Directory sync is Claude Code's syncing of a folder with a cloud session. Anchoring is how it keeps a fixed hold on a folder while placing files, rather than going by the folder's name each time. On some machines anchoring cannot be used. Examples are a missing native interface, a missing Linux process filesystem, an unstable folder identity, or a platform that is not supported.
On those machines, directory sync used to fall back to placing files by name, limited to the root folder. It now reports anchoring as unavailable and gives up instead.
Placing files by name now happens only when you set CLAUDE_CODE_DIR_SYNC_DISABLE_ANCHORING, and in that mode it is no longer limited to the root folder. The related error message now names that variable.
Placing files by name is less safe than anchoring, so refusing is the safer default. The cost is that on machines without anchoring support, files from the cloud may not be downloaded.
It is not clear what, if anything, is shown to you when sync gives up because anchoring is unavailable.