OpenAI releases GPT-5.5 Instant as new ChatGPT default, cutting hallucinations by 52.5%

OpenAI released GPT-5.5 Instant on May 5, 2026, replacing GPT-5.3 Instant as the default ChatGPT model. In internal evaluations, the new model produced 52.5% fewer hallucinated claims than its predecessor on high-stakes prompts covering medicine, law, and finance, and reduced inaccurate claims by 37.3% on user-flagged challenging conversations. On benchmarks, GPT-5.5 Instant scored 81.2 on AIME 2025 (vs. 65.4 for GPT-5.3), 76 on MMMU-Pro (vs. 69.2), and 51.4 on HealthBench (vs. 49.6). The HealthBench Professional clinical score jumped from 32.9 to 38.4. The model is more concise, asks fewer unnecessary follow-up questions, reduces gratuitous emoji use, and now pulls context from past chats, saved files, and connected Gmail for personalized responses. Personalization is initially available to Plus and Pro web users. GPT-5.3 Instant remains available for three months, and the model is accessible via the API as 'chat-latest'.