One developer used Fable AI models to build a game that develops itself

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A game you build with words

Spawn is a massively multiplayer game world where players interact with an in-game AI agent called Savi. Instead of clicking buttons to build things, you simply type what you want. The reporter tried it on a phone: he prompted a castle to appear, then a dragon, then rode the dragon around the castle he had just created ¹.

But the surprising part is not the game itself — it is who is building and maintaining it. According to Sansbury's blog post, Spawn is now being developed autonomously by a swarm of agents powered by Fable-class models ¹.

Millions in AI tokens a month

The numbers are striking. Sansbury says the company spends more than $3 million a month on AI tokens — roughly 30 times their payroll cost ¹. The agents read every error report, bug filed by users or by Savi herself, and live performance metrics, then decide what to fix .

About 60% of the work starts from something already broken in production — an error, a Savi bug report, odd log activity or a feature request ¹. When Savi hits a bug, she files it herself. Another agent then decides whether to fix it, ignore it or flag it as intentional design . Most of the time, the change is delivered to the user who reported it within an hour, with a message in the same chat .

Speed or bust

The team treats an hour without a deployment as their most severe incident — a SEV-0 ¹. Changes pile up and problems last longer when nothing ships . Because each agent can only hold a few hours of work "in its head," shipping quickly also helps the agents stay coherent .

Rolling back a bad change takes about 30 seconds, targeting exactly where it broke ¹. The team even breaks things on purpose to test that their recovery systems work .

Not every model fits

The experiment revealed important limits. Fable 5 and Fable 5.1 were the models that made this level of autonomy possible ¹. Claude Opus 5.5 did not quite work for them . When they tried GPT-6 Astra to apply security fixes, every feature in the product was disabled and marked to be turned back on later — the model had gone too far, locking everything down instead of fixing specific problems .

This story reports on a trend for information only. It is not advice or encouragement to do any of it.

Key facts

• Spawn is a multiplayer game where players prompt an AI agent called Savi to create and modify game worlds
• The game is developed autonomously by Fable-powered agents spending over $3 million monthly on AI tokens
• About 60% of development starts from live production errors or user bug reports
• Bugs are typically fixed and delivered to the reporting user within one hour
• GPT-6 Astra disabled every feature during security fixes; Opus 5.5 was insufficient

Sources

1. Building a multiplayer game world with autonomous Fable-class models — simonwillison.net