Still Sudoku's artwork: a single three by three Sudoku block with four of its nine cells filled in.

The constraint

Most Sudoku apps treat the puzzle as a vehicle for ads and upsells. Still Sudoku treats it as the point: a calm board worth sitting with, and enough quiet progression to come back to without ever being nagged about it.

Decisions

The generator guarantees one solution: Boards come from a backtracking generator that produces a single valid solution at the requested difficulty. No board is ever unsolvable or ambiguous, which is the one thing a solver cannot forgive.
The player decides how much help they get: Three error modes rather than one setting buried in a menu. Strict for a real test, Assist for guidance, Zen for a solve with no pressure at all.

What it does

Four difficulties, Easy through Expert
Three error modes: Strict, Assist and Zen
Notes mode, undo and redo, and a hint system
XP, levels, daily streaks and achievements
Six unlockable themes, including AMOLED and Ocean

How it is built

Clients: React Native (Expo)
Service: None. Fully offline.
Data: Zustand with AsyncStorage persistence
Distribution: App Store and Google Play
Stack: React Native · Expo · Zustand