WSL disk reclaim

WSL2 keeps each distro in an ext4 VHDX on the Windows drive. A non-sparse VHDX only grows: space freed inside Linux stays allocated on Windows until a manual compaction. agentihooks gc frees space inside WSL; making the VHDX sparse is what returns that space to Windows.

Check

From WSL:

findmnt -no OPTIONS /                     # 'discard' must be listed
powershell.exe -NoProfile -Command 'Get-ChildItem HKCU:\Software\Microsoft\Windows\CurrentVersion\Lxss | ForEach-Object { $p = Get-ItemProperty $_.PSPath; "{0}|{1}" -f $p.DistributionName, $p.BasePath }'

From Windows (PowerShell), with the distro’s BasePath:

fsutil sparse queryflag <BasePath>\ext4.vhdx

This file is NOT set as sparse means freed space never returns to Windows.

Make the VHDX sparse (one time)

Run from Windows PowerShell. Every WSL session stops in step 1.

wsl --shutdown
wsl --export <Distro> D:\backup\<Distro>.tar
wsl --manage <Distro> --set-sparse true

Some WSL builds ask for --allow-unsafe on the last command; the export in step 2 is the rollback (wsl --import). Then start the distro and release the space that is already free:

sudo fstrim -v /

Verify: fsutil sparse queryflag <BasePath>\ext4.vhdx reports the file as sparse, and deleting a few GB inside WSL shrinks the VHDX size shown in Explorer.

Why /tmp loses files on WSL

/usr/lib/tmpfiles.d/tmp.conf has D /tmp, so systemd empties /tmp on every VM boot: a crash, wsl --shutdown, a Windows reboot, an idle VM shutdown, or a WSL update. Keep working files under ~/scratchpad/<repo>/<task>/ (agentihooks scratch new), never in /tmp.