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How to check whether a web tool really runs locally

Any site can claim your file never leaves your browser. Here is how to verify it yourself in about thirty seconds, using a tab your browser already has.

Updated 2026-08-04

Plenty of web tools say your file is processed locally and never uploaded. It is an easy claim to make and, until recently, a hard one for a visitor to check.

It is not hard. Your browser ships with the tool for it, and the check takes about thirty seconds. This page explains how — including on this site, which is the point.

The thirty-second check

  1. Open the tool’s page, but do not add your file yet.
  2. Press F12 — or right-click and choose Inspect — and select the Network tab.
  3. Click the small ⃠ icon to clear the list.
  4. Now add your file and run the tool.
  5. Look at what appeared.

If the file is being uploaded, you will see a request with a POST method and a size roughly matching your file. A 4 MB photo produces a 4 MB request; there is no way to hide it, because the bytes have to physically leave your machine.

If the tool runs locally, you will see either nothing at all or small requests for code and fonts. Nothing will approach the size of your file.

What you should expect to see, and what is fine

Some requests are normal and not uploads:

  • Script and stylesheet files — the tool’s own code, loaded when the page loads
  • A WebAssembly module — often several megabytes, for tools doing image or video work. This is code coming down, not your file going up. Check the direction: the size appears under a request for a .wasm file, not under a POST containing your data
  • A machine learning model — same thing, sometimes 40–80 MB. Downloaded once, cached afterwards

What should make you stop:

  • A POST or PUT request whose size matches your file
  • A request to a domain that is not the site you are on, appearing at the moment you added the file
  • Any request at all after you clicked the button, on a tool claiming to do everything locally

The offline test

There is a blunter version that requires no reading at all.

Load the page, then disconnect from the internet — turn off Wi-Fi, or use the Offline option in the Network tab. Now use the tool.

If it works with no connection, it cannot be uploading anything. There is nowhere for the bytes to go. This test cannot be faked and requires no interpretation.

A tool that fails this test is not necessarily uploading your file — it might simply need to fetch code it has not loaded yet. Reload the page first, let it settle, then go offline and try again.

Why this matters more for some files than others

For a holiday photo, a server round trip is mostly inelegant. For other files it is a genuine disclosure:

  • PDFs are contracts, invoices, medical letters and payslips
  • CSV files hold customer lists, payroll and membership registers
  • Photos carry GPS coordinates in their metadata by default, so uploading one shares where it was taken as well as what it shows
  • Screenshots frequently contain someone else’s information along with your own

Many organisations have policies against uploading documents to third-party services, which is exactly the situation a locally-run tool is for.

Checking this site

Everything here works the same way, and we would rather you checked than took our word.

Open the Network tab, clear it, and use any tool on this site. You will see the page’s own code and — for the heavier tools — a WebAssembly module downloading. You will not see your file going anywhere, because there is no server to send it to. The site is static files on a CDN.

There is one honest exception. A small number of publishing tools fetch a public URL that you type in, which necessarily involves a request. Those tools say so on their own pages. The distinction is whose material it is: fetching a web address you supplied is not the same as uploading your private file, and nothing about those requests is logged.