Reddit.com :
The single biggest difference you will notice immediately is the style. GPT-5.5 Instant is downright aggressive about being concise. The era of « Certainly! I’d be happy to help you with that » followed by three paragraphs of useless preamble is over. OpenAI specifically tuned this to cut the fluff. They dropped the gratuitous emojis. They tightened the formatting. When I ask for a Python script or a PRD outline now, it just gives me the output. No transitions. No weird essay wrapping at the end telling me to let it know if I need anything else. It feels significantly more like a precision tool. Less like an overly enthusiastic intern trying to impress you.