Repair a video without uploading it: how local browser repair works

By the FixMP4 engineering team · October 1, 2026 · 5 min read

Most videos that break are personal: family moments, interviews, a dashcam incident, a confidential meeting. Many online repair and conversion services work by uploading the file to their servers. That is convenient, but it means your footage sits on someone else's computer, subject to their retention, security and policies. There is another way.

What "local processing" means

Modern browsers can read a file you select directly from your disk, in slices, without sending it anywhere. JavaScript and WebAssembly running in the page can then analyse and rewrite the data at near-native speed. The result is saved back to your disk through the browser's download or file-saving features.

The FixMP4 repair tool works this way:

1. You pick the broken file (and, for video, a reference).
2. The page reads the file's boxes and scans the media data locally.
3. It rebuilds the index and assembles a playable preview in memory.
4. When you export, it writes the repaired file straight to your disk.

The only network requests are the page itself, sign-in, and checking whether your subscription is active.

How to verify it yourself

• Developer tools: open the Network tab (F12 or Cmd-Option-I), start a repair and watch the requests. None carries your file.
• Go offline: load the repair page, then disconnect Wi-Fi. Diagnosis and preview still work.
• File size check: a site that uploads a 4 GB file takes minutes on most connections; local processing starts immediately.

Why upload-based repair is risky

• Retention: files may be kept for hours or days, sometimes longer.
• Breaches: any server can be compromised.
• Jurisdiction: files may be processed in a country with different privacy laws.
• Size limits: large files are often refused or require a paid tier.
• Speed: uploading and downloading large files takes time.

For footage of other people, children, number plates or business meetings, those risks matter, and in some contexts privacy regulations apply.

Limits of browser processing

• A desktop browser is more comfortable for very large files than a phone.
• You need free disk space for the exported copy.
• Browser memory limits mean the tool streams data rather than loading whole files; that is handled automatically.

Performance

Local processing is usually faster than uploading, and it does not depend on your upload speed, which on many home connections is a fraction of the download speed. Scanning a multi-gigabyte recording reads the file once from disk; on a modern laptop with an SSD that takes seconds to a few minutes, depending on size.

What we can and cannot repair

FixMP4 rebuilds MP4, MOV and M4A files with H.264, HEVC and AAC whose index is missing or broken, which covers most recordings cut off by a dead battery, crash or full card. It does not handle AVI, MKV, ProRes, RAW, AV1 or Opus, and it cannot recover data that is not in the file. See video repair tools compared for alternatives, and our privacy policy for the details of what our server does store (your account and subscription, never your files).

• privacy
• local processing
• browser
• security
• no upload