Authentication is not isolation: a request can be fully authenticated to tenant A and still touch tenant B through a shared index, a semantic cache, or a fine-tuned model. The strong answer names every shared surface and how you stamp tenant identity through all of them.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
Authentication is not isolation: a request can be fully authenticated to tenant A and still touch tenant B through a shared index, a semantic cache, or a fine-tuned model. The strong answer names every shared surface and how you stamp tenant identity through all of them.
The trap is answering with database row-level security and stopping, because the AI-specific leak surfaces (shared vector index, semantic cache, cross-tenant fine-tune, logs and eval traffic) are where real cross-tenant incidents happen. The senior move is making tenant ID a non-optional dimension on every store and every cache key, then proving isolation with a continuous canary test rather than asserting it. Watch for the candidate who fine-tunes one model on pooled tenant data and never realizes the weights now have no tenant boundary at all.
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