What
Before running a shell command, Claude Code checks it without running it to decide whether to ask your permission. As part of that it tracks which shell variables a command changes. After pushd and popd, commands that move between folders and remember earlier ones, it now records both DIRSTACK and zsh's lowercase dirstack as changed. dirstack is also added to the list of special zsh variable names.
Why
In practice, the permission check has a more accurate picture of what folder-stack commands change under zsh, a common shell.