Which model should I use? The one-page answer

Open ChatGPT or Claude and there is a little menu of model names that look like serial numbers. Ignore the names. There are really only three kinds, and the app usually labels them.

Fast. Answers instantly. Fine for rewriting, summarising, quick questions, drafts, and almost everything you do in a day.

Careful. Pauses to think before replying. Better at maths, planning, tricky code, anything with several steps where a wrong early move ruins the rest. Slower, and it uses more of your limit per question.

Cheap. The small ones: Mini, Flash, Haiku and friends. Nearly as good as Fast on everyday text, much better at not eating your allowance. The right choice for anything an agent runs on a schedule.

The three-line rule

1. Start on Fast, or Auto if the app has it.
2. Switch to Careful when the question has steps, numbers or consequences.
3. Switch to Cheap when you are about to do the same small thing fifty times.

That is it. I have never regretted following it and I have often regretted ignoring line 2.

When you are not sure

Ask the model you are already talking to. It is surprisingly honest about this.

Before you answer: is this question better suited to a fast model or a model that reasons
step by step? Answer in one line, then answer the question itself with whatever care it
needs.

If it says “this needs careful reasoning” more than once in a session, move to the careful model for that job and stop asking.

The names will have changed by the time you read this. The three kinds will not.