Developer Discusses The Use of AI in Life Sim InZOI
Large Language Models (LLMs) are one of the most common types of AI around, with popular examples like ChatGPT and Claude. Jaewoong Cho, from the Deep Learning Division of Krafton, the publisher behind inZOI, gave a recent talk at GDC 2025 about how a “small” version of an LLM is used to create character behavior in the modern life sim.
InZoi was released into Early Access in March 2025, with a full release tentatively set for March 2026. It lets players create not just characters but entire cities for those characters to live in, with control given over everything from buildings to the weather. Characters can be customized, too, but it’s their behavior that sets them apart from competitor titles like The Sims.
“SmartJoy is a language model based live simulation engine and we are completely redesigning live simulation games, and inZoi is the first one that can run entirely on a language model based engine,” Cho said in the GDC presentation.
“And with SmartJoy, we’re not just replicating what traditional engines do, we are introducing new features made possible by the language model. So SmartJoy can plan their day, make decisions based on its schedule and surroundings, and even write a freeform diary, all without pre-scripted content.”
AI Content
InZoi doesn’t just use AI for behavior, though; it’s woven into almost every aspect of the game. From textures to items, inZoi uses generative AI technology to create additional content. Players have expressed mixed reactions across social media platforms, although most reviews for the game itself are positive. To see broader examples across genres, check out games with AI for titles using this technology.
“The Text to Image (T2I) technology applied in inZOI was trained using publicly available images that are permitted for commercial use. Over the course of several months, we built our own dataset by selecting 20 million images from public images released under Creative Commons licenses permitting commercial use and modification.”
The inZoi dev team posted on the game’s official site about its use of AI, aside from Smart Joy, to clarify why and how it was and is used, both in prior development and its current state.
The team also outlined the use of “in-game 3D printing” that players can access: “Our 3D printing AI technology was trained on approximately 46,000 high-quality 3D models either owned by us or sourced from datasets that are also permitted for commercial use.”
SML vs LLM vs Utility AI
An SML (Small Language Model) is a streamlined version of an LLM, which means it’s more specialized, faster to customize, and more efficient to run. Smart Joy is an SML. Utility AI is common in video games and, basically, it works by assigning scores to actions.
For example, a character might have a hunger rating from 1 to 10, so when this reaches 8 they’ll be more likely to try and eat food. However, a utility AI has limitations that prevent it from acknowledging other game-related factors without extensive coding, making it okay for standard NPC behavior but not ideal for something as involved as a life sim.
By contrast, an SML like Smart Joy is capable of not just examining scores such as a hunger rating, but other things that might impact it, such as an upcoming dinner reservation. If a character is hungry but they’re supposed to go on a date that evening, a utility AI would ignore this condition and just make the character eat right away. Smart Joy understands that this breaks immersion in a life sim, so makes that character wait to eat later.
“If we turn on SmartJoy, then the Small Language Model chooses the action, what they do based on their current state and context, and they provide the inner thought regarding their action,” Cho said.
“Building on this simple foundation, we gradually introduced additional features. For example, the first addition was a reasoning generator, which generates the character’s inner thought for each selected action. Next, we edit the daily reflection agent. This enables (the character) to reflect on the day before going to sleep and captures the key moment and thoughts of the day. And finally, we introduce the daily planning agent, which takes insights from daily reflection and uses them to plan the next day more meaningfully.”
The Names Behind The Game
Krafton is the publisher of inZoi, and is based in South Korea. The game is technically developed by InZoi Studio, although this is an in-house team at Krafton.
Krafton was founded in 2018 via the consolidation of other game companies including Unknown Worlds and Bluehole, and is also the publisher of popular titles such as The Callisto Protocol and PUBG: Battlegrounds.
