On BYOK - I didn't want the added complexity of a backend proxy to handle requests and subscriptions, I've tried to reduce the friction needed, its just paste key, save, test... anyone done this in their apps, what worked?

TTS - I tried so many! Kokoro was great but memory was too high, Supertonic fits under 300mb ram, and is fine for my uses.

Theres always a second Stockfish analysis pipeline running alongside the game engine (Stockfish, Rodent IV or Fairy-stockfish) - that took some rework to make happen to not clash with the game engine!

Opening Identification Staleness - A position-hash index handles transpositions, and the detector reports honestly when a game has left theory. Lichess's explorer doesn't surface this, neither does ChessBase. Players know the feeling of "we're in theory, we're in theory, we're... wait, are we still in theory?" — the app answers that question explicitly.