How it works, explained
4 min read · Checked August 18, 2026
What on-device detection means
The engine runs on your Mac, Windows PC, or Android phone. It looks at what is on your screen, decides clean, suggestive, or explicit, and throws the frame away. What crosses to the internet is a heartbeat, an event, and your account. Never the picture.
On this page
The engine lives on the device
On Guard installs a small engine on the device itself. On a Mac or a Windows PC it reads the screen. On Android it reads the screen through the Accessibility permission you grant, along with the browser address bar and the name of the app in use. It samples frames as you scroll and browse, and a vision model on the device classifies each one in memory, then discards it. No frame is written to disk or uploaded to be looked at somewhere else. There is no cloud vision service in the loop.
What stays on your device, and what leaves it
Your device
What is on your screen
Frames are sampled as you scroll and browse.
On Guard engine
Classifies each frame locally. Nothing is uploaded.
A verdict
Clean, suggestive, or explicit. The frame is discarded.
Crosses the boundary
- A liveness heartbeat
- A detection event, with no content
- Your account, settings, and billing
Never crosses it
- Screenshots or frames
- URLs or browsing history
- What you were looking at
That is the meaning of on-device. It is not a marketing phrase. It is where the work happens, and it is why the privacy claims further down are structural rather than promises.
Three verdicts
- Clean. Nothing happens. This is almost everything.
- Suggestive. The deliberately soft tier: sexualized but not explicit. It earns a short nudge on your screen, and nothing else.
- Explicit. A gate goes up before you go further, you are offered something better to look at, and your ally gets one check-in text.
The tiers are not a moral ranking. They are three different actions, chosen so that the loud one fires only when the engine is confident, and the quiet one can afford to be more sensitive without bothering anyone but you.
What happens on a detection The gate, the redirect, and the text, step by step.
What leaves the device, exactly
The honest line
Some things must cross to our servers, or accountability could not work. Here is the exact list, so you never have to take "on-device" on faith.
Crosses to our servers
- A liveness heartbeat: when each device was last seen, and whether its engine and permissions are on. This is how "Gone quiet" is known.
- A detection event when a gate fires: that it happened, with no content attached. This is what sends the ally text.
- Your account, ally links, settings backup, and subscription state.
- Crash reports, with no screen content.
Never crosses
- Screenshots, frames, or pixels. The engine discards each frame after classifying it.
- URLs, page titles, or browsing history.
- What a detection was about: no site, app, image, or category, on any channel.
- Anything from the suggestive tier. Nudges are on-device only.
The backend cannot unblock anything, and detection does not depend on being online or paid up. Offline, the gate and the nudge still work on your screen. The check-in text needs a connection; if the device is offline at that moment, the text may not go out.
iPhone is different
iOS does not let an app read the screen the way a Mac, a PC, or Android does, so On Guard runs no detection engine of its own on iPhone or iPad. There, On Guard works through Apple's Screen Time controls: the Safari adult-content filter, and blocking or nudging the apps you choose. The ally check-in, your account, and your settings work the same on every platform. It is the detection layer that differs, because the operating systems differ.
Install On Guard on iPhone or iPad What the iPhone setup actually turns on.
Questions
Does On Guard upload my screen to check it?
No. Frames are classified on the device and discarded. There is no image upload and no image store to leak.
Can you see what I was looking at when a gate fired?
No. The event that reaches our servers says that a gate fired on a device. It carries no site, app, image, or category, and there is no field in the system to put one in.
Does the engine work offline?
Yes. Detection, the gate, and the nudge run on the device without a connection. The check-in text needs a connection, so a gate that fires while the device is offline may not text your ally.
Which platforms have the engine?
Mac, Windows, and Android. On iPhone and iPad On Guard uses Apple's Screen Time controls instead of a detection engine of its own.
Checked August 18, 2026 against the shipping app. If something here no longer matches what you see, email team@stayonguard.app.
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