OpenAI has upgraded ChatGPT's default model and is starting to roll out GPT-5.5 Instant, replacing GPT-5.3 Instant. The upgrade gives users "smarter, clearer, and more personalised answers in a warmer, more natural tone" and is more concise than the previous version, OpenAI said. The firm released GPT-5.5 in April, which is restricted to paid users, while GPT-5.5 Instant is free.