Unity 6.6 landed on 1 September with domain reload disabled by default in Play mode, Animation Rigging and Timeline promoted to core packages, and Content Directories replacing AssetBundles.
Unity released 6.6 on 1 September, with the 6000.6.0f1 installers dated the day before. The change most likely to alter your working day is a flipped default: entering Play mode no longer reloads the scripting domain, so scripts do not recompile and static state does not reset between test runs.
That is a real iteration win on a large project, and it is also the setting to read carefully before you upgrade. Unity's manual advises keeping domain reload off "to prepare for the forthcoming removal of Mono", so this is a direction of travel rather than a toggle you are meant to flip back. The practical consequence is that static fields keep their values across a stop-and-play, and static event subscriptions survive with them. Any singleton that quietly assumed a fresh start — a spawn manager, a character registry, an animation state cache — will now carry yesterday's state into today's test, and it will look like a gameplay bug rather than an editor one.
For anyone doing character work, the quieter item matters more. Animation Rigging and Timeline are now core packages, shipped with the editor instead of installed from the Package Manager. On paper that is a convenience. In practice it means constraint-based runtime rigging stops being an optional add-on someone has to justify adding to the manifest, and starts being engine furniture that every project has to hand. Unity also serialises Dictionary<TKey, TValue> fields natively now, with the Inspector showing them as a two-column list and validation happening at compile time — which retires a lot of hand-written serialisation glue in character and inventory data.
The build pipeline gets the other substantial work. Content Directories arrive as a replacement for the AssetBundle content build system, with automatic de-duplication and a new BuildPipeline.BuildContentDirectory API; teams already on Addressables can adopt the workflow through their existing APIs. Shaders can be compiled with the DirectX 12 compiler rather than FXC, and Unity cites one test project dropping from 1 hour 46 minutes to 32 minutes. A new Build Analysis window reads the reports written to Library/BuildHistory so you can see which assets are actually costing you.
Elsewhere: WebGPU is out of experimental and supported for production, with WebAssembly64 lifting the web memory ceiling from 4 GB to 16 GB, and the Compute Light Baker uses compute shaders and hardware-accelerated ray tracing in place of the Progressive Light Baker.
One caveat on timing. Unity classes 6.6 as a Supported release, not an LTS — it gets the same stability and critical updates as an LTS only until 6.7 arrives, and 6.7 is the version expected to carry the long-term badge. If you need something to sit on for the length of a production, that is the one worth waiting for.