# Unreal Engine 5.9 MegaJam Preview

**URL:** <https://forums.unrealengine.com/t/unreal-engine-5-9-megajam-preview/2852840>\
**Category:** Announcements\
**Tags:** unreal-engine, Release\
**Created:** [November 9, 2026, 2:50pm UTC](https://forums.unrealengine.com/t/unreal-engine-5-9-megajam-preview/2852840 "2026-11-09T14:50:41Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![RaghibK](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/raghibk/32/1220655_2.png) [@RaghibK](https://forums.unrealengine.com/u/RaghibK)\
**Post date:** [November 9, 2026, 2:54pm UTC](https://forums.unrealengine.com/t/unreal-engine-5-9-megajam-preview/2852840/3 "2026-11-09T14:54:59Z")

</div>

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](https://forums.unrealengine.com/t/current-available-quick-fix-solutions/2829) (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](http://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.

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