In my opinion the OS should broker the communication between harnesses (which is part of Siri), LLMs and app functionality. It can take into account the permissions, the user could configure which LLM to use at an OS system settings level and complex workflows can integrate permissions like FaceID only once as the harness then interacts with APIs exposed by applications (arguably already there with app intents).
Self promotion blog article:
https://braw.dev/blog/2026-04-06-mobile-os-as-agentic-broker...