How to fix error 4099 when syncing my music library to iCloud?
maddyvan
User level: Level 1 (8 points)
User level: Level 1 (8 points)syncing music library to iCloud
Posted on Oct 11, 2026 7:25 AM
1 reply
sooperfresh
User level: Level 2 (377 points)
User level: Level 2 (377 points)Oct 12, 2026 9:20 AM in response to maddyvan
4099 means Music lost its connection to the background service doing the upload, so it's not your songs. Quit Music, restart the Mac, then switch Sync Library off and back on in Music > Settings > General and leave Music open while it runs; if it still fails, Account > Sign Out and sign back in. If you mostly want the library playable on this Mac without waiting on iCloud, Katalog (musickatalog.com) runs alongside Music and plays the files straight from disk.
Originally posted at discussions.apple.com/thread/256374215