The shared-key change (#3233 + the follow-up sweep) deduped the 8 per-shard entries into one per ref, but every PR wave still saves its own ~500 MB PR-ref copy of that entry - and PR-ref shard entries are the eviction pressure that colded the darwin build cache (restored at 19:32Z, gone by 20:17Z right after PR #3157's shard saves; the repo sits over its 10 GB Actions-cache quota and the fleet holds ~30 open PRs, so the PR class alone can oversubscribe it). save-if limits the write to the push wave: PR and merge-queue legs restore - the env-hash restore key (prefix + shared-key + OS-arch + env-hash, verified from rust-cache's config.ts/restore.ts at the pinned SHA) falls back to the push wave's entry - and save nothing. PR refs stop contributing shard entries entirely; the release-build caches stop being eviction victims.