How do I actually confirm my file is not uploaded?

Open your browser's developer tools, go to the Network tab, clear it, then run a conversion. If a tool uploads your file there will be a request whose payload is the size of your file. There is no such request here — and this is the check worth running on any tool site, not just this one.

Is there a catch — ads, a trial, a file limit?

No trial and no tier. The build fails if a paid-tier flag or a usage meter appears anywhere in the source, and the site's own response headers deny the browser's Payment Request API, so a checkout could not run here even if someone added one.

Why would an upload-based tool ever be better?

Because a server has far more memory than one browser tab, and native code is faster than WebAssembly. On a very large video, or on a low-memory machine, a server-side tool may finish where this one runs out of room. That is a real trade, not a footnote.

Does it work offline?

Yes, after the first visit. The app precaches itself, so the tools keep working with the network off. The exceptions are the video engine and the AI model weights, which are tens of megabytes and download the first time you use them.

GoChronos vs Upload-Based Online Tools

What actually differs between a tool that runs in your browser and one that uploads your file to a server — with the code behind each claim.