How to verify a web tool never uploads your file (a 30-second DevTools test)

Rui Barreira — Thu, 16 Jul 2026 10:16:56 +0000

Every "free online PDF/image tool" promises your files are safe. Most still upload them to a server. Here's how to check for yourself in about 30 seconds — no trust required.

The Network-tab test. Open the tool, press F12 → Network, clear the log, then run the tool on a file. If your file is being uploaded, you'll see a request (usually a POST) carrying it. If the list stays empty of your file, the work happened entirely in your browser.

The air-gap test (even simpler). Load the page, then turn off your Wi-Fi / go offline. If the tool still works, it cannot be sending your file anywhere — there's no connection. Genuinely client-side tools keep working; server-based ones break immediately.

Why it matters. "Files deleted after 1 hour" still means your document — a contract, a passport scan, a medical form — sat on someone else's server. Client-side processing removes that risk entirely: the file never leaves your machine.

Disclosure: I build Brevio, a set of free browser-based tools (PDF, image, dev, calculators) designed to pass both tests — you can run the checks above on any of them, or see the privacy-proof page. But the tests work on any tool, and that's the point: don't take a privacy claim on faith when you can verify it in 30 seconds.

What tools do you always run the air-gap test on before trusting?

DEV Community: Rui Barreira

How to verify a web tool never uploads your file (a 30-second DevTools test)