- The most important feature for serious solvers: clean pencil mark management with auto-update
- Unlimited undo is non-negotiable — it removes the fear of mistakes that blocks experimentation
- Daily challenges with varying difficulty build consistent practice habits
- Ad interruptions during active solving destroy the flow state that makes Sudoku satisfying
- Multiple difficulty levels with accurate ratings let you progress at the right pace
The Features That Actually Matter
Pencil Mark Management
The single most important feature for serious solvers. A good implementation lets you enter candidates with one tap, updates all affected candidates automatically when you place a digit, and allows bulk candidate clearing. Auto-candidate fill (filling all valid notes at once) is a major time-saver on Hard and Expert puzzles.
Unlimited Undo
Without undo, every wrong placement can ruin an hour of progress. Unlimited undo encourages experimentation with advanced techniques like trial-and-inference — because if the assumption leads to a contradiction, you can simply undo back to the branch point. Apps that limit undo to 3–5 steps force restart behavior that is deeply frustrating.
Daily Challenge
A new puzzle every day at a set time creates a habit anchor — the same reason daily crosswords and word puzzles are so popular. The best daily challenge systems offer multiple difficulty options so you can choose your engagement level each day.
Accurate Difficulty Ratings
A puzzle labeled "Hard" should consistently require Hard techniques (pairs, locked candidates) and not be solvable with just scanning. Inaccurate ratings waste time and erode trust in the app. Good apps generate puzzles algorithmically and verify the required technique level before assigning a rating.
What to Avoid
| Feature | Why It's Problematic |
|---|---|
| Mid-game ads | Breaks focus and flow state |
| Forced hint system | Can train passivity instead of active solving |
| Incorrect solution highlighting | Encourages guessing rather than logic |
| Limited undo | Makes mistakes punishing rather than educational |
Sudoku.by is designed around these principles: no ads during solving, full pencil mark support, unlimited undo, accurate difficulty ratings, and a Daily Challenge at multiple difficulty levels. Read the feature guide for a full walkthrough.