Unreal Engine 5.9 MegaJam Preview

[Flak]

Download Unreal Engine 5.9 MegaJam Preview from the Epic Games Launcher, GitHub, or here for Linux.

This isn’t a typical Unreal Engine Preview.

We’ve been accumulating changes for rendering and animation in UE5 Main for an eventual 5.9 release next year. However, just a week ago, we decided to start working on a UE 5.9 Preview in time for the 2026 Epic AI MegaJam on November 12–15.

This UE 5.9 MegaJam Preview is the first Unreal Engine release built primarily with agentic workflows. A handful of people directed fleets of agents to port the AI tooling from our UE6 development branch and implement Web as a target platform. Agents wrote the web platform code, starting from UE4’s retired HTML5 platform, with engineers directing, reviewing, and testing the work.

We’re using this release to learn how to build the engine with multiple people directing swarms of agents working in parallel… and we still have lots to learn. Having said that, we’re already seeing benefits from this approach, and it allows us to deliver more UE5 releases with faster bug fixes and development on targeted features, all without pulling focus from UE6.

This Preview had about a week of development and a shorter QA pass than usual. We’re still cleaning up some of the agent-written code, and parts of the web renderer are prototypes we plan to refactor before the full 5.9 release ships. As with all Previews, expect rough edges, missing pieces, and things that break.

Test it out during the Epic AI MegaJam or on a copy of your project and tell us what bugs you find: we’ll be fixing issues throughout the preview period, with agents helping us triage and turn reports around quickly.

Key feature overview

Updates to Unreal MCP (Experimental)

Unreal MCP launches automatically with the editor, delivers more reliable connections, and expands its toolsets to support rendering, source control, profiling, debugging, gameplay testing, geometry, audio, UI, and level creation. New integrations with Lore enable AI agents to manage project changes and automate source control workflows, while the new Fab toolset lets agents search the catalog, browse your library, and install free or purchased assets directly into your projects.

Unreal Assistant (Experimental)

Work with your preferred AI coding agent, including Claude Code, Codex, Cursor, or OpenCode, directly in the in-editor assistant panel in Unreal Editor and connected to Unreal MCP to streamline agentic development workflows.

Expanded Python Scripting

Automate more of Unreal Engine with expanded Python APIs covering audio, MetaSounds, modeling, physics, Niagara, materials, PCG, networking, and UI. These capabilities also extend to AI agents, enabling them to perform more tasks directly in the editor.

Unreal Canvas (Experimental)

Explore AI-assisted content creation with Unreal Canvas, available as a web app and directly in Unreal Editor. Generate concept art, images, 3D meshes, and video using prompts, reference images, or your own scenes while preserving composition and camera perspectives. Keep track of your creative process through connected prompts and generation graphs. Connect the AI models you choose using your own API keys.

Run Your Projects in a Browser (Experimental)

Bring Unreal Engine experiences directly to web browsers with no installation required. Powered by WebAssembly and WebGPU, this experimental capability uses the mobile renderer and currently requires a source build and C++ project, with Chrome offering the best experience. Expect longer cook times and occasional performance issues as development continues.

And there’s more to explore! Discover what’s new in Unreal Engine 5.9 Preview, and check-out the UE 5.9 Public Roadmap for more information.

---

Please keep in mind that Preview releases are not fully quality-tested, are still under heavy active development, and should be considered as unstable until the final release. We recommend you make a copy of your projects for testing these builds, rather than converting your projects.

If you discover any issues with this Preview release, please report them using these guidelines: How to Report a Bug

[RaghibK]

The following are known issues to be aware of in the Unreal Engine 5.9 Preview 1 release.

Mac/Linux Precompiled/Launcher Builds

Known issue:
The premade builds on Mac and Linux are missing part of the Wasm (Web) support files.

Workaround:
Use a source build on Mac/Linux to target the Wasm platform. A Quick Fix Engineering (QFE) build is currently planned to release this week.

Web crashes and data loss

Known issue:
UE-412003 - Save games/settings lost on reload (Launcher-built C++ web games)

Workaround:
Append -project=../../..//.uproject to Page Args, preserving existing arguments, then repackage. For an already hosted build, append it to index.html’s ue-args tag and regenerate/remove index.html.br.

Known issue:
UE-411671 - Game crashes above 2 GB Wasm memory.

Workaround:
Reduce content usage, and follow other workarounds in this page.

Known issue:
UE-411514, UE-412029 - Mobile devices can crash/freeze during start with forced multithreading due to increased memory usage.

Workaround:
Keep the default single-threaded mode. With the Launcher engine, use Shipping (which uses much less memory), although it has fewer dev features (like in-game console and most commandline options). A source build can use Test config which could be a useful “middle of the road” solution.

Known issue:
UE-412148 - iOS: 5.9 Preview 1 web builds work best on iOS 27. On iOS 26’s version of WebKit, threads use a lot more memory than webKit 27.

