Your games deserve more than a one-time analysis.

Build a personal chess library from Chess.com, Lichess, or PGN. Find the turning point with Stockfish, ask Cassia for the reasoning, then explore or simulate the game that could have happened.

One connected review loop

Stockfish finds the move. Zen helps you learn from the game.

Zen Chess is designed as more than a one-time engine report. It keeps your games in a personal library, uses Cassia to explain why a move failed, lets you branch from the position with Explore, and lets Simulation continue games the original result left unfinished.

Keep the game

Your Chess.com, Lichess, and PGN games become a review history you can return to.

Understand the reason

Cassia connects the engine verdict to the threat, tactic, structure, or plan behind it.

Try another move

Explore a different choice on the same board without rebuilding the position or losing context.

Continue what was unfinished

If a player resigned or a better move went unplayed, Simulation carries the position forward at a chosen strength.

Where Zen began

A tournament game ended. The chess question did not.

At a local tournament, Zen’s developer had an opponent resign from a position he thought they might be winning. He kept returning to a tactic they could have tried and wondering how he could counter it. Zen began as a way to keep that position, explore the unplayed move, and continue the game without losing its context. AI came later, when Cassia was added to help explain the engine-grounded reasoning.

Why Cassia is different

Explanation follows chess analysis. It does not invent it.

Cassia is designed to connect engine-grounded facts to a reason a player can understand. The development note on Lichess documents that constraint, the limits of AI language, and why Cassia became part of the larger Zen workspace.