Making "files never leave your browser" verifiable with DevTools and CSP

"Files never leave your browser" is becoming standard copy for PDF tools, image editors, and document converters. But a trust claim and a verifiable fact are different things. Here's how to turn "zero upload" into something any user can audit in about two minutes, and how to enforce it at the browser level so it isn't just a promise. Step 1: Read the Network panel Open DevTools → Network, enable "Disable cache", reload. While processing a file, filter by "Fetch/XHR" and "Doc". A genuinely client-side tool should show only HTML/CSS/JS/WASM asset loads — no POST requests, no GETs carrying file content in query parameters. The non-obvious trap: third-party analytics, Google Fonts, and CDNs all show up as outbound requests. If you claim zero uploads, those count too. The honest move is to self-host fonts and scripts and drop analytics entirely, so the request list is genuinely short enough to eyeball. The Network panel is the human-readable check. The next part is what actually makes it hold. Step 2: Enforce egress with CSP connect-src This is the piece people get backwards, so it's worth stating precisely. CSP's connect-src is an egress allowlist the browser enforces before the request is sent . A fetch /XHR to an origin that isn't on the list is blocked by the browser and never leaves the machine. You'll see it fail in the console as a CSP violation, with no entry in the Network tab going out to that origin. This includes no-cors requests. no-cors is sometimes assumed to be an escape hatch, but it isn't one for this purpose. All no-cors does is let you issue a cross-origin request while making the response opaque (you can't read the body). It does not bypass connect-src : if the target origin isn't in your connect-src allowlist, the no-cors request is blocked exactly the same way — it never goes out. So you can't smuggle a file out to a third party with no-cors under a tight CSP. That's what makes CSP the actual proof, not just documentation. Tighten connect-src to 's