A co-op Sudoku for exactly two people. Every placement, note and mistake syncs live over WebSockets — solve together on two phones, or play solo with saved progress.
This board was generated by the app's actual engine — the same generator/solver that grades every puzzle valid with a unique solution. Tap a cell, pick a digit. The blue/lavender split is what two players see live on a shared board.
No accounts, no invites, no codes. Both players enter the same 4-digit code, pick their name, and they're in. Rate-limited so it can't be brute-forced.
Values, pencil marks, mistakes and the timer all sync over WebSockets. If your partner places a 7, you see it in under a second — with their colour.
Every puzzle is generated on the server: solved with constraint propagation, then checked to have exactly one solution before it's graded into easy → expert tiers.