Failed to get matching snapshot: Error getting main window kAXErrorServerNotFound

An XCUITest step fails with this line. Some Xcode versions write Failed to get matching snapshots:. Through WebDriverAgent and Appium, the same failure arrives as a stale element reference on a page-source or window-size request:

The previously found element "Application 'com.example.app'" is not present in the current view anymore. Make sure the application UI has the expected state. Original error: Error getting main window kAXErrorServerNotFound

Mobster reads screens through WebDriverAgent, so a read on your iPhone can hit this. On iOS 26.0.1 (23 Sep 2026), with Safari in front, WebDriverAgent took an invisible system overlay, com.apple.siri.IntelligentLight, for the app in front, and every screen read failed after about 8 seconds. Mobster sets the app it opened as WebDriverAgent’s active app and retries the read once. In this case, the retry took 0.58 s.

Why it happens

XCTest reads a screen by asking the app’s accessibility server for a snapshot, starting from the app’s main window. kAXErrorServerNotFound means no server answered for that app. Reported causes include:

What you see on the phone	The cause
The lock screen	The phone is locked. Accessibility reads don’t work on a locked iPhone
The Home Screen, another app, or nothing	The app crashed, quit, or opened in the background. XCTest is asking an app that isn’t in front
The app, under Siri, Control Center or a banner	WebDriverAgent picks the app in front by checking what sits at one point of the screen, `activeAppDetectionPoint`. Its default is `MIN(w, h) * 0.2, MIN(w, h) * 0.2`, near the top left, where banners and overlays sit
The app, and every other app fails too	In one report (Dec 2023), an Appium contributor found `testmanagerd` unable to write `com.apple.Accessibility` settings, and only a restart of the phone cleared it
The app, and only your app fails	Something in the app breaks its accessibility. In one report (Jan 2024), a React Native library that called `class_getClassMethod` caused it, and `class_copyMethodList` fixed it

Fix

Unlock the iPhone and look at it. Close Siri, Control Center and any banner. If the app isn’t on screen, open it again.
Bring the app to the front before you read it. In an XCUITest, call app.activate(), or app.launch(), before the query. Through Appium, run mobile: activateApp with {"bundleId": "com.example.app"}. Through WebDriverAgent itself:
curl -s -X POST http://127.0.0.1:8100/session/<session id>/wda/apps/activate \ -H 'Content-Type: application/json' -d '{"bundleId": "com.example.app"}'
Point WebDriverAgent at the app. Set defaultActiveApplication to its bundle ID, and activeAppDetectionPoint to a point inside the app’s content, as "x,y" in points. Mobster tries 160,320 first:
curl -s -X POST http://127.0.0.1:8100/session/<session id>/appium/settings \ -H 'Content-Type: application/json' \ -d '{"settings": {"defaultActiveApplication": "com.example.app", "activeAppDetectionPoint": "160,320"}}'
Through Appium, pass the same two settings to updateSettings. Mobster sets the detection point on every session, and names each app it opens as the active one.
Close the app in the app switcher and open it again. A restart of WebDriverAgent doesn’t clear a stuck accessibility server, and closing its app does.
If every app fails, Settings included, restart the iPhone.

Check it worked

curl -s http://127.0.0.1:8100/source returns JSON whose value is the screen’s XML, starting with the app’s XCUIElementTypeApplication. An XCUITest query on the same screen passes. Checked 5 Nov 2026 against Mobster 0.4.0. The commands were checked with Xcode 26.4. Back to Troubleshooting.

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Failed to get matching snapshot: Error getting main window kAXErrorServerNotFound

Why it happens

Fix

Check it worked

​Why it happens

​Fix

​Check it worked

Why it happens

Fix

Check it worked