Fix: Unity ScriptableObject Changes Not Persisting After Domain Reload

You wired up a tool that edits values on a ScriptableObject. The edits look right. Then a script recompiles, or you exit Play Mode, and every change is gone. Unity is doing exactly what it is supposed to do; the asset was never marked dirty.

The Symptom

You hit a button on a custom inspector or run a tool script. Fields visibly update in the Inspector. Five minutes later, after you click somewhere else, the values are back to what they were before. Or you mutate the asset from a runtime script in Play Mode and the changes vanish on stop.

What Causes This

Unity serializes assets only when the Editor decides they are dirty. Setting a field via reflection, SerializedProperty, or direct assignment does not by itself flip that flag — you have to do it explicitly. On a domain reload (script recompile, assembly definition change, exit Play Mode with reload enabled), Unity reloads the asset from disk and any unsaved in-memory mutations are discarded.

The Fix in Editor Tools

using UnityEditor;
using UnityEngine;

public static class EnemyConfigEditor
{
    public static void SetHealth(EnemyConfig cfg, int hp)
    {
        Undo.RecordObject(cfg, "Set Enemy Health");
        cfg.health = hp;
        EditorUtility.SetDirty(cfg);
        AssetDatabase.SaveAssetIfDirty(cfg);
    }
}

The four lines do four different jobs:

• RecordObject — lets Ctrl-Z undo the change.
• Direct field set — the actual mutation.
• SetDirty — marks the asset for serialization.
• SaveAssetIfDirty — writes YAML to disk now.

If you skip SaveAssetIfDirty, the change persists across script recompiles but only writes to disk when Unity next saves the project. Usually fine; for tools that operate on many assets, prefer a single AssetDatabase.SaveAssets() at the end.

The Fix for SerializedProperty Edits

If you are editing through a SerializedObject in a custom inspector, the right calls are serializedObject.ApplyModifiedProperties(). ApplyModifiedProperties calls SetDirty internally, so you do not need a separate SetDirty.

SerializedObject so = new SerializedObject(target);
so.FindProperty("health").intValue = 100;
so.ApplyModifiedProperties();   // dirties + records undo

Play Mode Is Different

In Play Mode, runtime mutations to a ScriptableObject persist until the domain reloads. Exiting Play Mode reloads the domain by default, which reverts the asset. To capture state from a Play Mode session:

1. Add an inspector button: “Capture Runtime Values”.
2. While in Play Mode, click it. The button reads the live fields and calls SetDirty + SaveAssetIfDirty so the asset is serialized before the reload.

Don’t Mutate ScriptableObjects in Builds

Once shipped, ScriptableObjects are read-only data. The asset bytes live in a binary blob; writing to them in a player has no effect across sessions and may quietly corrupt other instances that share the same backing memory. For runtime state, use a separate save system.

Understanding the issue

AI bugs are emergent. The code is correct in isolation; the behavior emerges from interaction with other systems. Reproducing means controlling the interaction; fixing means deciding which interaction was wrong.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Boundary conditions deserve specific testing attention. What happens when the input is zero, maximum, negative, or NaN? What happens at the start of a session vs hours in? What happens at the boundary between two systems handling the same data? These are where bugs hide and where regression tests are most valuable.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“SetDirty marks. SaveAssetIfDirty writes. ApplyModifiedProperties does both. Without one of these, every edit is in-memory only.”

Related Issues

For lost prefab edits, see prefab changes not saving. For undo behavior, see undo in custom inspector.

RecordObject. SetDirty. SaveAssetIfDirty. The bytes hit disk.