Epic Uses Claude Code and Codex to Build Unreal Engine

Epic Games CEO Tim Sweeney has confirmed that Unreal Engine, Fortnite, and other Epic projects are now built using AI code generation tools, marking a dramatic reversal from the company's 2025 stance. In a statement to This Week in Videogames and follow-up social media posts, Sweeney named Anthropic's Claude Code and OpenAI's Codex as the primary tools enabling what he calls a "huge pivot" toward maximum AI acceleration across Epic's engineering teams. Officially launched on October 28, 2026.

How AI Tools Rewrite Game Engine Code

Epic development lead Marcus Wassmer detailed the new workflow in statements about the next version of Unreal Engine. Engineers now prompt AI tools with high-level requirements and refine the generated output rather than hand-writing core systems. In one striking example, AI rewrote a significant chunk of the D3D12 rendering layer in approximately one week, a task that would take a single human engineer three to six weeks to complete manually. The tools span multiple domains:

• Claude Code and OpenAI Codex for core feature development and refactoring
• Automated bug detection and root-cause analysis for PC stability issues
• Shader compression acceleration
• Automatic fix generation for driver and hardware compatibility problems

Wassmer emphasized that the AI-based approach is "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."

Eliminating Decade-Old Bugs at Scale

One of the most significant wins involves non-deterministic asset cooking bugs, issues that had plagued Unreal Engine for nearly a decade. These bugs caused unpredictable build failures and forced developers into time-consuming manual workarounds. According to Wassmer, AI code generation tools have essentially eliminated this entire class of defects by rewriting the underlying asset pipeline logic with deterministic guarantees. The company has also deployed automatic monitoring of PC stability on Fortnite that detects issues with specific driver and hardware combinations, problems that previously took months to root-cause. The system now provides automatic root cause analysis and automatic fix generation, measurably improving the long tail of stability issues in both Fortnite and Unreal Engine.

From Manual Coding to Prompt Engineering

The shift represents a fundamental change in how Epic's engineers spend their time. Rather than writing boilerplate or debugging obscure edge cases, developers now focus on designing system architecture, writing clear prompts, and evaluating AI-generated solutions. Sweeney framed the transition as a retooling effort, not a headcount reduction: "The aim isn't to do the same stuff with fewer people, but to enable everyone here to build MORE and BETTER stuff, and to bring the benefits to all teams using Unreal." Epic has moved developers across the organization to maximize AI acceleration, ensuring every team can leverage AI code generation and AI code review capabilities in their workflows.

A "Vibe Coded" Launcher and Performance Claims

Epic's new Preview version of the Epic Games Launcher showcases the approach in a consumer-facing product. The company said the launcher was "vibe coded" because "programming is hard," claiming it boots from a cold start 46 percent faster than Steam and offers a 14 percent performance boost over Valve's storefront when restoring from the system tray. The launcher is described as "way faster and more responsive than the current one, and has a new look," though independent benchmarks have not yet verified Epic's performance claims.

Policy Clash With Valve Over AI Disclosure

Sweeney's announcement comes amid ongoing criticism of Valve's policy requiring game publishers to disclose generative AI use in Steam store listings. Last year, Sweeney stated Epic wasn't using AI tools for development, a position that applied to player-facing assets rather than internal tooling. Now, Sweeney has publicly embraced AI for core engine work and welcomed projects that use the technology, positioning Epic as a more AI-friendly alternative to Steam for developers building with generative tools.

What This Means

Epic's pivot signals a new phase in game engine development where AI code agents handle routine implementation while human engineers focus on architecture and validation. The productivity gains (60-70 percent time savings on the D3D12 rewrite, elimination of decade-old bug classes) suggest AI tools have crossed the threshold from experimental to production-critical. For Unreal Engine users, the promise is a more stable, modern, and feature-rich platform delivered faster than traditional manual development allowed. The next version of Unreal Engine, unifying Unreal Engine 5 and Unreal Editor for Fortnite, will be the first fully AI-accelerated release and a test case for whether AI-written game engine code can match or exceed human-written equivalents at scale.