Epic built the Unreal Engine 5.9 preview in about a week, mostly with AI agents, and shipped it with a known-issues list

Epic’s description is blunt about who did the work. A handful of people directed fleets of AI agents to port AI tooling from the UE6 development branch and to add the Web as a target platform. Engineers reviewed and tested what the agents produced. The release came together in roughly a week so it could be ready for the 2026 Epic AI MegaJam.

The jam is the reason for the rush. According to Epic’s MegaJam announcement, teams can have up to five people plus unlimited AI agents, the event runs November 12 through 15, and the prize pool is $50,000. Using AI in the jam is optional.

What shipped includes expanded Unreal MCP toolsets, AI coding agents working inside the Editor, AI-assisted content creation, and browser-based builds. Several are marked experimental, and the Web renderer is described as a prototype to be refactored before the full 5.9 release.

The list of problems came with it, on day one. Web games reportedly crash above 2 GB of Wasm memory, and the Mac and Linux launcher builds are missing some web support files. Epic said a fix build was planned for the same week.

The phrase “port AI tooling from the UE6 development branch” matters here. Tools Epic is building for its next major engine version arrived early, inside a 5.x preview, so people using 5.9 get a look at where the company is heading. That also explains the experimental labels: the code was brought across a version boundary in days, and the people who normally test such a move had less time than usual.

Developers didn’t all cheer. Coverage of the release describes the community as divided over how much of an engine update is aimed at AI features. The shorter QA window is the cost Epic admits to, and a user who installs a preview should read the known-issues list before the feature list.

Epic’s own jam rules add one more detail worth noticing: teams get “unlimited AI agents” on top of five people, so the company is building the engine with agents in the same week it invites contestants to build games with them. The release and the contest describe the same bet, that a small group directing agents can ship something a larger group used to.

The unanswered part is how the full 5.9 release will differ, since the web renderer is the piece Epic says it will refactor.

My money says other engine makers copy the format: a short, labeled preview timed to an event, with the bugs listed up front.

A week, and a list of what broke.