A production chess platform and a strong chess engine, in one repo. The site runs at chessgo.timanthonyalexander.de: play vs other humans with live matchmaking and server clocks, vs the AI, solve puzzles, and play three variants (Chess960, Crazyhouse, Duck Chess).
The engine is zugzwang, a from-scratch C++17 NNUE engine — every rule, the move generator, the evaluation, and the search written from scratch. It serves all of the site's rules and all of its AI over a stateless HTTP API, and speaks UCI for any chess GUI.
A perspective-relative NNUE (a 512-wide feature transformer with 16 king-buckets and Stockfish-style full-threat inputs, incremental int16 accumulator, native SIMD kernels) over a PVS alpha-beta search: iterative deepening, aspiration windows, a clustered transposition table, late move reductions, SEE-filtered captures and quiets, null-move pruning, reverse futility and late-move pruning, singular extensions, correction history, and continuation history — with magic bitboard move generation, and a concurrent search-context pool for the web service. It falls back to a hand-crafted eval when no net is loaded.
Strength was built by self-play SPRT — a change plays the previous version until the test decides it's an improvement or rejects it; nothing ships on a hunch. zugzwang is materially stronger than the previous engine on the same net and sits within ~150–200 Elo of full-strength Stockfish at equal movetime. No point rating is quoted.
zugzwang succeeded gomachine, the original from-scratch Go engine, in the
2026-07 cutover (it won the head-to-head on the same net). gomachine is now an
archived reference — its docs live under gomachine/ — while its realtime hub
stays on as the live-game infrastructure.
cd zugzwang && make # arch-detected native build → ./zugzwang
./zugzwang serve # HTTP engine API on :6476
./zugzwang # UCI, for any GUI (Arena, Cute Chess, lichess-bot)Point net.nnue at the evaluation net (symlinked to the prod net in this repo);
without it the engine runs on its hand-crafted eval. See zugzwang/CLAUDE.md for
engine internals.
No account needed to play — bot games, puzzles, and casual live games all work as a guest. An account adds ratings, required for rated games.
- Live games — rating-proximity matchmaking (the gap widens with wait, capped at 400), server clocks that start Lichess-style, chat, draw/takeback offers, resign, premoves, reconnect and resume. A rating-matched bot fills in if no human turns up.
- Ratings — Glicko-2 tracked separately for bullet, blitz, rapid, and classical, plus an isolated puzzle rating.
- Play the computer — a 700–2900 Elo strength slider, choose your color, untimed, with a live eval bar and premoves. Unrated.
- Puzzles — Puzzle-Rush-style timed sessions on Lichess-seeded, rating-matched tactics; theme filter; server-validated solutions; a deterministic daily puzzle.
- Analysis board — streaming eval, best-move arrow, principal variation, a branching move tree, full-game review with per-player accuracy, an opening explorer, and an optional Stockfish second opinion.
- Variants — Chess960, Crazyhouse, and Duck Chess.
- Also — Guess-the-Elo, a position editor, watch/spectate the strongest live games, profiles, leaderboards, a daily-activity streak, and an admin panel with anti-cheat review.
Five services plus MySQL: the PHP backend (BaseAPI, :6464), the React frontend
(Vite + Bun, :6465), the zugzwang engine (:6476), the realtime hub
(gomachine hub, :6467), and — for now — the legacy gomachine engine (:6466).
./mason serve --screen # PHP API on :6464
cd zugzwang && ./zugzwang serve # engine on :6476
WS_TICKET_SECRET=… gomachine hub # hub on :6467
cd frontend && bun run dev # frontend on :6465Open http://127.0.0.1:6465. Full setup and deployment are in docs/COMMANDS.md; the design is in docs/SPEC.md and docs/ARCHITECTURE.md.
zugzwang/— the engine (C++): rules, movegen, search, NNUE, HTTP serve, variants. See zugzwang/CLAUDE.md.app/— PHP backend (BaseAPI); routes inroutes/api.php.frontend/— React + Vite + TypeScript + MUI.gomachine/— the realtime hub (internal/hub) + the archived Go engine and its historical docs (gomachine/docs/,gomachine/engine/docs/).- CLAUDE.md — fast orientation for the whole codebase.
- docs/tasks/ — the open + done backlog.
GPLv3. See LICENSE. Derivative work stays open under the same terms.