Your workbook stays on your device

There is no upload. No server sees your file, because no server is involved in processing it. Here is what that rests on, so you can check rather than take our word for it.

The site is static files

Every page is a file served from a CDN. There is no database, no account, and no endpoint that accepts a workbook. The only code running on the server side is ten lines that send www and our second domain to this one; it never reads a request body. The work happens in WebAssembly running in a worker inside your browser tab.

Static hosting alone would not be enough

A static site can still ship a script that sends data somewhere. So two further measures apply. The site sends a Content-Security-Policy that names every place it is allowed to reach, and on top of that the page and the worker install a guard, before any file is read, that refuses requests anywhere else and refuses any request to this site carrying a body.

One analytics script, and what it can see

Cloudflare Web Analytics counts page views. It sees the address of the page, the referring link, and coarse device information, the same things any web server logs. It sets no cookie and follows nobody between sites.

It cannot see your workbook. Your file is read inside a separate worker, and that worker's guard has no exceptions at all: no analytics host, no other host, nothing. The file name, the file contents and the verification report never cross out of it. Nothing else is loaded, no tag manager, error reporter, font CDN or embedded widget.

What you can check yourself

Open your browser's developer tools, switch to the Network tab, and shrink a file. You will see the page's own assets, one request to the analytics beacon when the page loads, and nothing during the work itself. Working offline is a stronger check: load the page, disconnect, and shrinking still works.

The report is yours

The verification report lists the parts inside your workbook, so it is generated locally and downloaded locally. It is never transmitted.