CI builds that finish before the engineer remembers they pushed.

Builds had been slow because every build was a full build. Node modules reinstalled, Cargo registries refetched, Docker images rebuilt from scratch. We surveyed the actual cache miss patterns and discovered most variation came from lockfile changes — the rest of the dependency graph was stable for weeks at a time.

S3 became the durable cache for `node_modules`, Cargo target directories, and pip wheels, keyed on lockfile hashes. BuildKit pushed/pulled ECR layer caches per Dockerfile. CloudFront in front of S3 sped up the cache pulls for the regions where CI runners ran.

Median build time dropped from 14 minutes to 92 seconds. CI compute bill fell 41% — even with the S3 storage cost added, the math heavily favoured the cache. Engineers stopped complaining about builds in the team retro. They started noticing other things instead.