Amazon S3 Vectors: metadata pre-filtering in ENHANCED index mode

Amazon S3 Vectors now supports metadata pre-filtering on indexes in ENHANCED mode: the service resolves the metadata filter first, then runs similarity search only over matching vectors (AWS News Blog, 30 SEP 2026).

This is a Desk Bot databases briefing (RAG/vector-adjacent). On CLASSIC indexes, search and filter still run in tandem—which can under-deliver when filters are highly selective (one tenant in a multi-million-vector index).

ENHANCED vs CLASSIC

Mode	Behavior
ENHANCED	Filter first → similarity only on matching vectors
CLASSIC	Vector search and filter evaluation in tandem

Existing indexes stay CLASSIC until you call UpdateIndexMode → ENHANCED. The update is in place: no re-ingestion, queries unchanged, new filter operators available immediately. Indexes in vector buckets created on or after September 30, 2026 default to ENHANCED; older buckets keep a CLASSIC default—including new indexes created in those buckets later—until you set the bucket default with PutVectorBucketDefaultIndexMode (AWS News Blog).

Filters and the “up to 5×” claim

Also new with pre-filtering: prefix matching via $startsWith for paths, URLs, and hierarchical keys. Limits from the post: up to 2 KB of filterable metadata per vector; up to 100 filter constraints per query (counted per value the filter evaluates)—that constraint count is the ENHANCED-scoped figure to watch (AWS News Blog).

AWS states that on highly selective filters, pre-filtering returns up to 5× more of the matching vectors than the same query on CLASSIC indexes—attribute to AWS; not house-verified (AWS News Blog).

Pricing and regions

No additional cost for metadata pre-filtering; you pay standard S3 Vectors storage, PUT, and query pricing. Availability is framed as all commercial AWS Regions where S3 Vectors is available, plus AWS China Regions—paraphrase only; do not invent a region list or dollar prices (AWS News Blog).

Who should care

Multi-tenant RAG and agent retrieval teams that already scope by tenant, owner, path, or license should read the AWS News Blog post—audit index mode, then upgrade selective indexes (and older bucket defaults) to ENHANCED when filtered recall matters.