Tagging a resource and activating a cost allocation tag are two different operations, and AWS never tells you when you have only done the first. A practical guide to the activation gap, what backfill can and cannot repair, the five places coverage leaks, and a retrofit order that actually converges.
A backfill that exits zero can still be missing a week of data, and nothing will tell you. This is a practical guide to the failure families behind silent gaps: pagination drift under a mutating source, retries that duplicate pages, prefix layouts designed for writes instead of reads, the seam where backfill meets live ingest, and the storage class rules that make mistakes expensive. Includes deterministic key derivation, S3 conditional writes, Athena partition projection, and a per-window manifest pattern that turns completeness into something you can query.