Bix and fair play
Difficulty changes its strategy, not what it is allowed to see.
What Bix can see
Bix receives its own tiles, the public chain, its legal moves, the number of tiles left to draw, and the selected difficulty. Its move picker receives no opponent hand, draw-pile contents, draw order or private game history.
How it chooses
Beginner uses simple pip-based choices. Intermediate balances pip totals with future moves in its own hand. Advanced also considers which tiles remain unknown, based on the standard set and visible tiles. This estimates possible replies; it does not look up which unknown tiles you actually hold.
Bix is a custom heuristic opponent. Its levels are relative settings within DoubleSix, not an external rating or a claim to be the strongest domino engine.
Bix draws blindly from the shuffled boneyard. Under the draw-until-playable rule, it keeps every tile drawn along the way, including non-matching tiles. A matching draw does not mean it selected a tile by looking at its face.
How we check the boundary
The move picker accepts a separate, limited observation object. Automated tests make hidden-tile access throw an error, then compare its decisions across different hidden deals while keeping the visible position unchanged. The checks run at every difficulty and for both seats.
Online games and offline practice
In online play, the server holds the complete deal to enforce the rules. A player receives their own tiles and the public board; opponents’ remaining tiles are revealed only after the hand ends. Spectators receive neither hand while play is in progress.
Offline practice stores the complete game on your own device so it can run without a server. The Bix move picker still uses the same restricted observation, but offline saves can be inspected or changed by someone controlling that device.
What this does not prove
These are code-level protections and regression tests. They are not an independent security audit or cryptographic proof of a particular deal. Online play still relies on trusting the deployed server and its rules implementation.