Plan mode or a skill? When to reach for which
Plan mode and skills solve different problems. Plan mode is read-only research before a change; a skill is a procedure you have written down to reuse.
Not the same axis
People reach for "a skill" when they mean "think first", and for plan mode when they actually have a repeatable procedure. The two solve different problems.
Plan mode: look before you leap
Plan mode tells Claude to research and propose changes without making them — it reads files and runs read-only commands, then writes a plan, but does not touch your source. Enter it with Shift+Tab or by prefixing a prompt with /plan. It is for the front of a risky or unfamiliar change: you want a plan you can read and approve before a single edit lands.
Skills: a procedure you wrote down
A skill is a SKILL.md you have authored — a checklist or multi-step procedure Claude loads on demand when its description matches, or when you type /name. It is for the work you keep doing the same way. The test: if you have pasted the same instructions into chat three times, that is a skill.
The distinction
Plan mode is ephemeral thinking about one specific change. A skill is a durable procedure reused across many. One is a mode you enter and leave; the other is a capability that persists. Confuse them and you either write a skill for a one-off (overkill) or re-explain the same procedure every time (waste).
They compose
They stack neatly: a skill named "plan this feature" can drive plan-mode-style research as its first step. The mode is how Claude works right now; the skill is what you have decided is worth doing the same way every time.