By default, Marvin syncs data to the cloud and to all your devices.  But if you want to keep all your tasks on one device, you can disable cloud sync in Marvin's settings.  Check out our Privacy Policy for more info on how we protect your data, but of course the best way to maximize privacy is to keep all your data on your device!

First open settings in the top-right menu:

Next click the "On" toggle on the right to switch it to off:

You can re-enable sync at any time, and all of your tasks will sync up among your devices!  You may experience some strangeness with recurring tasks and day notes (and similar) since Marvin will have to choose (seemingly at random) which version to keep.
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If you want to keep separate tasks on each of your devices and not sync on any of them, you'll have to disable cloud sync individually on each device.