'I took those files and deleted them': Former Sims 4 developer says he totally rewrote the sims' AI in the middle of development and 'that was a scary thing to do'

It's an accepted fact by Sims players that no matter how well you think you know the game, sims have a mind of their own. Sometimes you can wrestle them into behaving the way you want with arcane tricks of furniture placement or by installing mods. If you're one of Maxis' programmers, though, and you're in an "ask for forgiveness instead of permission" kind of mood, maybe you just rewrite the entire under-the-hood AI in about a week during development.

Former Maxis developer David "Rez" Graham is now the director of game programming at San Francisco's Academy of Art University but worked QA back on 1999's Sim City 3000, and by the time of The Sims 4 was serving as lead AI programmer. That's AI in the "programmed NPC behavior" sense, not in the generative AI sense.

One of the stories he likes to tell from that time: completely overhauling the AI logic inside every sim's brain right in the middle of development.

"The core of the AI for The Sims 4 took me a month—maybe not even that—to write, and then two to three years of iteration," Graham said in an interview with PC Gamer. "At one point, I rewrote the core of the AI, just rewrote it, and that was a scary thing to do."

Graham had two computers at work, he recalled: one for development and one that just ran the build of the game. "I'm not happy with what's happening," he said of playing the build of The Sims 4 at that time. "I mean, it's working. I'm just not happy with it. [Sims] weren't choosing the behaviors that I wanted. They weren't solving for their motives in the way that I wanted to solve for them, and I'm just like 'there is such a better way to do this.'"

Graham didn't go completely rogue in the way you'll sometimes hear from the war stories of developers at big studios in the aughts, but he did take a gamble. The Sims 4 was approaching an alpha state, at which point there was no way he'd get approval to flip the table on the whole AI system. For about a week, Graham spent his nights working away on a separate development branch until around 11pm until it was in a state he was happy with.

He went to his boss, The Sims 4's lead gameplay engineer Peter Ingebretson, and claimed he wanted to rewrite the AI. "He gave me this look. It's a look that I'll treasure forever, of: 'what you just said sounded crazy, but also I'm willing to hear you out.'"

Graham spent the next hour convincing Ingerbretson, mathematically, of how he wanted to rebuild The Sims 4's logic, which is when he revealed the truth: "Here's the cherry on top: I've already done it though."

Graham got approval for the extra time needed to finish shoring up his new AI logic and time with a member of Maxis' QA team to do the initial round of debugging before taking his changes to the wider team. "To everyone's credit, they instilled a huge amount of trust and said: yeah, okay, let's do it," Graham said. One big stack of bugs later, he was ready to make the swap to the new AI for The Sims 4.

"It was one of the best projects, one of the most interesting projects, I ever worked on because as a developer most of the time you walk into a project that has some establishment," Graham said. "For The Sims 4 there was a very, very basic, weighted random [system] that chose actions and nothing else. There was very little in the AI. So it was the first time that I got to really sit down and build the AI from nothing. I mean I took those files and deleted them.

"I don't know how many AI programmers get to do that."