Workaround:
On iOS 26: Turn off Project Settings > Platforms > Wasm > Threading and sound > iOS > Audio Thread (single-threaded) (bSingleThreadedAudioThread=False) and package again; or, without repackaging, open the page with ?audiothread=0 (or set the ue-audio-thread meta to 0 in index.html); or try a Shipping build which will use less memory and you may stay in budget while having an audio thread. Sound stays on either way.

Known issue:
UE-411884 - Phone rotation during loading breaks the image/touch controls.

Workaround:
Wait about 10 seconds before rotating, or reload.

Known issue:
UE-411441 - AMD browser GPU crashes on the experimental desktop rendering tier.

Workaround:
Use the default rendering tiers.

Known issue:
UE-411439 - On the Mobile Forward tier (the one phones use), metallic and glossy surfaces near a Reflection Capture can render black.

Workaround:
Turn on Project Settings > Rendering > Mobile > Clustered reflections (When using ForwardShading), restart the editor, and cook again. The Mobile Deferred tier isn’t affected.

Known issue:
Android phones and iPhones older than iPhone 15 Pro have no BC/DXT texture support. A build cooked with BC textures only stops at a compatibility card on those phones.

Workaround:
Cook with ASTC textures (-cookflavor=ASTC plus -WasmAddTextureSet) for iPhone testing. Recommended: You can use the web-texture-sets LLM skill to help make a dual-texture build.

Known issue:
UE-411710 - Mobile: starting a second Lyra match after returning to the front end can crash the tab due to memory usage.

Workaround:
Reload the page between matches.

Known issue:
UE-411672 - Bink video isn’t supported on the web, and calling Bink Media Player Blueprint functions in a web build is expected to crash

Workaround:
Skip Bink calls on Wasm, for example behind a Get Platform Name check.

Build and hosting blockers

Known issue:
itch.io rejects oversized browser games: 500 MB total, 200 MB per file. Lyra and Development builds exceed these limits.

Workaround:
Use Shipping and reduce content; otherwise request higher limits, use another host, or provide a downloadable build. You can also try removing .br files if there is a non-.br version of the file- they are optional files used just to improve initial download speed.

Known issue:
UE-412027 - Projects created with new_template_project.py --web-ready can’t join a server, through the WebSocket relay or otherwise: the web build has no IpNetDriver.

Workaround:
In the project’s .uproject, remove “Wasm” from the PlatformDenyList of OnlineSubsystemUtils and OnlineSubsystemNull, then package again from the start (build, cook and stage). Projects created without --web-ready aren’t affected.

Known issue:
UE-411774 - Launcher builds fail with WebMinimal/DesktopMinimal trim profiles.

Workaround:
Remove ApplyTrimProfile; configure plugins in the .uproject and use supported per-target size options.

Known issue:
UE-411525 - Incremental packaging can ship stale content with -NoCleanStage.

Workaround:
Remove the switch or clean the staging folder after content changes.

Known issue:
UE-411308, UE-411939 - Opening the packaged index.html from disk (file://) shows a cross-origin isolation error or a hosting card instead of starting the game.

Workaround:
Start the game with Play_.bat in the packaged folder, or serve the stage with python serve.py --root Wasm --port 8800.

Known issue:
UE-411973 - After an interrupted download, every reload in that browser session downloads the game again (45-49 s instead of about 3 s).

Workaround:
Restart the browser.

Known issue:
UE-411974 - Play_.bat fails from a network share (UNC path) and no server starts.

Workaround:
Copy the packaged build to a local folder.

Known issue:
UE-412037 - Launcher engine: changing the Wasm SIMD or allocator settings in a Target.cs (bSimd128, bSseEmulation, Malloc) will generate code incompatible with what ships in the Launcher, which could corrupt memory or crash.

Workaround:
Keep these settings at their defaults with the Launcher engine; change them only with an engine built from source.

Editor crashes and destructive scripting

Known issue:
UE-411502 - Python Blueprint root swaps can crash the editor.

Workaround:
Use a C++ base class that owns the required root component instead of swapping Blueprint-added roots.

Known issue:
UE-410941 - Undo after scripted Deformer Graph additions can crash and lose unsaved work.

Workaround:
Add variables/component bindings through the Deformer Graph editor’s Explorer > Add menu; the manual path supports Undo/Redo.

Known issue:
UE-411097 - Dataflow Add/Remove Option Pin crashes when a comment box is selected.

Workaround:
Avoidance: deselect comment boxes and select only the relevant Dataflow node before changing pins.

AI agents and the Unreal MCP server

Known issue:
UE-412135 - New projects give AI agents no tools to edit the level: the Unreal MCP server is on by default, but the Toolset plugins (AllToolsets, EditorToolset, BlueprintToolsets, ConsoleToolset, LiveCodingToolset) are off, so list_toolsets offers no actor, level, asset, Blueprint or Python tools, and nothing tells the agent why.

Workaround:
Before connecting the agent, enable All Toolsets in Edit > Plugins, or add { “Name”: “AllToolsets”, “Enabled”: true, “TargetAllowList”: [ “Editor” ] } to the .uproject, then restart the editor and reconnect your agent. Do this once per project.

[user_b7524e9b77c9fbd47ad1797454da6c7df2b8695d6c7063b06bff7f (丘木晨)]

沙发