Last Week in Unreal: The 5.9 Branch Goes Public, and ue6-main Drops Cooking in the Editor (November 2 to November 8, 2026)

Week of November 2 to November 8, 2026 | 1,476 commits on ue6-main, 2,450 on the new public 5.9 branch, 730 more on ue5-main, 2 on 5.8 | Branch: ue6-main

Overview

The week’s news is 5.9. Epic posted: “It’s official: 5.9 is happening and it’s coming in Preview next week. It’ll include improved AI workflows, web as a target platform, rendering and animation updates, plus more.” The same post said Epic was “making a few changes in 5.9 branch over on GitHub,” and the public 5.9 branch took 2,450 commits between Monday and Sunday. There is no 5.9 tag or GitHub release yet, so nothing here is released. What the branch shows is what Epic is preparing for that Preview, and the commits call it “UE 5.9 Preview 1” in places. The section near the end goes through it.

The short version: the headline 5.9 feature is a web platform, a WebGPU target built with emscripten. Its own documentation sets the terms for Preview 1. It needs a C++ project, the desktop renderer tier is not part of it, Nanite, virtual shadow maps, and hardware ray tracing are not supported on it, and “Preview 1’s web path is written for an agent doing the work, with a person reviewing.” The branch also raises the iOS minimum from 15.0 to 17.0 and turns on the AI Assistant and Model Context Protocol plugins by default, and it still builds with the VS2022 compiler, which ue6-main dropped three weeks ago.

On ue6-main, the change most likely to break a workflow is in the cooker: “all project cooks must now run through the cook commandlet” (84cd4dab). The cook incremental version changed three times, legacy ray traced sky light was removed, and a UBA fix stops credential environment variables from reaching remote helpers.

At 1,476 commits, ue6-main came in just below last week’s 1,506. Among named subsystems, build leads at 129, followed by editor at 110, rendering at 91, core at 61, and platform at 40. If you ship on 5.8, the 5.8 branch took two localization commits and nothing else. There is still no 5.8.5 tag.

Development Trends

The cook invalidations are back

Pattern: the engine’s CookIncrementalVersion GUID, which the source says to change “when all packages in all platforms in all projects need to be invalidated in the next incremental cook,” changed three times this week (b7d89bd1, d6ecb330, 2441c767). The messages read “Primary asset scanning rules not picked up,” “Shader change not picked up,” and “Duplicate primary asset ids not picked up.” Two weeks ago it changed four times, and last week not at all.

Interpretation: each bump is a dependency incremental cook did not track, fixed by invalidating everyone. The same week removed cooking in the editor process entirely (84cd4dab) and fixed a case where incremental cooks missed a recook, with the repro “cook a Niagara system, edit its renderer MIC’s parent material, cook incrementally: the system is not recooked” (6ea64332). Epic is narrowing the cooker to one supported path and still closing gaps in it, and three of those gap fixes cost build farms a full recook.

Who cares: cook and build engineers, anyone with incremental cooks on a farm that syncs often.

Signal strength: Strong.

Agent tooling, now on two branches

Pattern: counted by commit path the same way as before (the experimental Toolsets, ToolsetRegistry, and AIAssistant plugins, ToolsetCore, and AsyncFunctionResult, with mesh modeling toolsets excluded), the ue6-main cluster came to 91 this week, against 60 last week. Low-tier file lists are capped, so 91 is a floor.

Interpretation: the bigger change is that this work now has a release target. The 5.9 branch took “The UE6 AI stack at //UE6/Main” in one landing and then enabled AI Assistant and Model Context Protocol by default, “5.9 only; UE6/Main keeps both plugins disabled by default.” The 5.9 section has the details. The agent tooling we have been counting on ue6-main since June is about to reach studios in the 5.9 Preview.

Who cares: tools programmers, pipeline teams with editor automation, and anyone deciding whether to let agents into their editor.

Signal strength: Strong.

