I built an API that takes a FEN or PGN and returns every tactical pattern it finds — forks, pins, skewers, discovered attacks, back rank mates, and more (10 patterns total).

It uses a two-phase approach: fast geometric detection, then sequence confirmation to filter out false positives.

You can try it directly in the playground (no signup, no API key): https://chessgrammar.com/playground

Paste any FEN or a full PGN and see what it finds. The playground also has a bug report button if something looks wrong.

API docs: https://chessgrammar.com/docs

Built with Python (python-chess), deployed on Vercel. The engine runs custom heuristic algorithms — no Stockfish dependency at runtime.

Curious to hear feedback, especially from anyone who's worked on chess analysis tools.