Upcoming Unity Editor Change Affecting Package Compatibility: Unity 6.6 Fast Enter Play Mode

2026-07-06 · Favo Yang · 2 min read

Starting with Unity 6.6, Fast Enter Play Mode is enabled by default for new projects. For package authors, the main compatibility change is that packages should no longer assume entering Play Mode performs a domain reload and resets C# static state.

When domain reload is disabled, static fields and static event subscriptions can survive between Play Mode sessions. This is good for iteration time, but it can expose hidden assumptions in packages that keep mutable static state, static singletons, static caches, static Unity object references, or static events.

If you maintain a package, the practical migration path is:

1. Audit mutable static fields and static singletons.
2. Check static caches, especially caches that store Unity objects or objects created during Play Mode.
3. Review static events and delegates so stale subscribers do not accumulate between Play Mode runs.
4. Add explicit reset or cleanup code where the package expects a fresh Play Mode session.

Unity's migration-style manual page, Enter Play mode without domain reload, is the best starting point. It explains what persists when domain reload is off, when manual cleanup is needed, and how Unity's automatic static cleanup attributes work.

For packages that support Unity 6000.5 or newer, Unity provides lifecycle APIs for this cleanup. Use OnEnteringPlayModeAttribute when a package needs to initialize or reset systems as Play Mode starts, and use AutoStaticsCleanupAttribute for static fields that should be cleaned up automatically.

Because these APIs are only available in Unity 6000.5 and newer, packages that also support older Unity versions should guard the new attribute with a version define:

using Unity.Scripting.LifecycleManagement;
using UnityEngine;

public partial class MyCounterClass : MonoBehaviour
{
#if UNITY_6000_5_OR_NEWER
    [AutoStaticsCleanup]
#endif
    public static int cleanedUpCounter = 0;

    private void Start()
    {
        Debug.Log(cleanedUpCounter);
        cleanedUpCounter++;
    }
}

For implementation details and ordering, refer to Unity's Domain and scene reload execution order reference. That page shows which steps Unity runs or skips as domain reload and scene reload settings change.

This Play Mode change is also part of a broader Unity runtime transition. If your package depends on low-level runtime behavior, reflection, generated code, or editor/runtime boundary assumptions, also read Unity's Path to CoreCLR, 2026: Upgrade Guide for wider migration context.

For OpenUPM package maintainers, this is a good time to add a compatibility pass to your next package release: test repeated Play Mode entry in a Unity 6.6 project, verify that package state starts clean each time, and keep the cleanup code conditional when your package still supports older Unity versions.