Cloud file sync signs uploads with your device and says when Anthropic has switched it off

What

With --cloud, Claude Code can keep a folder on your machine in sync with a cloud session. That sync has been hardened.

• Device signing: file uploads (put, upload-begin and upload-commit) are now signed with the key of the device the session is bound to, and carry device_attestation and device_attestation_issued_at. Attach and headless attach pass the bound device's id and signer through.
• No usable device key: a folder session with no bound device id stops file sync with reason device_key_unavailable, and an unsigned result maps to the same cause. A separate message covers a machine that cannot sign as the bound device.
• Off switch: when the server refuses with the reason "synced_file_lane_denied" (HTTP 400), the cause is now switched_off_by_anthropic instead of the general "sync is not available for this session". You see "file sync was switched off on Anthropic's side for now; the cloud session keeps whatever files it has - re-open the session later to try again", or, at attach, "File sync was not started: it is switched off on Anthropic's side for now. Attach the folder again later to retry." This shows as a warning, and the paused or refused status elsewhere in the sync view reflects it.
• The sync worker's device-status, failure-tracking and restore logic was rewritten. It stands down after 3 reads without a status and adds held-turn and back-online states.
• The attach path checks tengu_violin_soundpost, with a fallback of off. No reading of it has been taken under this release.

Why

Cloud file writes are now tied to the machine the session is bound to, so on a machine without a usable device key sync stands down instead of uploading. When sync stops on Anthropic's side, you can tell that apart from a problem on your own machine and know a later retry may work.