How do I choose between a fast model and a thinking model inside ChatGPT, Claude or Gemini?

Use the fast setting for everyday questions and switch up when the task needs working out, such as a plan with trade-offs or a long document. In ChatGPT, the model menu has a Thinking slider (Instant, Medium, High, Extra High, plus Pro on Pro plans), and since September 2026 Plus and Pro no longer switch to thinking on their own, so choose it yourself. Free and Go users get a Think option. In Claude, the menu next to Send sets the model and an effort level from Low to Max; more effort is more thorough but slower and uses your limits faster. In Gemini, the free app runs a Flash-Lite model and Google AI Pro adds Gemini 4 Argon and Google's Pro model.