When macOS Settings shows System Data at 50, 80, even 150 GB, most guides tell you to "reinstall macOS" or buy a cleaner app. Neither is necessary. System Data is just a label for everything that is not apps, photos, or media — and most of it is deletable cache once you know where to look.

Step 1: map the bucket yourself

Apple's number comes from a rough calculation; measure the real thing instead:

sudo du -xsh ~/Library /Library /private/var/folders 2>/dev/null | sort -h

Then drill into the biggest one:

du -xsh ~/Library/* 2>/dev/null | sort -h | tail -15

On a typical AI-developer Mac this surfaces the same repeat offenders:

Directory	Typical size	Verdict
~/Library/Caches	2-30 GB	Delete contents — apps rebuild them
~/Library/Application Support	5-60 GB	Selective: AI tools live here, see guides below
~/Library/Containers	1-15 GB	Sandboxed app data; check before deleting
~/Library/Developer	3-40 GB	Xcode simulators & DerivedData — safe after projects close
/private/var/folders	1-10 GB	System temp; clears on reboot, do not touch manually

Step 2: delete in the safe order

1. Caches first — quit apps, then clear ~/Library/Caches/*. Zero risk, instant win.
2. Developer debris — rm -rf ~/Library/Developer/Xcode/DerivedData/* and old simulators via xcrun simctl delete unavailable.
3. AI-tool data — this is where Claude Code transcripts, Cursor session data and Ollama models hide. Follow the per-tool steps in our cleanup hub instead of blind-deleting.
4. Time Machine local snapshots — tmutil listlocalsnapshots / then tmutil deletelocalsnapshots <date>; snapshots alone can hold tens of GB.

Step 3: only then reach for a tool

After manual triage, a visualizer shows what is left. For a one-sitting survey we use DaisyDisk (macOS) — its sunburst map makes the remaining 20 GB legible in minutes. Maintenance suites such as MacCleaner PRO (Nektony) bundle the same discovery with scheduled cleanup, which suits people who do not want to think about it again.

What NOT to delete

Stay out of /System, /usr, /private/var/db and anything SIP protects — deleting there breaks the OS, and the space comes back after the next update anyway. If System Data climbs back within days, the cause is almost always one app (Docker and Xcode are the usual suspects); measure again with du and you will find it.