What
The Monitor tool watches the output of a command. The written instructions Claude receives for it have been rewritten. The new text includes the line "The script runs in the same shell environment as Bash." The rewrite covers the guidance on watching for every occurrence of something indefinitely. That guidance used to point to never-ending commands such as tail -f, inotifywait -m or while true.
This change was seen under certain account settings. Some other model configurations got a different edit to the same instructions, so the wording you get may depend on the model in use.
Why
Claude follows these instructions, so a rewrite can change how it sets up and runs Monitor. What the new wording changes in practice has not been measured.