xcodebuild prints this (com.apple.dt.deviceprep code -3) when the phone is locked while Mobster starts its helper. Mobster shows Unlock your iPhone. Mobster's helper starts as soon as it is unlocked.
Why it happens
iOS won’t start UI automation on a locked phone. Mobster never enters your passcode and never unlocks the phone.Fix
- Unlock the iPhone and keep it unlocked.
- Wait. Mobster builds again by itself once the phone reads unlocked, and the helper starts about 2 seconds later.
- For a phone that locks during unattended tasks, lengthen Settings › Display & Brightness › Auto-Lock.
Check it worked
mobster doctor prints ✓ WebDriverAgent. See also the helper waits for unlock.
Checked 26 Oct 2026 against Mobster 0.4.0. Back to Troubleshooting.