OpenAI released GPT-5.5 Instant on May 5, replacing GPT-5.3 Instant as ChatGPT's default and rolling out to the API as the alias 'chat-latest'. The company reports 52.5% fewer hallucinated claims on high-stakes prompts in medicine, law, and finance, 37.3% fewer inaccurate claims on user-flagged conversations, and noticeably more concise output — 30.2% fewer words and 29.2% fewer lines on an open-ended advice benchmark.
The change that matters most is the new memory tooling. GPT-5.5 Instant can search across past conversations, uploaded files, and Gmail to ground its answers, and ChatGPT now displays the memory sources behind a response so users can delete or correct outdated facts. That is a small UI move with a large epistemic implication — for the first time, a default consumer chatbot is showing its work on what it remembers about you.
OpenAI is keeping GPT-5.3 available to paid users for three months. The pattern echoes its handling of every default-model swap since GPT-4o: a brief overlap, then a forced cutover. Power users have already begun complaining that GPT-5.5 Instant feels different on coding edge cases and creative writing — the standard response to any default change at OpenAI's scale.
A note for learners: 'fewer hallucinations on high-stakes prompts' is the more important benchmark than raw IQ scores. If you use ChatGPT for medical, legal, or financial questions, retest the same prompts you ran on the old model — the regression patterns are usually subtle, and OpenAI's headline numbers are aggregates, not guarantees on your specific use case.