exe.dev is great, but the VMs are not really "apps". They are durable computers / VMs.

An example of a "sandboxed agent app", would be: give the app all your past emails. An agent scans them and finds sales emails you need to follow up on. It shows you the suggested follow ups in a UI, and you approve/reject them. Then, it mass sends the approved emails and emits an update to your CRM with the changes.

The sandbox is deleted when the app runs. It's ephemeral for the lifecycle of the app. And you can re-run the same app repeatedly with new inputs, but it gets the same clean starting slate.