AI defeats Stratego champion by exploiting clues from hidden pieces

A research team from Carnegie Mellon, MIT, New York University and Stanford has built Ataraxos, an AI that defeated leading Stratego player Pim Niemeijer 15 games to one, with four draws. The result matters because Stratego’s hidden pieces, bluffing and long games have made it unusually difficult for computers to master, despite earlier successes in chess, Go and poker.

Ataraxos trained through 163 million self-play games using 16 GPUs and a budget of a few thousand dollars. Its key addition is a second neural network that estimates the opponent’s hidden pieces from their moves, allowing the AI to sample plausible board positions and search ahead without evaluating every possible setup. The researchers also adjusted its learning strategy over time, making larger changes early in training and smaller ones later.