Troubleshooting
General troubleshooting
Common issues and solutions: slow inference, black exports, missing features, browser errors.
The AI is taking a very long time
First run: The model weights download once on first use — ~80 MB for Fast, ~180 MB for Best Quality. On a slow connection this can take several minutes. The progress bar shows download status.
Subsequent runs: If inference is slow after the first run, your device may not support WebGPU. Check the badge in the top bar — if it says WASM, inference runs on CPU, which is slower (typically 15–60 seconds vs 2–5 seconds on WebGPU). See Browser support for which browsers enable WebGPU.
Large images: Images over 2000 px wide take longer to process. This is normal.
The page says "model failed to load"
- Check your internet connection — the model weights download from Hugging Face CDN on first use
- Try refreshing the page
- Check your browser console (F12 → Console) for a specific error
- If you're on a corporate or school network, your proxy or firewall may be blocking
huggingface.co. Try a different network. - If you use a content blocker extension, temporarily disable it and try again
My exported PNG shows a black background in Photoshop
This is the premultiplied alpha problem. See Why does my export show black in Photoshop? for a full explanation.
Short answer: another tool in your workflow may have incorrectly re-saved the file. If you downloaded directly from NSS and opened it straight in Photoshop without any intermediate tools, open a bug report via contact with the file attached.
The upload zone won't accept my file
- HEIC not converting: HEIC conversion requires a WASM module that loads on first use. Wait a moment for it to initialise and try again.
- File type not supported: Animated GIF and animated WebP are accepted (first frame only). Video files are not supported.
- File too large: There's no enforced size limit, but very large files (300+ MB RAW) may exceed available browser memory. Try downscaling first.
- Magic byte mismatch: If a file is renamed (e.g., a PNG renamed to
.jpg), the tool detects the real type and processes it correctly, but may show a type warning.
The brush tool isn't visible / editor won't open
- The editor only opens for images that have finished processing. Check the queue status — look for a green checkmark.
- Some browser security policies block OffscreenCanvas. Try Chrome if you're on Firefox.
- If the editor is blank, try refreshing the page and re-opening the image.
My session disappeared after a refresh
Session data is stored in IndexedDB. If it's gone after a refresh:
- Some browsers clear IndexedDB in private/incognito mode on close
- Storage quota may have been exceeded (check browser settings → site data for the site)
- Some aggressive privacy extensions clear IndexedDB automatically
To avoid losing work, export your results before closing the tab.
The app isn't working offline
The service worker caches the app after your first visit. If offline mode isn't working:
- Make sure you've visited the app at least once with a working internet connection
- Open DevTools → Application → Service Workers and check the worker is registered and active
- Try clearing the cache and reloading (this will re-download everything on next load)
See Working offline for more detail.
I see a "SharedArrayBuffer not available" warning
SharedArrayBuffer requires the page to be served with specific HTTP headers (COOP + COEP). NSS sets these headers correctly, but some browser extensions or proxies strip them. If you see this warning, multi-threaded WASM inference is disabled and the app falls back to single-threaded, which is slower.
Try disabling extensions temporarily to diagnose. Chrome extensions that intercept headers are the most common cause.
Something else is wrong
For bugs, unexpected behaviour, or questions not covered here:
- Check the browser console (F12 → Console) for error messages
- Contact us with a description of the issue, your browser version, and OS