TL;DR

5.9 is official, and its branch is public. Epic says it is “coming in Preview next week.” There is no tag or release yet. The section on 5.9 covers the web platform’s requirements, the iOS 17 minimum, AI plugins on by default, and the toolchain.
ue6-main removed cooking in the editor process. “all project cooks must now run through the cook commandlet.” In-editor cook on the fly is deleted “without deprecation” (84cd4dab).
Three full recooks. The cook incremental version changed three times (b7d89bd1, d6ecb330, 2441c767).
UBA stopped sending secrets to remote helpers. Variables ending in _TOKEN, _SECRET, _PASSWORD, and similar no longer reach remote workers (f79979f1).
Legacy ray traced sky light is gone. It “is covered by Lumen by default” (057f1be8). The matching ray traced AO removal was backed out the same day and did not ship (a391cd94, fcaaf637).
Defaults and floors moved. Vulkan split barriers are off by default (47f21e81). The minimum GDK is now 260403 (551ea77a). Android uses AutoSDK directory -29 (d33069cd).
If you ship on 5.8: nothing new on the 5.8 branch beyond localization, and no 5.8.5 tag. The 5.8 section lists what to cherry-pick.

Highlights

Cooking in the editor process is gone

What changed

“Cooker: Remove support for cooking in the editor process (UUnrealEdEngine::CookServer, ECookMode::CookOnTheFlyFromTheEditor/CookByTheBookFromTheEditor); all project cooks must now run through the cook commandlet” (84cd4dab). The commit says this “follows up changes in 5.7 that deprecated CookByTheBook in editor,” and that it also “deletes (without deprecation) the ability to do CookOnTheFly in editor, which was previously supported by setting CookerSettings.bEnableCookOnTheSide=true.”

What still works. “Cooks of individual packages can still be executed in editor via UE::Cook::Internal::CookAsset, but the behavior may differ from the project-wide cook for some classes.”
Saved launcher profiles. Profiles that used an in-editor cook mode are remapped, with a warning: “Cooking in the editor process is no longer supported. This profile’s saved cook mode now cooks By the Book or On the Fly in a separate cook process.”
Removed API. The diff removes the launcher profile’s OnIsCookFinished() and OnCookCanceled() delegates. It also removes bEnableBuildDDCInBackground from the cooker settings, which the message does not mention.

Why this is important

Launch On profiles keep working, but they now start a separate cook process. Editor plugins or tools that cooked in the editor process, or that hooked the removed launcher delegates, will not compile. If anyone on your team used cook on the side, it is gone with no deprecation period.

Who should care

Tools programmers with editor cook integrations, build engineers, anyone who uses Launch On.

Urgency

Act now if you have custom tooling around in-editor cooking.

Cook: three invalidations and a missed-recook fix

What changed

Full recooks. Three changes to CookIncrementalVersion (b7d89bd1, d6ecb330, 2441c767), each for something incremental cook had “not picked up.”
A missed recook. Objects created during PreSave lost their cook dependencies, so “cook a Niagara system, edit its renderer MIC’s parent material, cook incrementally: the system is not recooked” (6ea64332).
Stale chunk assignments. A previous cook’s chunk assignment could be honored by staging and IoStore. The cook now writes a drop op and will “Fail the cook if the drop op cannot be written, since output staged with a stale assignment would otherwise report success” (4d36541f).
API break. IPackageWriter::WriteBulkDataPayloadInfo is now pure virtual, so “Every writer must now decide whether to use it” (5d153b7a).

Why this is important

Three bumps in one week means a farm that syncs daily can pay a full recook more than once. The missed-recook and stale-assignment fixes come from the same place: an incremental cook that finished with stale output. The stale-assignment case, in the commit’s words, “would otherwise report success.”

Who should care

Cook engineers, CI owners, anyone with a custom package writer.

Urgency

Act now on recook capacity. Track the rest.

