Where should document tools explain what stays on your device?

I have been thinking about privacy guarantees in document tools. Saying “no upload” is only meaningful when the processing path actually keeps file bytes in the browser, not just when a provider is used.

For a small web app, what do you consider the clearest way to communicate that boundary? A simple data-flow diagram, a per-tool explanation, or something users can verify in DevTools? I’m interested in what builds trust without making the interface feel like a security whitepaper.