TL;DR: app-layer filtering is a single point of failure. Push tenant isolation into Postgres with RLS — and watch out for the security definer trap in your vector-search function.
Your WHERE clause is not a security boundary
My app is an AI wellness coach for parents. Every user's data is about the most private thing they have: how they're actually coping. Their check-ins, their bad nights, the things they'd never say out loud. The whole product runs on retrieval — when someone talks to the coach, the system pulls their relevant history out of a vector store and grounds the response in it.
Which means the single scariest bug I can imagine isn't a crash. It's user A asking a question and the retrieval quietly returning a snippet of user B's private history. No error. No stack trace. Just one person's worst night surfacing in another person's conversation.
In a multi-tenant app, that bug is one forgotten line of code away at all times. Here's how I make sure it can't happen — and the part of it that no tutorial warns you about.