UBA and Horde: credentials stay local, and a Linux SDK fix

What changed

Credentials. “The session server copies the host process environment to every remote helper,” so “a bearer token in the host environment (for example the Horde token a distributed shader compile logs in with) reached every remote worker’s environment.” Variables whose names end in “_TOKEN, _SECRET, _SECRET_ACCESS_KEY, _PASSWORD, _APIKEY or _API_KEY (any case)” are now left out (f79979f1). The rule is by suffix, so a secret named another way still travels.
Wine. UBA can access the Linux file system directly when running under Wine, measured at “open+read+close of 2000 files: 483ms vs 20ms direct.” It is “Off by default, enabled with UBA_WINE_DIRECT_FS=1” (76456632).
Spurious Visual Studio errors. On machines with SentinelOne installed, UbaObjTool printed exception callstacks, “2,124 for 1,062 libs in one editor build,” and “MSBuild turns these into Visual Studio errors even though the build succeeds” (a4033883).
Horde UBA cache. “A cluster without UBA cache settings made the uba-cache endpoint return 500, and Unreal Build Tool builds failed after retries.” Also, “A partly filled uba block is now an error when the global config loads” (345e22b5).
AutoSDK on Linux and Mac hosts. A Windows-only “runas” verb meant “any HostLinux or HostMac AutoSDK with a setup.sh took down every UBT and UAT process on that host” (e2aba1e6).

Why this is important

If you run distributed UBA builds with tokens in the environment, every remote helper has been receiving them. Sync this, and consider rotating anything that may have been exposed on shared helpers. The Horde config change can turn a config that loaded last week into an error, so check it before you deploy.

Who should care

Build and DevOps engineers, Horde administrators, security owners.

Urgency

Act now on the credential fix if you use remote helpers. Act now on the Horde config check.

Ray tracing: sky light removed, AO stays for now

What changed

Sky light. “[HWRT] Remove deprecated RT Skylight feature.” The commit says “This is covered by Lumen by default, which has been the recommended path for several releases” (057f1be8). It was backed out once and relanded the same week. The diff removes r.RayTracing.SkyLight and its tuning CVars along with the .Supported switch added last week.
AO did not go. “[HWRT] Remove legacy RT Ambient Occlusion” (a391cd94) was backed out the same day (fcaaf637) with no reland, so ray traced AO and r.RayTracing.AmbientOcclusion.Supported are still there.
BLAS compaction. Compaction moved from the RHI into the engine and is on by default: “r.RayTracing.Compaction.Explicit, default on; set 0 at startup to roll back to RHI compaction” (d4651cb5).

Why this is important

Any config that still sets the ray traced sky light CVars now sets nothing. The AO removal will likely return, since the commit calls it “deprecated by Lumen,” so treat it as borrowed time. Explicit compaction changes memory and timing for every ray traced project after a sync.

Who should care

Rendering leads and technical artists on ray traced projects.

Urgency

Act now if your configs use ray traced sky light. Track AO.

Defaults and floors that moved

What changed

Vulkan split barriers. r.Vulkan.AllowSplitBarriers goes from 1 to 0. The commit: they add “to cost over regular barrier for nothing” and “destroy performance when multiple instances are running on a single machine.” Epic calls it temporary, “until we control better where to use them” (47f21e81).
Async loading order. Two new settings, s.PrioritizeExternalReads and s.BoostSharedShaderMapIoRequests, both default to true. The s.PrioritizeExternalReads help text describes 0 as “strict FIFO order,” so FIFO is now the opt-out (22b81471).
Clang warning. “Enable -Wsuggest-override … by default for BuildSettingsVersion.V9 and later. Older build settings versions keep it off” (b5a48408).
Minimum GDK. “update MinGDKEdition to 260403” (551ea77a). The diff moves MinGDKVersion from 250404 to 260403, so build machines with an older GDK fall out of the supported range.
Android AutoSDK. “Android: Use AutoSDK directory -29,” which adds the same SDK for Linux hosts. “Every other branch keeps -28” (d33069cd).
ITF visualizers. Editor.EnableITFVisualizer goes from false to true (fe13c293).
Sequencer audio. Audio sections now always restore state on completion, and the “legacy When Finished completion mode dropdown” is hidden (31cc76ca).

Why this is important

The GDK and AutoSDK changes break build machines that are not updated, not code. The Vulkan change helps test farms that run several Vulkan instances per host. The Sequencer change can alter how existing sequences end, with no edit.

Who should care

Build engineers with Xbox or Android agents, Vulkan teams, cinematics teams.

Urgency

Act now on GDK and AutoSDK mirrors. Track the rest.

Worth Tracking

A badge that ended the week where it started. A generative AI provenance badge for the Content Browser was added off (c7bee7dd), turned on (73450868), and turned off again with “Hide AI badge by default” (4ac8854c). Net: a new feature that is off.

