.py — the app's two interactive panels as a self-contained notebook cell or standalone HTML page
• piece_art.py — draws pieces.py's cburnett pieces in matplotlib from PNGs
• bridge.py — the game state + the JSON API the UI calls (LensApi).
• ui.py — the whole interface (HTML/CSS/JS) as one string including guide to change template.
• pieces.py — SVG piece set as data URIs.
• app.py — app launcher (native window via pywebview); the chessformer_lens command is its main().
• web.py — the same app served to a web browser (chessformer_lens --web, or web.serve() in a notebook).

These come with concise but detailed docstrings.

General color guide

Blue and orange is used for diverging signed values; negative is blue, positive is orange. Viridis is used for magnitudes like attention weights. Green has to do with a specific chess move. Red has to do with undesirable things. The black and white halo lens denotes a query square.

Notes

Created by David Litman

Maia-3 comes from: Chessformer / Maia-3 (Monroe et al., ICLR 2026).

BT4 comes from the Leela Chess Zero project (lczero.org); the smolgen / attention-body design it uses is described in the Chessformer paper and in lc0's own writeups.

Both Maia-3 and Leela BT4 are supported. Both treat each square as a token—which allows for beautiful board readable attention patterns. The core now works through model adapters, so other models can plug in; for models whose tokens aren't squares, the board views show a "not available yet" notice for now.

Please don't hesitate to give me feedback or thoughts by email or at my personal website. I hope for this to be a useful and intuitive tool for the community.

Citing

If chessformer_lens contributes to published work, a citation is very appreciated and helps others find it! Use GitHub's "Cite this repository" button, or DOI 10.5281/zenodo.21877655.