Plan mode means asking an AI agent to first propose what it will change, where and how it will test it, without editing anything yet. You review the plan, correct it and only then let it act. Some tools ship it as a separate mode; in others, asking is enough.
A question from the app. Pick an answer.
For which of these tasks is asking for a plan first overkill?
It touches money and several parts of the app. There a plan lets you stop mistakes before they cost you.
It is small and you can check the result at a glance. Planning it costs more than doing it.
Many files and high risk: if it goes wrong, nobody gets in. Exactly where seeing the plan first pays off.
A useful plan can be reviewed: it says which steps it will take, which files it touches and how it will check that it worked. Say you ask a coding agent to add free shipping over a certain amount in an online store. A good plan names the cart calculation, the checkout screen and the tests it will run.
Then you compare it against two things. Against the project rules, the ones you wrote down for example in CLAUDE.md: if the plan adds a date library and the project already uses another, tell it. And against what is missing: if it says nothing about orders already paid, ask whether it touches them. Red flags: steps like "I will adjust whatever is needed", files unrelated to the request, or no step to test.
In tools that ship it as a mode, the agent can usually read the project but not edit it until you approve. If yours does not have one, one sentence does it: "Before editing, tell me which files you will change, in what order and how you will test it. Do not touch anything until I say so."
Key difference: a tool mode can block editing; a request in the chat only guides the agent, which may skip it. If the change is sensitive, prefer the mode or permissions that ask for your approval before editing.
It helps when the change touches several parts, money, user login or user data: there, fixing the plan costs one sentence and undoing changes costs hours. Skip it when the change is small and you can check the result at a glance, like renaming a variable inside a single file. Rule of thumb: if reading the plan takes longer than reviewing the change, skip it.
Common mistake: "I approved the plan, so we are done." The plan says what it intends to do, not what it did. A good plan can be carried out badly, so at the end you still review the diff and the tests.