A legal FEN is not a game: state identity in a browser chess extension

Description and use

ChessNavio’s extension validates a live game state using FEN, boardEpoch, configRevision, jobId, and platform game ID before rendering Stockfish suggestions, preventing false positives on non‑game boards.

Problem

A browser chess extension must reliably identify when a board represents a live game, avoiding false positives on lobbies, broadcasts, or rematches that share the same FEN.

Outcome

By combining DOM signals, engine state, and platform identifiers, the extension now accurately distinguishes live games from non‑game boards, preventing misleading move suggestions.

Workflow

• 1. Verify the current URL matches a known game route for the site.
• 2. Ensure exactly one visible board of the expected type exists.
• 3. Check that the game has started (e.g., presence of move list or controls).
• 4. Retrieve the platform game ID if available.
• 5. Capture the board’s FEN, boardEpoch, configRevision, and jobId from the DOM and engine state.
• 6. Store a PresentationIdentity object with these values.
• 7. When the engine returns a bestmove, compare its FEN, epoch, jobId, and configRevision to the stored identity.
• 8. If all match, render the suggested move and caption; otherwise discard the result.

Evidence

If any of that is missing, nothing renders. I'd rather the panel sit there doing nothing than draw an arrow on a lobby or a broadcast.

Source

The type we compare at presentation time isn't a FEN and isn't a session snapshot: ...

Source