Plan Mode puts Claude in a read-only state. It can read files, search code, and reason about changes, but it can't write, edit, or run commands, so nothing gets modified.
Enter it by pressing Shift+Tab twice, or start a session with claude --permission-mode plan. Once Claude has a plan, press Ctrl+G to open it in your editor and modify it directly before anything runs. For headless use: claude --permission-mode plan -p "Analyze the auth system" runs a full read-only analysis and prints the plan. I reach for this before touching an unfamiliar codebase, planning a multi-file refactor, or scoping out what a complex change will involve.