OpenAI Releases GPT-5.5 Instant as New Default ChatGPT Model, Replacing GPT-5.3

Summary

OpenAI released GPT-5.5 Instant as the new default ChatGPT model for all users, replacing GPT-5.3 Instant. The model delivers improved accuracy in sensitive domains — law, medicine, and finance — with claimed hallucination reduction alongside maintained response speed. Mathematical reasoning improved significantly: AIME 2025 score went from 65.4 to 81.2, and MMMU-Pro multimodal benchmark from 69.2 to 76. A major new feature is enhanced cross-session personalization — GPT-5.5 Instant can search p