Follow-ups on earlier posts. Last week we reported the DWARF 5 change as backed out. Its refactor relanded this week (bc671693), with DWARF version 4 kept on Android, Apple, and Linux. The UBT Horde demand reporter has not come back, UE_USE_COMPACT_SET_AS_DEFAULT and WITH_RUNTIME_FEATURE_LEVEL_SWITCH are still 0, and the 14.50.35723 MSVC minimum still stands on ue6-main.

What We Ignored

This week we ignored most of the 1,476 ue6-main commits: Toolsets, MCP, and Python API exposure, Verse and UEFN work, Lore revision control changes, SceneGraph and TEDS, Test Automation Hub, Mutable and MeshPartition churn, and narrow crash fixes. There were 32 commits tagged [Backout], against 23 last week. Among the backouts that did not reland, besides ray traced AO, are the GameplayBots experimental plugin (809eaa38, backed out by bf37aff9) and the SceneGraph packed mesh (b297cc1a, backed out by 3dd900fb).

For teams shipping on 5.8

Nothing moved on 5.8. The public 5.8 branch took two Localization Automation commits and nothing else. Build.version still reads 5.8.4, and there is no 5.8.5 tag. Last week a ue5-main commit described itself as “Hotfix for 5.8.5,” and nothing this week repeats it. Apart from the AssetRegistry fix we covered last week, which was already on 5.8, none of the fixes we recommended in the last two editions are on 5.8, so the rest are still cherry-pick material.

Where the fixes are now. With the 5.9 branch cut, fixes land on ue5-main and on 5.9. Of the commits on ue5-main that are not on the 5.9 branch, most are 5.9 web platform work. The ones below matter to 5.8 users. We compared each one’s diff context against the source at the 5.8.4-release tag, and every hunk matched unless we say otherwise. That means it should apply, not that it compiles, so build and test it.

Worth cherry-picking onto 5.8.4

UBA credentials. The same fix as on ue6-main: “a bearer token in the host environment … reached every remote worker’s environment” (e6ae90eb). If you run remote helpers on 5.8, take this first.
Packaged Win64 games exit with a crash code. The audio mixer’s device notification client made the process exit “with ECrashExitCodes::CrashReporterCrashed (777003). The log ends cleanly (“Log file closed”), no crash folder is written, and only the exit code shows the crash” (ae3910d8). If your automation checks exit codes, this is a false failure you may already be seeing.
Mutable ragdolls detach from the root. In Mutable customizable objects, “Fix physics asset constraints that use body index 0 being dropped,” which made “partial ragdolls detach from the kinematic root.” The commit calls it a “Regression from 5.7” (a181c6df). Skip it if you do not use Mutable.
Xcode 27. “Remove legacy OS X 10.9 targeting so CrashReportClient builds with Xcode 27” (27460478).
UAT boolean switches. “a boolean switch given as -Name=false or -Name=0 is off, not on.” Before, -iterate=false turned incremental cooking on. It “Applies to every boolean switch it parses (93 call sites)” (fe616141). Check any pipeline that passes =false before you take it, because its behavior will change.
FBX embedded files. On the 5.9 branch, an FBX file’s embedded files are now rejected if they resolve outside the source folder: “Potential malicious embedded files detected” (8f27cf21). The FBX part matches 5.8.4. The glTF part of the same commit does not, so take the FBX files and resolve the rest by hand.
Linux file deletion and iOS remote builds. Dangling symlinks broke “Gauntlet’s artifact copy on Linux” (21065f28), and iOS remote builds failed to link without GPUReshape on the remote Mac (5616eb4f).
Crashes. A null shader compiler environment from game-thread material lookups such as Landscape grass (26ab33b2), a worker-thread crash in mesh card builds (8ae4a3b6), renaming a morph target to an existing name “deleting its data” (6030315b), and Niagara “access violation binding a stale global distance field texture” (83231c54). From the 5.9 branch: “crash and corrupted weights” applying skin weight profiles to unlimited-influence meshes (788b3323), and “Metal: Fix out of bounds texture binding writes” (bdb0a7be).

