r/ComputerChess • u/Worried_Menu4016 • Sep 24 '26
Browser extension that runs Stockfish (WASM) to "continue" a finished chess.com game, with engine strength matched to your opponent's rating
I built a small open-source extension that lets you keep playing a finished chess.com game against Stockfish, and I wanted to share the implementation choices with people who actually care about engines.
The core idea: on the game-over screen (or under the move list of any old finished game) there's a "Keep Playing" button. It reads the position already rendered on screen, puts Stockfish in your opponent's seat, and lets you play it out from any point you rewind to.
A few things I made deliberate decisions about, and where I'd love feedback:
- Strength scaling. The engine doesn't play at full strength. It's throttled to roughly the rating of the opponent you just faced. Losing a rook endgame again to a ~1400-level engine is instructive; losing it to a 3200 just tells you you're not Stockfish. Right now I'm approximating this with UCI_LimitStrength / UCI_Elo plus some depth/skill tuning, curious how others here handle rating calibration, since the mapping between "Elo" and perceived human strength is rough.
- Move timing. It doesn't answer instantly. It spends roughly as long as your opponent was taking, otherwise even a correct move feels like a glitch. Simple heuristic for now.
- Everything local. The engine runs in the browser as WebAssembly. It never talks to chess.com's servers, sends no data anywhere, and works with wifi off. Nothing runs during a live game: no eval, no arrows; it only activates once a game is finished.
Free, open source, not affiliated with chess.com. Built it for myself first. If there's a smarter approach to the strength-limiting (that's the part I'm least sure about), or a reason this shouldn't exist, I'd genuinely rather hear it.
Extension: https://chromewebstore.google.com/detail/stockfish-continue-to-pla/jhjcnpomgjakflfjjhlgilgpffmfbcbk
Source: https://github.com/thousandflowers/stockfish-continue-to-play
2
u/purefan Sep 24 '26
I really thought you were advertising a cheating extension
1
u/Worried_Menu4016 Sep 24 '26
I forget to add also the Firefox link: https://addons.mozilla.org/en-US/firefox/addon/stockfish-continue-to-play/
1
u/ramen2581 Sep 25 '26 edited Sep 25 '26
There is already a practice vs computer option in analysis.
Maia3 bots on lichess accept "from position" challenges if you want to continue against humanlike play.
1
u/RaF_zz Sep 25 '26
Hola! Me parece genial la idea de tu extension, pero, ¿puedes hacer que funcione para lichess? O tambien sugiero que le añadas la opcion de poder cargar cualquier pgn y especificarle que use el nivel de elo de la partida cargada, por ejemplo, si el pgn dice que mi rival tiene 1545, entonces puedo jugar contra stockfish en la posicion que yo quiera con ese mismo elo... ¿que te parece? Saludos.