Live Video Preview
Real-time screen mirroring for physical iOS devices over USB. See exactly what’s on the device screen in a native macOS window — useful for watching what happens when your agent interacts with the device, or just keeping an eye on things.
Tell your agent:
“Show me the device screen” “Start a live preview of my iPhone”
A native macOS window opens showing the device’s screen in real time. The preview updates at whatever frame rate the device provides (typically 60fps).
To stop:
“Close the preview” or just close the window.
What You Need to Know
Section titled “What You Need to Know”USB Only
Section titled “USB Only”CoreMediaIO screen capture requires a wired USB connection. Lightning and USB-C both work. Wi-Fi does not. If you’re working with a physical device wirelessly, preview won’t be available.
First-Launch Permission
Section titled “First-Launch Permission”The first time the preview app runs, macOS will ask for screen recording permission. Grant it, or the capture will fail silently. This is a standard macOS privacy prompt — you’ll find it in System Settings > Privacy & Security > Screen Recording.
Device Trust
Section titled “Device Trust”Your iPhone must trust the Mac (“Trust This Computer?” dialog). If you haven’t accepted this, the device won’t appear for preview.
”Headphones or Other Device” Dialog
Section titled “”Headphones or Other Device” Dialog”On iOS 18 and later, when the preview capture session starts, your iPhone may show a dialog asking whether you’re connecting headphones or another device. Tap “Other” (or whatever the non-headphones option is) to dismiss it. This is iOS reacting to the CoreMediaIO capture session — it only appears once per connection.
Multi-Device
Section titled “Multi-Device”You can preview multiple devices simultaneously — each gets its own window. Just ask your agent to start preview on each device.
Behind the scenes, Quern staggers the capture sessions by 1 second between devices to work around a CoreMediaIO race condition. This is automatic.
Manual Control
Section titled “Manual Control”Once a preview window is open, the Devices menu in the menu bar shows all connected USB devices. Click to toggle preview on/off. Devices with active previews are marked with a checkmark.
Orientation
Section titled “Orientation”CoreMediaIO always captures in the device’s native orientation (portrait). When your app rotates to landscape, the captured frames show the rotated content within the portrait-oriented buffer — it’ll look sideways.
The preview window doesn’t auto-rotate currently. What you see is the raw frame buffer from CoreMediaIO.
Limitations
Section titled “Limitations”- USB only. Wi-Fi connections don’t support CoreMediaIO screen capture.
- View only. You can’t interact with the device by clicking on the preview window. (This is planned — mapping clicks to taps via WDA.)
- No audio. Video only.
- macOS only. CoreMediaIO is an Apple framework.
- No brightness control. But the preview always shows full brightness via CoreMediaIO regardless of the device’s actual screen brightness. So for dark-room scenarios: face the device down, the preview still works fine.