> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mobster.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# WebDriverAgent is not answering at http://127.0.0.1:8100

> Mobster's helper isn't running on your iPhone, or the USB relay to it isn't. Open Mobster for Mac or run mobster serve --manage-device.

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](https://docs.mobster.dev/troubleshooting/developer-mode-disabled.md) and [Enable UI Automation](https://docs.mobster.dev/troubleshooting/timed-out-while-enabling-automation-mode.md).
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](https://docs.mobster.dev/troubleshooting.md#iphone).


This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.