Epic's Engineers No Longer Write Most of Unreal Engine's Code. Claude and Codex Do.

Epic Games has confirmed what its executives have been hinting at all year: artificial intelligence is now writing most of the code behind the world's most widely licensed game engine. Marcus Wassmer, Epic's executive vice president of development, told the newsletter This Week In Video Games that the company is using AI tools — naming Anthropic's Claude Code and OpenAI's Codex — to develop code and features for Unreal Engine, including the next major version, Unreal Engine 6.

"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." He was quick to add that Epic is not "one-shotting" the engine: engineers iterate with the models to keep the codebase within what he called its memory, performance and scalability envelopes.

The concrete results Epic cites are unusually specific. Wassmer said a rewrite of a significant portion of the engine's D3D12 rendering layer — work that would have taken a single engineer three to six weeks — was completed by the team in about a week, with better CPU performance and fewer bugs. AI assistance also practically eliminated the engine's non-deterministic asset-cooking bugs, a class of maddening, timing-dependent defects Epic had fought unsuccessfully for nearly a decade. Shader compression work accelerated too, which Epic says will shrink download sizes for future engine updates.

The tools are also on guard duty. Wassmer said AI monitors Fortnite's stability across players' PCs, flagging the specific driver and hardware combinations that cause crashes. Almost all current development happens in the dev streams attached to Fortnite, Unreal Editor for Fortnite and UE6; a rumored Unreal Engine 5.9 may surface as a modest update for specific systems, including AI-developed changes.

The confirmation is a striking reversal for a company whose CEO, Tim Sweeney, denied just a year ago that Epic was using AI for engine updates. Sweeney has since confirmed the practice and become one of the loudest pro-AI voices in games, even criticizing Valve's requirement that developers disclose generative AI use on Steam pages. Earlier this month Epic rebuilt the Epic Games Store launcher with AI — boasting faster boot times than Steam, though social media users posted clips of the new launcher hallucinating, and Valve quickly matched the boot-time difference.

Reaction among developers has been predictably sharp: some question whether anyone at Epic fully understands the code it ships, others ask why studios should pay Unreal licensing fees for an engine they could in principle vibe-code themselves, and Epic's own staff have raised job-security concerns. The company has also delayed an unannounced Disney and Marvel crossover shooter to move it to fully AI-first development, where directors reportedly told staff there would be no more hand-written code.

Epic's answer to all of it is speed. "We are more concerned about the risks of moving too slowly than we are about AI skepticism," Wassmer said — arguably the bluntest statement yet from a major software company about why it is handing its codebase to models. If UE6 ships on schedule and stays stable, expect the argument to be settled in the marketplace rather than on social media.