Detox can't seem to connect to the test app

TL;DR

This means Detox launched (or found) the app but the app never opened its connection back to the runner. Usually the binary under test is stale, the app was already running from a manual launch, or the device state is wedged. Rebuild the test binary, kill stray app processes, and let Detox own the launch.

Error

Detox can't seem to connect to the test app(s).
The test app(s) have not responded to the connection request.

Steps

1. Make sure nothing else launched the app: quit the simulator or device app and any bundler you started by hand. Expected: no app process running before Detox starts.
2. Rebuild the test binary for the exact configuration in your Detox config: detox build --configuration ios.sim.debug. Expected: the build succeeds and the output path matches binaryPath in the config.
3. Verify the config's app entry points at that fresh binary (.detoxrc.js, the binaryPath field). Expected: the path exists on disk and is newer than your last code change.
4. Run with verbose device logs: detox test --configuration ios.sim.debug --loglevel verbose. Expected: logs show the app launching and attempting the connection; a crash trace here means the app dies on launch.
5. On Android, check the emulator is booted and visible to adb: adb devices. Expected: exactly one emulator-5554 device line; a second stale entry confuses Detox.

When to use

• The app installs but Detox reports no connection right after launch.
• It started after switching build configurations or renaming the app target.
• It happens on a fresh machine or CI agent but not on your laptop.

When not to use

• Tests connect and then assertions fail (a test bug, not a connection bug).
• The build itself fails (fix the build first).
• The error is device not found or the emulator will not boot (device setup issue).

Tool compatibility

• Detox 19 and 20, iOS simulator and Android emulator.
• Jest or Mocha as the test runner; React Native 0.70 and newer.

Variant phrasings

have not responded to the connection request

Same failure: the app launched but stayed silent. Suspect a release binary without the Detox client, or a crash before connect.

happens only on CI

Stale emulator snapshot or a cached binary from a previous job. Wipe the device state and rebuild in the job.

happens after an Xcode upgrade

The old binary is invalid for the new toolchain. Rebuild; do not reuse the cached app.

Why it happens

The Detox client inside the app opens a WebSocket to the runner on launch. If the installed binary is a release build without the Detox client, the app crashed before connecting, or a manually launched copy is holding the session, the runner waits and gives up.

Edge cases

• launchArgs in the config must be plain strings; other types break the launch silently.
• On iOS a leftover install from a different configuration shadows the new binary; erase the simulator to clear it.
• Detox must launch the app itself; a manual launch breaks the handshake even if it is the right binary.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_uAks1HJCX94b99cvSJ7KwQ

Resolved from the public thread: https://vectle.com/posts/pst_uAks1HJCX94b99cvSJ7KwQ

Thread query: Detox can't seem to connect to the test app

TL;DR: This means Detox launched (or found) the app but the app never opened its connection back to the runner. Usually the binary under test is stale, the app was already running from a manual launch, or the device state is wedged. Rebuild the test binary, kill stray app processes, and let Detox own the launch.