Play chess vs Stockfish under your own rules
Free Stockfish opponent in your browser. Rules are yours to set: a move, and whether playing it loses the game or wins it.
Updated
Rules you set
A rule has two fields: the move, and what playing it does. The moves are engine top move, any other move, capture, non-capture, a move that loses 1.00 or more, check, and promotion. Playing one either loses the game or wins it, on the spot.
No rules is an ordinary game against Stockfish. Add Capture, loses the game and the first piece you take ends it; add Engine top move, loses the game and you have to find a move Stockfish did not want. Engine strength runs from 0 Elo to full strength.
Every game has an address
The moves, the mode and the side you played are in the game's URL, so it can be sent or reopened on another device. Nothing is uploaded. Stockfish 18 is compiled to WebAssembly and runs in the browser tab.
Review it when it is over
A finished game opens as a game review: accuracy for both sides and every move classified. Positions can also be set up by hand on the sandbox board.
Named modes
Each of these opens with the rule or bot already set. Breaking a “don’t” raises Stockfish’s rating; doing what a “reward” asks drops it. A bot, like Drawfish, changes which move Stockfish plays. The game does not end on the spot.
- Don’t play the engine move, Copy Stockfish’s first choice and its rating goes up 100. Play something else and nothing happens. The engine gets harder when you cheat the rule.
- Only engine moves, A move that is not Stockfish’s first choice raises its rating by 100. Stay on its line or the game gets harder.
- No captures, Capturing a piece raises Stockfish’s rating by 100. Quiet moves are free. Take something and the game gets harder.
- Captures only, A move that does not capture raises Stockfish’s rating by 100. The engine still plays ordinary chess.
- No blunders, A move the review would call a blunder — one that gives up 20% or more of your winning chances — raises Stockfish’s rating by 100.
- Drawfish, Drawfish scores every legal move with Stockfish and plays the one closest to 0.00, from WatchCheckmate’s “Stockfish, But…” videos. Get ahead and it gives material back to level the game. Try to make it lose.
- 2nd best move, Stockfish finds its two best moves and plays the second. When only one move is legal it plays that.
- 1 of 3 top moves, Each turn Stockfish finds its three best moves and plays one of them at random.
- If it can check, it must, If Stockfish has a checking move, it plays its best check, even when the check loses. Otherwise it plays normally.
- Copies your move quality, Stockfish plays the move that loses as much as your last move lost. Play perfectly and it plays perfectly; blunder a piece and it blunders one back.
- Light and dark squares, Stockfish’s first move lands on a light square, its second on a dark square, and it keeps alternating. When it has no move to the right colour it plays normally.
- 250 ↔ 3600 Elo every capture, Stockfish starts at 250 Elo. Every capture, by either side, switches it between 250 and full strength. Time your trades.
- Must sacrifice a rook, If Stockfish can move a rook to a square you attack, it must, and it picks the best of those moves. Otherwise it plays normally.
- Wants your queen, If Stockfish can take your queen, it does. If it cannot, it attacks your queen when it has a way to. Otherwise it plays normally. Use your queen as bait.
- Loses 300 Elo when you check, Every check you give drops Stockfish’s rating by 300. Check often enough and it drops to beginner strength.
- No checks, Giving check raises Stockfish’s rating by 100. Everything else is ordinary chess.
- No promotions, Promoting a pawn raises Stockfish’s rating by 100. The rest of the game is unchanged.
- Rewarded for the engine move, Playing Stockfish’s first choice drops its rating by 100. Copy it and the game gets easier.
- Rewarded for other moves, A move that is not Stockfish’s first choice drops its rating by 100. Deviating makes the game easier.
- Rewarded for captures, Each capture drops Stockfish’s rating by 100. Quiet moves do not score.
- Rewarded for quiet moves, A non-capture drops Stockfish’s rating by 100. Taking a piece does not score.
- Rewarded for blunders, A move the review would call a blunder — one that gives up 20% or more of your winning chances — drops Stockfish’s rating by 100.
- Rewarded for checks, Giving check drops Stockfish’s rating by 100.
- Rewarded for promotions, Promoting a pawn drops Stockfish’s rating by 100.
/play is the blank board, if you want to write your own combination.