Sims don't plan anything. The Sims approximates how humans make decisions, but at any moment, they're constantly reassessing what they should do one objective at a time. This fundamental design choice has defined the series for decades, yet it wasn't inevitable. David "Rez" Graham, now the director of game programming at San Francisco's Academy of Art University, served as lead AI programmer for the original release of The Sims 4 over a decade ago.
Graham wanted something different. He had always wanted to have some level of planning in The Sims. The lead AI programmer on The Sims 3 attempted it, after which Graham himself attempted it on The Sims 4. The idea was straightforward: let sims autonomously decide to go to bed early so they could exercise before work in the morning, or manage their daily schedule the way actual people do.
It didn't work. Playtests revealed that planning caused confusion because players didn't know why their sim was doing what they were doing. Playtesters felt like they didn't have as much control. When sims make multi-step plans invisible to the player, the game stops feeling predictable and starts feeling arbitrary.
"It's always shot down, rightfully so, because the problem with game AI is you have to telegraph it," Graham said. He described watching someone ride up an escalator just minutes before reaching the third floor, then immediately turning around and riding back down. That's exactly the unpredictability that makes real life frustrating but authentic. In a game, it's just confusing.
The underlying system Graham built reflects this insight. Sims constantly reassess what they should do one objective at a time. They run AI on a cadence and say 'what is the next thing that I want based on my motives, traits, whatever?' A sim with high hunger will seek food. One with low bladder will find a bathroom. These impulses drive behaviour moment by moment, which makes them feel reactive rather than robotic.
Getting to this design meant taking risks during development. The core of the AI for The Sims 4 took Graham a month to write, and then two to three years of iteration. At one point, he rewrote the core of the AI entirely, which was a scary thing to do. The game was approaching alpha when he approached his boss with the idea. With no way to get approval to flip the table on the whole AI system at that stage, for about a week Graham spent his nights working on a separate development branch until around 11pm until it was in a state he was happy with.
The gamble paid off. He went to his boss, the Sims 4's lead gameplay engineer Peter Ingebretson, and claimed he wanted to rewrite the AI. Ingebretson gave him a look saying: 'what you just said sounded crazy, but also I'm willing to hear you out.'
This design philosophy highlights a broader truth about game AI: the most sophisticated systems aren't always the most fun. Perfect planning makes sims more human-like in some ways, but less fun to watch and control. The chaos of moment-to-moment decision-making, while occasionally frustrating, creates the emergent comedy that has defined the series. Players can fill their sim's action queue with tasks in a sensible order, maintaining control. The sim itself stays reactive. That balance is what makes the game work.