How do I prove that what my agent remembers belongs only to me?

AI agents deployed to thousands of users share underlying infrastructure: vector stores, semantic caches, and retrieval indexes. When tenancy boundaries are not enforced correctly, facts from one user's session appear as retrieved context in another user's response. There is no error or alert, just a subtly wrong answer carrying someone else's private data. A 2026 benchmark paper on cross-session threats in LLM agents explicitly models this failure class because no standard architectural primitive reliably closes it across all retrieval and caching layers simultaneously.