Detox synchronization timeouts happen when the test runner waits for the app to become idle and the app keeps reporting pending work. Debugging the timeout starts with the busy-resource log, because that output names whether Detox is waiting on animation, layout, timers, network requests, or another app-side signal.

The --debug-synchronization option controls how long an action or expectation can run before Detox asks the app why it is still busy. A lower value, such as 5000, makes the busy-resource block appear earlier during a focused rerun, while --loglevel trace and failure-only artifacts keep enough context for the app log and screenshot review.

Treat the first repeated busy resource as the investigation target instead of raising the test timeout immediately. A spinner animation, a missing error state after a server failure, a recurring timer, or a long-polling request each needs a different fix, and the retest should show that the same spec passes without the busy-resource block repeating.

Related: How to debug flaky Detox tests
Related: How to disable animations for Detox
Related: How to wait for UI elements in Detox

Steps to debug Detox synchronization timeouts:

  1. Re-run the failing spec with synchronization diagnostics enabled. 
    $ npx detox test --configuration ios.sim.debug e2e/checkout.e2e.js --testNamePattern "submits order" --debug-synchronization 5000 --loglevel trace --record-logs failing --take-screenshots failing --artifacts-location artifacts/sync-timeout/
FAIL e2e/checkout.e2e.js
  Checkout
    x submits order (12034 ms)

detox[71248] i The app is busy with the following tasks:
- UI elements are busy:
  - View animations pending: 1
  - Layers pending animations: 2

DetoxRuntimeError: Test Failed: No elements found for matcher: by.id("orderConfirmation")
Artifacts saved to artifacts/sync-timeout/

    Use the real configuration and spec name from the failing run. Detox prints synchronization status by default after a longer wait, but --debug-synchronization 5000 makes a focused diagnostic rerun easier to read.

  2. Locate the repeated busy-resource line in the saved Detox log. 
    $ grep -n "View animations pending" artifacts/sync-timeout/detox.log
artifacts/sync-timeout/detox.log:96:  - View animations pending: 1
artifacts/sync-timeout/detox.log:97:  - Layers pending animations: 2

    For network-related timeouts, look for repeated URL lines instead of animation lines. A request to a long-polling endpoint, websocket fallback, or unreachable server should be fixed or excluded from synchronization rather than hidden with a larger timeout.

  3. Match the busy-resource signal to the failure screenshot or device log. 
    $ grep -n "CheckoutService" artifacts/sync-timeout/device.log
artifacts/sync-timeout/device.log:1542: CheckoutService request failed with 504 Gateway Timeout
artifacts/sync-timeout/device.log:1543: CheckoutScreen left loading spinner visible

    If the Detox log only reports UI animation while the device log shows a server, parser, or app error, fix the app error path so the loader leaves the screen.
    Tool: Application Log Pattern Analyzer

  4. Isolate the busy animation with a test launch argument when the app already supports one.
    e2e/checkout.e2e.js
    beforeEach(async () => {
  await device.launchApp({
    newInstance: true,
    launchArgs: { disableAnimations: 'true' },
  });
});
 
it('submits order', async () => {
  await element(by.id('submitOrder')).tap();
  await expect(element(by.id('orderConfirmation'))).toBeVisible();
});

    Use this branch only for animations that do not change the feature being tested. If the app does not already support a no-animation launch argument, add that app-side switch first.
    Related: How to disable animations for Detox

  5. Use manual synchronization only for a screen that cannot be made idle. 
    await device.disableSynchronization();
 
await waitFor(element(by.id('orderConfirmation')))
  .toBeVisible()
  .withTimeout(8000);
 
await device.enableSynchronization();

    Call device.enableSynchronization() only after the app returns to a screen that can become idle. Re-enabling synchronization while the same endless animation, timer, or request is still active can block until Detox times out again.

  6. Re-run the same spec after the targeted fix or isolation. 
    $ npx detox test --configuration ios.sim.debug e2e/checkout.e2e.js --testNamePattern "submits order" --debug-synchronization 5000
PASS e2e/checkout.e2e.js
  Checkout
    PASS submits order (6421 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total

    A passing rerun with no repeated busy-resource block proves the selected timeout signature was removed for this spec. Keep the diagnostic artifacts until the fix is reviewed, then remove the temporary artifact folder from routine runs.

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