CoplayDev/unity-mcp · error · Exception

Preview loading timed out.

Error message

Preview loading timed out.

What it means

Thrown by NativePreviewGenerator.WaitAndWritePreviews (NativePreviewGenerator.cs:254) as a generic Exception after InitialPreviewLoadingTimeoutSeconds (10s) elapse without AssetPreview producing a single preview for the current batch (remainingObjectCount never drops below initialObjectCount). Unlike the Validate() errors, this is thrown inside GenerateImpl()'s try/catch, so it is captured into result.Exception with result.Success = false rather than propagating. It indicates Unity's AssetPreview subsystem is not delivering textures in time.

Source

Thrown at TestProjects/AssetStoreUploads/Packages/com.unity.asset-store-tools/Editor/Previews/Scripts/Generators/NativePreviewGenerator.cs:254

        private async Task WaitAndWritePreviews(List<PreviewMetadata> objects, List<PreviewMetadata> generatedPreviews)
        {
            var initialObjectCount = objects.Count();
            if (initialObjectCount == 0)
                return;

            await WaitAndWritePreviewIteration(objects, generatedPreviews);
            var remainingObjectCount = objects.Count;

            // First iteration may take longer to start loading objects
            var firstIterationStartTime = EditorApplication.timeSinceStartup;
            while (true)
            {
                if (remainingObjectCount < initialObjectCount)
                    break;

                if (EditorApplication.timeSinceStartup - firstIterationStartTime > InitialPreviewLoadingTimeoutSeconds)
                    throw new Exception("Preview loading timed out.");

                await WaitAndWritePreviewIteration(objects, generatedPreviews);
                remainingObjectCount = objects.Count;
            }

            if (remainingObjectCount == 0)
                return;

            while (true)
            {
                await WaitForEndOfFrame(1);
                await WaitAndWritePreviewIteration(objects, generatedPreviews);

                // If no more previews are being loaded, try one more time before quitting
                if (objects.Count == remainingObjectCount)
                {
                    await WaitForEndOfFrame(1);
                    await WaitAndWritePreviewIteration(objects, generatedPreviews);

View on GitHub (pinned to c21bf496bc)

Solutions

1. Let Unity finish importing the target assets before generating (wait for the import queue to drain), then re-run.
2. Enable ChunkedPreviewLoading with an adequate ChunkSize so the AssetPreview cache (sized ChunkSize*2) is not starved.
3. Switch to WaitForPreviews=false, which uses WritePreviewsWithoutWaiting and does not poll/timeout.

Example fix

// before: waitForPreviews true, chunkSize 0 -> validation throws first; if set but previews stall, you hit the timeout
var s = new NativePreviewGenerationSettings { InputPaths = paths, OutputPath = out, ChunkSize = 5, WaitForPreviews = true };

// after: non-blocking path avoids the polling timeout entirely
var s = new NativePreviewGenerationSettings { InputPaths = paths, OutputPath = out, ChunkSize = 100, WaitForPreviews = false };

// Nothing to validate pre-call; instead ensure assets are imported first.
if (EditorApplication.isCompiling || EditorApplication.isUpdating)
    throw new InvalidOperationException("Wait for Unity to finish importing/updating before generating previews.");

// GenerateImpl captures this into result.Exception; check the result, not an exception:
var result = await generator.Generate();
if (!result.Success && result.Exception?.Message.Contains("timed out") == true) { /* let imports finish, then retry or fall back to WaitForPreviews=false */ }

When it happens

Trigger: Running the native generator with WaitForPreviews=true on assets whose previews AssetPreview never loads within 10 seconds. Common when target assets (large models/textures/materials) are still importing, when the AssetPreview texture cache is starved by a too-small ChunkSize, or under Unity Editor streams where AssetPreview loading is known to be inconsistent (see the code comment 'works inconsistently across Unity streams').

Common situations: Generating previews immediately after an AssetDatabase.Refresh / import that hasn't finished; very large asset sets; a Unity version with a flaky AssetPreview; running headless/batchmode where preview generation is unreliable.

Understand the failure class

• Timeouts: ETIMEDOUT, deadlines, and hung requests — what actually expires when a request times out.

Related errors

• Width should be larger than 0
• Height should be larger than 0
• Depth should be larger than 0
• Native width should be larger than 0
• Native height should be larger than 0

AI-assisted analysis of CoplayDev/unity-mcp@c21bf496bc (2026-08-13). Data as JSON: /api/errors/6d98aba43c322cf1. Report an issue: GitHub.