Proton Chess

Proton Chess is an experimental open-source chess engine written in C++20 with supporting Python tooling for testing, data generation, and future neural-evaluation work. The current codebase is a classical search engine with a generated opening-book workflow and a small training scaffold for later NNUE-style and deep-eval integration.

What is in this repo

• A UCI engine with legal move generation, perft support, iterative deepening alpha-beta, quiescence, aspiration windows, a transposition table, null-move pruning, and LMR.
• A baseline evaluator plus GPU-ready deep-eval interfaces.
• Match, watcher, and play-against-engine scripts.
• A generated opening-book pipeline built from official Lichess broadcast PGNs.
• Training schema and export stubs for future neural models.

Current strength

This is still an early engine. It is competitive against weaker engines and lower strength-limited Stockfish settings, but it is not yet close to unrestricted top engines.

Current verified checkpoints:

• python tools/check_perft.py build\proton_chess.exe
• python tools/crosscheck_movegen.py build\proton_chess.exe --count 40 --plies 10 --seed 17
• Sunfish, 8 games, 0.2s/move, plain starts: 3 wins / 1 loss / 4 draws
• Stockfish 18 with UCI_Elo=1800, 8 games, 0.2s/move: 5 wins / 1 loss / 2 draws
• Stockfish 18 with UCI_Elo=1900, 8 games, 0.2s/move, mini_suite: 5 wins / 2 losses / 1 draw
• Stockfish 18 with UCI_Elo=2000-2100: competitive in some short samples, but still unstable across reruns
• Stockfish 18 unrestricted and Koivisto 9.0: still losing

Build

cmake -S . -B build
cmake --build build --config Release

Run

.\build\proton_chess.exe

Supported engine commands:

• uci
• isready
• ucinewgame
• position startpos moves ...
• position fen <fen> moves ...
• go depth <n>
• go movetime <ms>
• go wtime <ms> btime <ms> winc <ms> binc <ms>
• perft <depth>
• d

Opening book generation

The engine can load weighted UCI opening lines from openings/book_lines.txt. To regenerate that file from official Lichess broadcast archives:

python -m pip install zstandard
python tools\build_opening_book.py `
  --download-url https://database.lichess.org/broadcast/lichess_db_broadcast_2026-02.pgn.zst `
  --download-url https://database.lichess.org/broadcast/lichess_db_broadcast_2026-01.pgn.zst `
  --download-url https://database.lichess.org/broadcast/lichess_db_broadcast_2025-12.pgn.zst `
  --max-games 30000 `
  --max-ply 18 `
  --root-branching 16 `
  --branching 6 `
  --min-count 4 `
  --min-ratio 0.02

Downloaded .zst source files are cached under openings/data/ and ignored by Git.

Tooling

Syntax check:

python -m compileall python tools

Run a small match against Sunfish:

python tools\run_match.py build\proton_chess.exe tools\sunfish_uci.cmd --games 8 --seconds 0.2

Run against strength-limited Stockfish:

python tools\run_match.py build\proton_chess.exe external\bin\stockfish18\stockfish\stockfish-windows-x86-64.exe --games 8 --seconds 0.2 --opponent-option UCI_LimitStrength=true --opponent-option UCI_Elo=1800

Watch a live match:

.\watch_stockfish.bat

Play against Proton locally:

python tools\play_against_engine.py build\proton_chess.exe --human-color white --seconds 0.2 --pgn .\out\human_vs_proton.pgn

Repo layout

• src/: engine source
• python/: training and export scaffolding
• tools/: perft, movegen cross-check, and match runners
• openings/: opening suite and generated book output
• configs/: evaluation harness configs

License

This repository is licensed under the MIT License. See LICENSE.