A read-only agent mode that investigates and proposes a plan without editing anything, so you approve the approach before any code changes.
The separation matters because the costly agent error is not a bad line of code, it is a confident wrong approach applied across thirty files. Plan mode surfaces the approach while it is still a paragraph you can reject in ten seconds.
It works best when the plan names the files it will touch and the checks it will run. A plan that says “refactor the auth layer” is not reviewable; one that lists five files and the test that must pass afterwards is.