Epic Games has stopped being coy about it. Asked directly on X whether the company uses AI to write engine code, CEO Tim Sweeney answered: "Yes, we are now using AI tools to develop code for Unreal Engine." A day later, executive vice president of development Marcus Wassmer filled in the details for This Week in Videogames — and the numbers are the story.

"Yes, we are vibe-coding things," Wassmer said. "That means our engineers are no longer writing the bulk of the code, but they are still guiding the tools to get the architectural result we want. We're not 'one-shotting' the engine; we're iterating with AI to ship features developers want and keep the engine within the memory, performance, and scalability envelopes."

He named the tools as Anthropic's Claude Code and OpenAI's Codex, and said "almost all development happening right now is being done in the dev streams attached to Fortnite, Unreal Editor for Fortnite, and UE6."

Three concrete results were offered. First, the rewrite of a significant portion of the D3D12 rendering layer — work Wassmer estimated at three to six weeks for a single engineer — was completed in about one week, with better CPU time and fewer bugs. Second, shader-compression work accelerated, shrinking patch sizes. Third, and most striking to anyone who has shipped a large game: non-deterministic cooking bugs, a plague Epic has fought "for a decade," were "almost completely" eliminated. Asset cooking — turning source assets into platform-ready data — is exactly the kind of sprawling, stateful, poorly documented code where ordinary agents struggle, which makes the claim worth watching rather than dismissing.

Wassmer also framed the strategic logic in unusually blunt terms. "We are more concerned about the risks of moving too slowly than we are about AI skepticism," he said. "We don't think it's viable to take a halfway approach to AI in development."

The disclosure lands in the middle of a fight Sweeney picked himself. On 23 October he attacked Steam's AI-disclosure requirements as "just as partisan as mandatory political essays," arguing they hand ammunition to a "hate mob." Epic had also run an AI game jam and then walked back the requirement, and said it had "vibe coded" the Epic Games Launcher to be faster.

There is an irony worth naming: Epic now volunteers, in detail, the very disclosure it says Valve should not compel. The difference, presumably, is who holds the pen.

For developers the practical question is not whether Epic uses agents but what the gains cost. "Almost all" of Unreal's development running through AI-assisted streams implies a review and verification load that Epic has not quantified, and a rendering-layer rewrite is exactly the sort of change where "fewer bugs" is a claim best tested by shipping. Wassmer's answer to doubters is the one that will settle it: "The best way to counter any skepticism is through the quality of the games and tools we're able to deliver."

Unreal Engine 6 is not out yet. When it lands, the commit log will not say which lines a model wrote — but the frame times, the patch sizes and the crash reports will.

Sources