AI Dungeon 2 is a retro-inspired text-based mobile game that lets the player do, essentially, whatever they would like to do. Hobby programmer turned CEO Nick Walton built his AI Dungeon games using an algorithm developed by OpenAI called “GPT-2” which is capable of writing large sections of understandable, readable and grammatically logical text. There were some concerns from OpenAI that sharing their algorithm could have consequences (it can be used to generate “fake news“) but Walton harnessed it to deliver an open-ended adventure that is only limited by the player’s imagination (okay, and sometimes the game’s build itself).

That time I joined a band of Orcs as their drummer and toured the kingdom. 😂

Read the rest of this adventure from AI Dungeon 2 here: https://t.co/3EALbctcUE pic.twitter.com/LBregimM2j

— Nick Walton (@nickwalton00) November 26, 2019

According to the official description’s on its GitHub home site, AI Dungeon 2 is “a first of it’s [sic] kind game that allows you to enter and will react to any action you can imagine.” Another example of this type of AI-powered text-based game is the aptly named GPT Adventure, modeled after the subconscious-probing Mind Game from Orson Scott Card’s novel Ender’s Game and made by a neuroscience graduate student at Northwestern University named Nathan Whitmore.

You MUST play AI Dungeon 2, a text adventure game run by a neural net.@nickwalton00 built it using @OpenAI‘s huge GPT-2-1.5B model, and it will respond reasonably to just about anything you try. Such as eating the moon.https://t.co/3ovYyEMWpf pic.twitter.com/c4DGJieaAE

— Janelle Shane (@JanelleCShane) December 6, 2019

There are many games that give players their fair share of choices, but ultimately the player is limited by the game’s expectations and its rules–most games have to show you, and generating infinite graphical possibilities isn’t feasible yet. AI Dungeon 2 is different because the player feeds it whatever command they like and the game will use its neural network to figure out how to make the player’s wildest ideas come to life in the text-only wold of the game. The game starts by giving players choices on the setting and their character in order to generate an opening for the player’s story, and the trick is that as the story progresses the algorithm forgets a little bit of what it told you earlier. This means that sometimes the story goes wide and all outside, but AI Dungeon 2 does its best to keep up with even the most vivid of imaginations. Research scientist and AI writer Janelle Shane was able to eat the moon in her AI Dungeon 2 story.

AI Dungeon 2 is publicly released! You can download it on iOS now and Android in another day or two. As a holiday gift we’ve decided that it will be completely free till the end of the year so try it out now! https://t.co/bed8zMJrbr

— Nick Walton (@nickwalton00) December 17, 2019

According to Walton’s post on Brigham Young University’s Perception, Control, and Congnition Laboratory blog, the first iteration of AI Dungeon had game-generated options only; it was “a deep learning generated text adventure” that was restricted by its “computational cost.” AI Dungeon 2 allows the player to submit their own options, and the GPT-2 model picks up the story from what the player chooses to do or say. AI Dungeon 2‘s model size is bigger than its predecessor, using “OpenAI’s largest 1.5B parameter model and fine-tun[ing] it on a collection of text adventures obtained from chooseyourstory.com.”

Speaking to The Verge, Nick Walton said that he was surprised by the interest in AI Dungeon 2. “I thought people would enjoy it but I’ve been really blown away by how much,” he says. Walton also told The Verge that he enjoys seeing what stories players and the AI come up with, saying that it “never gets old” as each player’s ideas are different and affect their game’s AI in unique ways. “There was one where someone told the NPCs in the game that they were in a game,” Walton said. “The NPCs got really depressed and sad which made them feel surprisingly lifelike.”

AI Dungeon 2 got to be a bit too popular to support browser-based play. The game is available now for iOS and on Google Play.