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xcodebuild prints this under Testing failed: when it starts a UI test runner on an iPhone or a simulator. It then exits with code 65. Words in angle brackets are what Xcode fills in:
<Runner> is the test app Xcode installed, such as WebDriverAgentRunner-Runner for WebDriverAgent. The last line can also read ** TEST FAILED **. Xcode 8 printed it as Error Domain=IDETestOperationsObserverErrorDomain Code=6 "Early unexpected exit, operation never finished bootstrapping - no restart will be attempted", or with Code=5. Mobster starts the helper app on your iPhone (WebDriverAgent) with xcodebuild test-without-building, so this line can appear in the helper’s log, logs/wda-runner.log in the data folder. Mobster restarts the helper by itself. If it stops with an error three times, Mobster for Mac shows The helper app didn't start. When Mobster recognizes the cause in the log, it shows that cause’s fix. Otherwise it shows The helper app keeps stopping. Unlock your iPhone and check Developer Mode. On a simulator, mobster verify ends couldnt_run with WebDriverAgent stopped before it was ready: and the log’s last error: line.

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Why it happens

Xcode installs the runner, launches it, and waits for it to connect back to the Mac before the first test. This handshake is called bootstrapping. The runner quit or crashed before it finished, so Xcode stopped and won’t try again. Read the Underlying Error in parentheses for the cause: The first four rows are quoted from Xcode 26.4’s XCTHarness framework. The next two come from iOS, and Xcode passes them on.

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Fix

  1. Read the Underlying Error and find it in the table above. If it links to a page, follow that page.
  2. If the message names a log file (please attach the log file at …/Session-….log), open it.
  3. Watch the phone’s log while the runner starts. Open Console on the Mac, select the iPhone, choose Start Streaming and search for -Runner.
  4. Compare the iPhone’s iOS version with your Xcode’s iOS SDK version:
    With Xcode 26.4, the first command prints 26.4. The second prints the phone’s version, such as • osVersionNumber: 18.3.2. A phone newer than the SDK is the mismatch in the 6 Oct 2026 report above.
  5. Restart the iPhone. Unlock it and keep it connected.
  6. Delete the runner from the phone: touch and hold its icon, then Remove App › Delete App. Build it again and start it again.

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Check it worked

The runner’s log shows Test Suite 'All tests' started, and xcodebuild keeps running instead of exiting. For WebDriverAgent, curl http://127.0.0.1:8100/status answers with "ready" set to true, and mobster doctor prints a ✓ beside Phone connection. Checked 5 Nov 2026 against Mobster 0.4.0. The commands were checked with Xcode 26.4. Back to Troubleshooting.