Take if you hit them

Cook. Niagara GPU scripts could be missing from one platform’s shader library when a cook targets two shader platforms of the same feature level, “a different subset from cook to cook” (36cb35b7). It was backed out and restored within hours, and it forces a Niagara GPU script recompile and recook on every platform. A geometry collection cooker version change fixes a “non-deterministic cook,” and forces those to recook (249fd8f5).
Rendering. Volumetric lightmap streaming broke “on Vulkan with VK_EXT_descriptor_buffer bindless (NVIDIA 595.x, T4/L4)” (0a4ce01c). Lumen compacted traces dispatched past 65,535 groups, and “D3D12 has the same 65,535 limit, where drivers tolerate the out-of-spec dispatch” (19fc56bc). No native symptom is reported.
Mover travel crash. In the Game Animation Sample, “a cooked desktop build with the stomp allocator crashes on every travel” (510080d3). It affects Mover data that holds a Blueprint user-defined struct. A companion commit (87c9f5bf) frees the state machine’s data earlier, but says that on its own it “moves the crash to the other holders.” 510080d3 is the one that fixes it.
Unattended editor runs. Scripted runs log instead of opening modal dialogs, which includes a “Mac commandlet crash” (20fb6598).

Looks like a fix, but is not one for 5.8.4

The CEF v147 update “plus fix for CVE-2026-111497” (4634a95d) replaces the 147.0.14 binaries that 5.9 uses by default. 5.8.4 defaults to CEF 128, and its experimental option points at a different 147 build, so none of it applies to 5.8.4.
Several commits say they restore “5.8” behavior, such as “native platforms default to 5.8’s encoders again” (93615354). They undo regressions that exist only in 5.9, so there is nothing to take.
The EOS SDK 1.20.0.3 binary update (a269cf19) replaces binaries that 5.8.4 does not have. None of the file entries it swaps match the hashes at 5.8.4, so treat it as a separate SDK upgrade, not a cherry-pick.

UE 5.9: what the public branch shows

Where it stands. Epic’s post: “It’s official: 5.9 is happening and it’s coming in Preview next week. It’ll include improved AI workflows, web as a target platform, rendering and animation updates, plus more. We’re making a few changes in 5.9 branch over on GitHub, and more will come in over the next few days.” The public 5.9 branch was cut from ue5-main at the start of this week and took 2,450 commits by Sunday. There is no 5.9 tag and no GitHub release. Everything below is what the branch holds, not a release.

Two cautions on the numbers. 5.9 contains everything that landed on ue5-main since 5.8, most of it before this week, so 2,450 is this week’s activity, not the size of 5.9. And most changes landed in two or three streams at once (5.9, Dev-5.9, and ue5-main), so there are roughly 1,000 to 1,200 distinct changes behind those commits, depending on how duplicates are matched. About half carry a “[UE 5.9]” tag. One engineer wrote 757 of the 2,450 commits, most of them the web platform and AI work.

The web platform. The branch landed “Engine/Platforms/HTML5 (the web platform: UBT/UAT platform, RHI, shader format, toolchain kit, page, harness, deploy tools)” (3b214252), then renamed it: “Rename the HTML5 platform to Wasm” (4a4c2e68). Its README at the head of the branch says “The Wasm platform builds a project into a web page: a .wasm game (emscripten, wasm64 with threads), a WebGPU renderer with two tiers, and a page that loads, caches and starts it.” The terms it sets for Preview 1, all quoted from that README and the support matrix next to it:

Renderer. Two tiers, both on the mobile renderer: “mobile forward (SP_WEBGPU_ES3_1, for phones and low-end GPUs) and mobile deferred (SP_WEBGPU_ES3_1_DEFERRED, for desktop-class GPUs).” “The desktop tier is experimental and not part of Preview 1,” so Lumen on the web exists only in that tier, which is off by default.
Not supported. Nanite, virtual shadow maps, and hardware ray tracing are “not supported” on every web tier. Meshes render their Nanite fallback, and shadows use cascaded shadow maps.
Projects. “A content-only (Blueprint-only) project is not supported for the web in this preview and needs one C++ class added.” The editor now greys out the web platform for content-only projects (c8cf8580).
Toolchain. “UE 5.9 with the web libraries (Windows host),” and “emscripten 6.0.10 (Config/Wasm_SDK.json; no other version is accepted).”
Browsers. “Chrome (Windows, desktop) | supported; the reference.” Edge on Windows and Chrome on macOS are listed as supported. Safari on macOS is “supported and tested,” but a large project’s Test build does not load in Safari 26.6, which is “under investigation.” Chrome on Linux needs Vulkan flags. Firefox on Windows “runs, slower start.” Safari on iOS is “a phone browser,” but “the 3D scene draws black with no WebGPU errors,” which is also under investigation. “Phones and tablets” are “warn by default.”
Agents. “shipping a game on the web in Preview 1 takes an agent (an LLM coding assistant) doing the work with the skills this platform ships,” and “Preview 1’s web path is written for an agent doing the work, with a person reviewing.”

