ChessMate — Best Move for chess.com

src/img&videa/Pasted image.png Chrome/Edge extension (Manifest V3) that shows the best move on chess.com using local Stockfish. Works on live games and analysis. No internet needed for the engine — Stockfish runs entirely on your device.

Warning: using engine assistance in rated online play violates chess.com Terms of Service and can get your account banned. This tool is meant for analysis and learning. Use at your own risk.

Demo

Install

1. Open chrome://extensions (or edge://extensions)
2. Enable Developer mode (top right)
3. Click Load unpacked and select the chessmate folder
4. Open chess.com — arrows appear automatically

How it works

chess.com page
    │  content.js: reads board FEN from DOM, draws arrow overlay
    ▼
background.js: routes requests, keeps offscreen document alive
    ▼
offscreen.html + offscreen.js: runs Stockfish (Web Worker + WASM)

• FEN extraction: data-fen attribute → web component game object → piece DOM (div.piece with square-NN classes + data-piece attribute) → turn from running clock (clock-player-turn) or move list. Pieces are searched through shadow DOM too — live 1v1 games render the board inside a chess-board shadow root, bot games use light DOM.
• Orientation: orientation attribute → flipped class → geometric vote on piece positions (works even when a1 is empty)
• Engine: stockfish-18-lite-single (single-threaded WASM, ~7 MB, GPLv3). Command: go depth N movetime M, result: bestmove + score
• Overlay: fixed SVG layer with arrow + eval badge, tracks board position and size, respects flipped boards
• Live play: only shows the move when it's your turn (optional)
• Stale results: request-id based, results are dropped if the position changed while the engine was thinking

Stealth protocol (anti-detection)

Reduces the chance of being flagged by chess.com Fair Play. None of this is a guarantee — engine use in rated play always carries risk.

• Hide mode — always (default) / only while holding Alt / only while mouse is over the board. Alt and hover modes keep the screen clean for streams/screenshares: hold Alt or hover the board to peek.
• Think delay — random delay (0–8 s, min–max range) before the arrow appears in live games, so your reaction time looks human. Off by default; ignored in analysis view.
• Your-turn only — in live games the arrow never appears on the opponent's turn (and analysis stops running).
• Instant hide — press Alt+Shift+S anywhere on chess.com to toggle the arrows on/off immediately (useful if someone enters the room).
• Blur safety — the arrow hides automatically when the window loses focus.

Settings (popup)

Setting	Default	Description
Master switch	on	Enable/disable arrows
Mode	both	Live + Analysis / Live only / Analysis only
Depth	16	Engine search depth (8–24)
Analysis time	2.5s	Max search time in analysis view
Live time	1.5s	Max search time in live play
Show arrows	always	Always / while holding Alt / on hover
Think delay	0–0s	Random delay before showing in live
Only your turn	on	In live games, hide on opponent's turn
Show eval	on	Score badge next to the arrow

Demo video

Watch the original MP4 (with sound)

Files

chessmate/
├── manifest.json              MV3 manifest
├── src/
│   ├── content.js             FEN extraction + arrow overlay
│   ├── background.js          Message router + offscreen lifecycle
│   ├── offscreen.html/.js     Stockfish worker host
│   ├── engine/                Stockfish 18 lite (GPLv3)
│   └── popup/                 Settings UI

Test

Engine + routing and content-script logic are covered by automated tests run against Chrome for Testing:

node /tmp/opencode/test-e2e.js          # engine E2E (offscreen worker)
node /tmp/opencode/domtest/test-content.js  # FEN + overlay logic