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.