> Adding tags like "Warning: Untrusted content" can help
It cannot. This is the security equivalent of telling it to not make mistakes.
> Restrict downstream tool usage and permissions for each agentic use case
Reasonable, but you have to actually do this and not screw it up.
> Harden the system according to state of the art security
"Draw the rest of the owl"
You're better off treating the system as fundamentally unsecurable, because it is. The only real solution is to never give it untrusted data or access to anything you care about. Which yes, makes it pretty useless.