Limits of Homebrew's Download Cache on Stale Formula Bottles

Answer

Homebrew should prune its download cache using a combined age‑and‑size policy: remove bottles older than 30 days or when the total cache exceeds 10 GB.

Confirmed facts

• Homebrew stores bottles and source tarballs in ~/Library/Caches/Homebrew.
• The built‑in brew cleanup removes outdated files that are not needed for any installed version, but it does not prune based on age or size unless run manually.
• Users can inspect the cache with brew cache and clear it entirely via brew cache --clear.

Likely explanation

Stale bottles remain in the cache until a cleanup operation is performed, causing the cache to grow linearly with the number of distinct formula versions installed. Without automatic pruning, users must run brew cleanup periodically to avoid unnecessary disk usage and to guarantee that brew install fetches the latest version.

Steps to evaluate the policy on your system

1. Check current cache size and age:

   du -sh ~/Library/Caches/Homebrew
   find ~/Library/Caches/Homebrew -type f -mtime +30 -print | wc -l
   

2. Run the existing cleanup to see what would be removed:

   brew cleanup -n   # dry‑run
   

3. Simulate the proposed policy by moving old bottles to a temporary directory and verifying that brew install fetches fresh bottles when the simulated policy would have pruned them.

Communication of automatic pruning

If Homebrew enables automatic pruning, it should emit a brief notice after the operation, e.g.

==> Auto‑pruned 1.2 GB of stale bottles (older than 30 days or cache >10 GB)

and document the behavior in brew help and the FAQ, noting that users can disable the feature with HOMEBREW_NO_AUTO_CACHE_PRUNE=1.