Skip to main content
You see this: Mobster blocks a live run with WebDriverAgent is not answering at http://127.0.0.1:8100, and mobster doctor prints ✗ WebDriverAgent not answering at http://127.0.0.1:8100.

​
Why it happens

WebDriverAgent is the helper Mobster installs on the iPhone. Port 8100 on this Mac is the USB relay to it. Either the helper stopped, or the relay did.

​
Fix

  1. Open Mobster for Mac, or run mobster serve --manage-device. Either starts both the helper and the relay, and restarts them if they stop.
  2. Unlock the iPhone and keep it connected. If the helper keeps stopping, check Developer Mode and Enable UI Automation.
  3. For a simulator, run mobster sim prepare --device NAME.

​
Check it worked

mobster doctor prints ✓ WebDriverAgent. Checked 26 Oct 2026 against Mobster 0.4.0. Back to Troubleshooting.