Yeah that's an active research topic for teams of PhDs, including some of Google's brightest. And the current approach even with added barriers may just be fundamentally untrustable. Read the links from my earlier comment for background.
If the shim does use an LLM but no uncontrolled data is allowed in, this is not a problem.
You literally have to fully prevent all outside input, or you have to prevent all exfiltration routes including web page reading (even the choice of links to follow is an exfiltration mechanism). At that point, what's left? What do you think will be on your allowlist?
I seriously doubt the early adopters of these software bundles use their assistants like with such restraint (https://xcancel.com/summeryue0/status/2025774069124399363), and that idealized image of these access control shims is not realistic.
No, literally no one understands how to solve this. The only option that actually works is to isolate it to a degree that removes the "clawness" from it, and that's the opposite of what people are doing with these things.
Specifically, you cannot guard an LLM with another LLM.
The only thing I've seen with any realism to it is the variables, capabilities and taint tracking in CaMeL, but again that limits what the system can do and requires elaborate configuration. And you can't trust a tainted LLM to configure itself.
https://simonwillison.net/2025/Jun/16/the-lethal-trifecta/
https://simonwillison.net/2025/Jun/13/prompt-injection-desig...
https://simonwillison.net/2025/Apr/11/camel/