Fix: Unity Asset Pipeline V2 Import Loop

Here is how to fix Unity Asset Pipeline V2 stuck reimporting assets in an endless loop. The Editor freezes, progress bar cycles, and saving the project takes forever. The cause is recursion from a custom importer or postprocessor that triggers another import as a side effect.

The Symptom

Importing assets cycles indefinitely. Editor.log fills with import messages for the same files. CPU usage spikes. Cancel does not stop it.

What Causes This

SaveAssets in import. Calling AssetDatabase.SaveAssets inside OnImportAsset triggers a write that re-imports the file.

Modifying .meta in postprocess. Updating the asset’s .meta file from a postprocessor changes the import hash; Unity re-imports.

Cross-asset writes. An importer for A that writes to B causes B’s importer to write to A; loop.

The Fix

Step 1: Keep ScriptedImporter side-effect-free.

using UnityEditor.AssetImporters;
using UnityEngine;

[ScriptedImporter(1, "mydata")]
public class MyImporter : ScriptedImporter
{
    public override void OnImportAsset(AssetImportContext ctx)
    {
        var obj = ScriptableObject.CreateInstance<MyData>();
        // populate obj from file
        ctx.AddObjectToAsset("main", obj);
        ctx.SetMainObject(obj);
        // NO: AssetDatabase.SaveAssets()
        // NO: AssetDatabase.WriteImportSettingsIfDirty()
        // NO: System.IO.File.WriteAllText for meta files
    }
}

Step 2: Move side effects to menu items. Asset post-processing that modifies other assets (generating spritesheets, batch transforms) should be a manual menu item, not an automatic postprocessor.

Step 3: Use OnPostprocessAllAssets correctly.

static void OnPostprocessAllAssets(string[] imported, string[] deleted, string[] moved, string[] movedFromPaths)
{
    foreach (var p in imported)
    {
        if (p.EndsWith(".png"))
        {
            // Read-only operations OK
            // AssetDatabase.LoadAssetAtPath, etc.
        }
    }
    // Avoid AssetDatabase.SaveAssets here
}

Step 4: Use DependsOnArtifact for clean dependencies.

ctx.DependsOnArtifact(externalArtifactGuid);

Declare dependencies explicitly so AP2 reimports your asset when the dependency changes — without you needing to write to it.

Step 5: Bisect to find the culprit. Disable Editor scripts in chunks. When the loop stops, you know which postprocessor caused it.

Understanding the issue

Asset pipelines transform source content into runtime data. Each stage can lose information, change behavior, or introduce platform-specific variations. Bugs at this layer are often invisible until the cooked build runs.

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

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

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

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

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

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

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

Diagnosing this class of bug benefits from a structured approach: confirm the symptom, isolate the variables, hypothesize the cause, and verify the hypothesis before writing fix code. Skipping the isolation step is the most common mistake; without it, fixes often address symptoms while the underlying cause continues to produce other variations.

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

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

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

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

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

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

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.

“ScriptedImporters describe how to import. They do not write meta files or trigger other imports. Side effects belong in menu items.”

Related Issues

For AssetBundle name updates, see AssetBundle Name. For domain reload phantom overrides, see Phantom Overrides.

No SaveAssets in import. No meta writes. Use DependsOnArtifact. The loop stops.