The actual switch
HKLM\SYSTEM\CurrentControlSet\Control\FileSystem → LongPathsEnabled = 1 (machine, admin) and the consuming app must declare longPathAware in its manifest. Both halves required; half-done ≠ half-fixed.
Why per-account tools decline it
- Admin scope needed for the registry half — one write violates the per-user promise.
- The manifest half is compiled in per app — a tool can't toggle that for other apps.
- Bad tweakes toggle the registry anyway; some apps then write long paths on app-tiers that weren't declared ready. Support pain follows.
The bounded, safe plays
- Find: scan bounded like profile cleanup and report paths ≥ 240 chars so the user knows before Sync tools stumble;
- Shape: cleanup deletes own-account files under deep temp trees via Unicode-safe APIs — legal regardless of the machine switch;
- Defer: actual LongPathsEnabled decisions belong to admin baselines; a good per-user tool documents the ceiling rather than exceeding it.
ProofTune ProjectEngineering notes — every claim here names the bytes a real tool touches. Verify first, install second.