Last week, Reed Wilen, an elite gamer who uses the handle “Chicago” in Rocket League, a popular vehicular-soccer game, encountered a strange and troubling new opponent. The player seemed like a novice at first, moving their rocket-powered vehicle in a hesitant and awkward way. Then they caught and balanced the ball perfectly on the hood of their car, and dribbled it with superhuman skill towards the goal at high speed.
Not only was the other driver clearly a bot—it was also ridiculously good. “It is very confusing to play against,” Wilen says. “Its perfect dribbling would cause havoc on almost every player.”
Wilen is one of a number of elite Rocket League players to have recently encountered the bot in competitive play. It is not yet good enough to beat all comers, but it can play to a high level, allowing less skilled players to cheat their way to a higher ranking.
Rocket League is frenetic and extremely tricky to play. Each player controls a car capable of impossible acrobatics inside an arena where gravity and physics are apparently set to ludicrous mode. The objective is to use your vehicle to maneuver a giant ball past your opponent and into their goal, a task that requires considerable skill and patience. Sometimes two players work together as a team, making huge leaps, desperate parries, and accidentally colliding, all while trying to anticipate and counter their opponents’ own antics.
Top Rocket League players will often launch their cars through the air to move the ball toward the goal, but Wilen says the bot he faced appears to have been trained specifically to carry it on the ground. “The bot doesn’t really flip around too often and doesn’t jump in the air,” he says, apparently because it hasn’t been programmed to, or learned how to do so. “Instead, it waits for the ball to come down, where it catches it on top of the car and performs a perfect dribble towards the opposing team’s net,” Wilen says.
The bot that Wilen and others have come up against is called Nexto. It picked up the ability to dribble and score using an artificial intelligence approach known as reinforcement learning, which has underpinned research breakthroughs that let computers master other difficult games such as Go and Starcraft. The technique has also been applied to more practical areas, including chip design and data center cooling in recent years. Reinforcement learning entails creating a program that can perform a task at a basic level and improve by responding to feedback as it practices.
The company behind Rocket League, Psyonix, part of Epic Games, allows players to deploy bots to practice against. In 2020 it made an application programming interface (API) available to help developers build bots more easily. Last April, a group of Rocket League enthusiasts with coding skills announced RLGym, an open source library for building reinforcement-learning bots for Rocket League. Later in the year, the group released several open source AI bots—including an especially skilled dribbler called Nexto.