Skills are packaged instructions Claude can use. Skills sync is Claude Code downloading them and putting them in place on your machine, in repeated rounds. It is tied to the CLAUDE_CODE_SYNC_SKILLS environment variable, a named setting given to Claude Code when it starts.
Each round now keeps track of which skills failed to land, meaning failed to end up in place. A skill whose download failed while being unpacked, or while being written locally, is held back from later rounds. A round with such a skill now ends with a "held back" status and reports how many skills were held back. Before, failures were counted but the skill was not held back, and the round ended as either no changes or a download failure.
Failed downloads also report more detail: which step failed, the system's error code, and what was already in the spot the skill was meant to go.
A skill that cannot be put in place no longer gets retried on every round, and it no longer stops Claude Code from marking its stored copy of skills as complete.