Epic says AI coding tools killed Unreal Engine bugs that resisted a decade

Epic Games has confirmed that AI tools such as Claude and Codex have eliminated "non-deterministic cooking bugs" in Unreal Engine that the company had tried to stamp out for roughly ten years. Marcus Wassmer, Executive Vice President of Development at Epic, made the statement in response to questions from This Week in Video Games, giving the first detailed account of how Epic itself uses AI to build its own software.

The confirmation follows Epic's broader AI-first pivot. The company has said it will center Unreal Engine on AI development, use generative AI to build game assets, and update the Unreal Engine EULA to allow AI training on engine output. Until now, Epic had not specified how AI would shape the engine's own development.

What did Epic actually say?

"We are definitely using AI to develop code and features for Unreal Engine," Wassmer said. "Almost all development happening right now is being done in the dev streams attached to Fortnite, UEFN [Unreal Editor for Fortnite], and UE6."

Wassmer also left the door open to an interim release. He said there is "a possibility of a 5.9" — a relatively modest update for specific systems that would include AI-developed changes. His priority framing was blunt: Epic is working "as hard as we can on UEFN and UE6."

Where is the concrete payoff?

Two operational wins stand out.

• Bug elimination. AI tools have resolved cooking bugs — defects in the engine's asset-cooking pipeline — that had persisted for a decade. These are the "non-deterministic" kind, meaning they failed to reproduce reliably and therefore resisted manual debugging.
• Stability monitoring. AI now automatically monitors Fortnite's PC stability. It flags driver and hardware combinations causing problems, work Wassmer says would have taken more manual effort to root out.

The tools do more than write code, Wassmer argued. "The tools are helping us not just develop code and features, but to keep the engine more stable as we work, trim, and streamline it, and really modernize it in a way that we haven't been able to before given the size, complexity, and age of the engine code." That is the core claim: AI lets Epic refactor a large, aging codebase faster than human teams could.

How does this connect to 'vibe coding'?

Epic recently said it rebuilt the Epic Games Launcher through "vibe coding," claiming the launcher now runs notably faster. Wassmer confirmed the same process extends beyond the Epic Games Store. Engineers are not writing the bulk of the code. Wassmer insists they still control the scope of where and how AI is applied.

What does this change for studios building on Unreal? Potentially a great deal. If AI-assisted modernization trims and stabilizes the engine faster, licensees inherit those gains in UE6 and any 5.9 release. The promised EULA change on AI training output also carries direct licensing implications for teams shipping on the engine.

Is Epic worried about developer skepticism?

Surveys in recent years have shown game developers are worried about generative AI. Wassmer shows little hesitation, saying the benefits to Epic outweigh any concerns.

"We are more concerned about the risks of moving too slowly than we are about AI skepticism," Wassmer said. "We think that in pretty short order doing AI development for basically all software will be ubiquitous because software done correctly with AI will simply be better and be more capable of responding quickly to customer feedback than software done without. The best way to counter any skepticism is through the quality of the games and tools we're able to deliver."

That is a strategic bet, not just a tooling choice. Epic is wagering that speed of iteration, accelerated by AI, matters more than industry unease.

The signal to watch now is Unreal Engine 5.9. Whether Epic ships that modest interim update, and how much of its code is AI-developed, will offer the first measurable read on how deeply AI now sits inside the engine's release pipeline.