Epic Games is done hedging about AI. Tim Sweeney confirmed on X this week that the company is “now using AI tools to develop code for Unreal Engine.” Marcus Wassmer, Epic’s executive vice president of development, then filled in the details, and they go further than most studios would say in public. Epic’s engineers no longer write the bulk of the engine’s code. They point AI tools at the architecture they want and keep iterating until the output fits inside the engine’s memory and performance budgets.
Wassmer was blunt about it. “Yes, we are vibe-coding things,” he said, naming Anthropic’s Claude Code and OpenAI’s Codex as the main tools. Almost all of Epic’s current development runs in the code streams tied to Fortnite, Unreal Editor for Fortnite (UEFN) and Unreal Engine 6, the merged engine Epic unveiled at Unreal Fest in Chicago. None of this came out of nowhere. Back in June, Wassmer’s UE6 roadmap post said Epic had opened up code generation and AI analysis across its backend, engine and game teams. This week’s statement reads like the first scorecard.
What AI Actually Fixed Inside Unreal
The examples Wassmer picked are deep plumbing. These are the parts of the engine every Unreal studio depends on and almost nobody wants to touch.
First, cooking. Before a game ships, Unreal “cooks” its assets into platform-ready formats. When an asset that hasn’t changed cooks to slightly different bytes from one run to the next, it ends up in the next patch anyway. Epic has fought these non-deterministic cooking bugs for about a decade. Wassmer says AI tools have now almost eliminated them.
Second, shaders. Work on shader compression moved faster with AI help, and together with the cooking fix it cuts patch sizes significantly. For a game like Fortnite that updates constantly, that’s real bandwidth and real storage saved on every player’s machine.
Third, and the one engine programmers will argue about: a rewrite of a large part of the D3D12 rendering layer. That’s the code that turns Unreal’s rendering commands into DirectX 12 calls on Windows and Xbox. Wassmer had estimated the job at three to six weeks for one engineer. It took about a week, used less CPU time and came out with fewer bugs.
Why Unreal Licensees Should Pay Attention
Unreal isn’t only Epic’s engine. Thousands of studios license it, from solo developers up to the biggest publishers, and Epic takes a 5% royalty on gross revenue past the first million dollars per product. So when Epic says AI tools now write most of its new code, every UE6 licensee inherits that code in the renderer and the cook pipeline, whether or not their own team ever opens an AI tool. Studios with source access can read every line on GitHub. Few will.
The speed matters as much as the code. If a renderer job that should take a month takes a week, Epic’s engine roadmap compresses, and Unity and the big in-house engines are now competing against that pace. Epic also wants to sell the workflow itself. Unreal Engine 5.8 and UE6 are getting an MCP server so developers can plug in Claude, Gemini or whichever model they prefer, and Epic says those tools stay optional.
There’s a risk on the other side. AI-written engine code that ships a bug into thousands of games at once would be a much bigger story than one studio’s bad patch. Epic is betting its own review and testing catch that first. So far the numbers it’s sharing say fewer bugs, not more.
Epic Picks a Fight Over AI Disclosure
The timing isn’t an accident. Days earlier Sweeney went after Steam’s AI disclosure requirement, comparing it to forcing developers to write mandatory political essays. Before that, Epic announced an AI game jam and then dropped the rule that entrants had to use AI. And the week before that, the company claimed it had vibe-coded the Epic Games Launcher to run faster than Steam.
Wassmer’s line is that Epic worries more about moving too slowly than about AI skeptics, and that a halfway approach to AI in development doesn’t work. His answer to the skeptics is the quality of the games and tools Epic ships. Being upfront about it, he argues, beats pretending.
That leaves an awkward question for storefronts. Steam’s disclosure form is built around AI content that players see and hear. Engine code sits a layer below, inside every Unreal game on the platform. The disclosure rules weren’t written with that in mind.
UE6 is where the bet gets tested.