If you lose a device that has a passkey, you can still sign in with your password. Once signed in, go to Settings > Login & Security to delete the passkey associated with the lost device and add a new one on your current device. If your passkey is backed up through your device's cloud account (e.g., iCloud Keychain or Google Password Manager), it may also be available on your other devices automatically.
What happens if I lose my device?
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