What
When Claude reads a file, Claude Code now checks a per-session record of files already 'held' from an earlier write to that same path. On macOS specifically, it goes further and computes the file's real on-disk identity (its device and inode numbers, the operating system's way of uniquely identifying a file even if the path changes) for the resolved path and for a fallback path used when a file isn't found, and passes that identity along with the read.
Why
This guards against subtle mixups where a path read from disk isn't actually the same file that was just written, for example if something was moved, replaced, or symlinked in between, keeping Claude's view of a file's contents consistent with what it just wrote on macOS.