Playable — two phones
Marubatsu 2.0
Online 4×4 tic-tac-toe
A four-by-four board where you place two marks a turn and four in a row wins. Everyone already knows the old game, so the one changed rule is the whole product.
- Rule changed
- 1
- Ways to play: alone, the AI, or two phones
- 3
- Interface languages
- 9
01
The rule is the asset; the app is disposable.
The same ten win lines are written identically in the local game, the AI's search and the online room — and they outlived three rewrites of everything around them.
02
Three equal doors, not a settings screen.
An "Option A / Option B" layout made joining a room feel like configuring one. Creating a room is now the main action, and joining by code is a quiet second line.
03
The opponent is not named after a vendor.
It tries Gemini and falls back to a local search when the key or the reply is missing. Naming it after a model was an implementation detail, and only half true.
Where it belongs
- Two strangers with five minutes A waiting room, a train, a queue: one room code and a board they both already understand.
- A two-player game on a brand's own site Nine languages, no download, and a server that only has to referee.
- The pattern under any shared-board game Swap the rules and the same engine runs a quiz, a draft, or a card game across two phones.
- Input
- Two phones, one phone passed between two people, or one person and the AI
- Output
- One board, kept in step across both screens in real time
- Runs on
- Next.js · Firebase Realtime Database · Gemini, with a local search behind it
- Made
- 2025–2026, by our founder with a young co-designer at home
Bring this to something you are building.
Tell us the game and the place it lives, and we will tell you what it takes to get two phones holding one board. We reply from New York.