Every assistant that answers from SharePoint works from an index: a copy of your documents, split into passages. What matters for access control is whether that index also stores who may see each passage.

Design 1: the permission list lives in the index

Azure AI Search has a SharePoint indexer with permission metadata ingestion, a preview feature of Azure AI Search, not of Microsoft Copilot. During ingestion the indexer "computes the effective ACL for each file" and "stores the permissions as metadata on each indexed document". At query time the engine matches the asking person against that stored list.

That list is a snapshot.…