The web defaults moved back and forth during the week, which is normal for bring-up: the mobile deferred tier was made opt-in (6cf8f608) and then cooked by default again the same day (3610db99). Read the branch head, not the history, if you are evaluating it.

AI workflows. The branch took “The UE6 AI stack at //UE6/Main,” covering the Python scripting plugin, Model Context Protocol, the toolset registry, AI Assistant, and the Unreal Console (ec7f3964). Then: “Enable AIAssistant and ModelContextProtocol by default.” The commit says “Enabling ModelContextProtocol does not start the MCP server: bAutoStartServer still defaults to false,” and “5.9 only; UE6/Main keeps both plugins disabled by default” (4a9a457c). Both plugins are still marked experimental. Before that, a set of 5.9-only additions to the MCP server, including a bearer token and a read-only mode, was removed so the plugin “matches //UE6/Main” (440b8d52).

Upgrade notes we found in this week’s commits. Compared against 5.8.4:

iOS minimum 17.0. The diff moves MinSoftwareVersion from 15.0 to 17.0, and the commit removes bUseSceneBasedLifecycle because “the UIScene-based lifecycle is now always used” (93daf84b).
JSON import is strict. “a string imported into an integer or bool property must parse completely … (5.8 used FCString::Atoi64 / FString::ToBool and accepted any text)” (3cc36d66). Data that relied on lenient parsing can fail to import.
Buoyancy. A new p.Buoyancy.RegisterWithWaterBodiesOnly defaults to 1: “Buoyancy’s sim callbacks run only in worlds that have a water body. 0: in every world, as in 5.8” (009aaa80).
Lumen. r.Lumen.RadianceCache.MaxFramesAccumulated is 4 by default (5ce504ac). It was reverted (6498ab39) and restored (79a35442) within the same block of changelists, and it does not exist in 5.8.4, so this is new temporal accumulation in the radiance cache, with the trade-off named by the commit that added it: “the expense of increased GI lag” (1356bbda).
Lore. Epic’s revision control plugin arrives from //Fortnite/Main, and “LoreSourceControl is enabled by default (like Perforce and Git)” (f0c828be). It is marked experimental.
Toolchain. The branch bans MSVC 14.51, “VS2026 18.10.x: Optimizer miscompile causes memory corruption” (e247d73b). Its minimum MSVC is still 14.38.33130, as in 5.8.4, so the VS2022 compiler still builds 5.9, but only from Visual Studio 2022 17.14 on, and several 14.39 to 14.44 builds stay banned. Compared with 5.8.4, the minimum Windows SDK is 10.0.22621.0 instead of 10.0.19041.0 and the minimum Visual Studio 2022 is 17.14 instead of 17.8, though those landed on ue5-main before this week.
UBA Cloud is not in the build. An implementation landed Monday and was backed out Friday, “replaced by UBA team implementation” (3793974c). The replacement is “Alpha, not part of any engine build” (5c597406).

Closing

If you are planning a 5.9 upgrade, wait for the Preview, but check the iOS minimum, JSON imports, and the web platform’s C++ requirement now. If you track ue6-main, look for editor cook integrations and plan for recooks. If you ship on 5.8, the branch took only localization updates.

Commit SHAs are cited inline, in parentheses, next to each claim. They reference the ue6-main branch of EpicGames/UnrealEngine, except in the 5.8 and 5.9 sections, where they reference commits on ue5-main and the public 5.9 branch.

Last Week in Unreal: The 5.9 Branch Goes Public, and ue6-main Drops Cooking in the Editor (November 2 to November 8, 2026)

Overview