with bubblewrap it's better to pull a rootfs from dockerhub (eg. debian:unstable) then bootstrap it into a fully fledged distro rootfs living in its own folder. install the AI agents right into it, then create launch scripts that invoke bwrap with the distro rootfs (readonly) and a custom read-write /home/user and run whatever you want inside it - it will not see anything important outside the directory you give it. you can also run multiple agents each invisible to the others.
for bonus points you can uplift the bwrap container into an actual sandbox by invoking gvisor (`runsc ... do ...`) from inside it, or a virtual machine monitor like muvm. I'm really fond of this pattern because you can trust bwrap to set up the environment, then you just need a sandbox tool to lock it down.
bwrap by itself will probably be sufficient against most adversaries as assuming proper config it would require committing to using a linux kernel 0day to escalate privs.
Thanks for the suggestions. I've used debootstrap to build a Debian rootfs for bwrap before, but my threat model is simpler: nothing sensitive lives outside $HOME on my machine. So I just ro-bind the system dirs I need and give the sandbox a tmpfs home (one-shot apps) or a persistent fake home (stateful apps, under ~/.var/app/<appname>). This is good enough for my case.
The gvisor layering looks promising though. I'll take a look and see if it would be useful.