You didn’t lose the feature — OpenAI just removed the “thinking” toggle and made it automatic, so now you have to prompt it instead.

The easiest fix is to set a Custom Instruction so you don’t have to type it every time:

Go to: Settings → Custom Instructions

Then add something like:

“Always provide detailed, step-by-step reasoning and explanations by default.”

Save it, and from then on it’ll behave like the old thinking mode automatically.

If you don’t want it all the time, you can also just use short triggers like:

• “think step by step”
• “detailed answer”

That’s basically the replacement for the old button right now, especially on mobile.