Models
OpenAI Upgrades ChatGPT's Default Model to GPT-5.5 Instant — 52% Fewer Hallucinations
OpenAI has replaced GPT-5.3 Instant with GPT-5.5 Instant as ChatGPT's default model for all users, delivering significant accuracy gains and a new memory personalization system.
By Patrick T ·
OpenAI has quietly replaced the default ChatGPT model for hundreds of millions of users. GPT-5.5 Instant is now live — and the numbers behind the upgrade are striking.
In internal evaluations, GPT-5.5 Instant produced 52.5% fewer hallucinated claims than its predecessor, GPT-5.3 Instant, on high-stakes prompts covering medicine, law, and finance. Inaccurate claims on especially challenging conversations dropped by 37.3%. For a model used by this many people daily, those are not marginal improvements.
What's Actually Changed
The upgrade is not just about accuracy. OpenAI has also overhauled how the model handles responses — trimming verbosity, cutting gratuitous emojis, and reducing the tendency to ask unnecessary follow-up questions. The result is a model that gets to the point faster without losing substance.
Improvements span visual reasoning, math, and science benchmarks. The model is also better at deciding when to trigger a web search rather than relying on its training data — a meaningful change for users asking about current events or recent developments.
Memory Gets Transparent
The headline feature may be the new memory sources system. For the first time, ChatGPT will show users exactly what context shaped a personalised response — which saved memories, which past chats, which connected files. Users can see the sources, delete them, or correct them if they're outdated.
This is a significant transparency move. AI personalisation has long operated as a black box — the model 'knows' things about you, but you can't see what or why. Memory sources changes that dynamic, at least partially.
GPT-5.5 Instant produced 52.5% fewer hallucinated claims than GPT-5.3 Instant on high-stakes prompts covering medicine, law, and finance.
OpenAI internal evaluation, May 2026
Rollout and API Access
GPT-5.5 Instant is rolling out now to all ChatGPT users, replacing GPT-5.3 Instant as the default. In the API, it's accessible as `chat-latest`. Paid users will retain access to GPT-5.3 Instant for three months before it's retired.
Enhanced personalisation from past chats, files, and connected Gmail is rolling out first to Plus and Pro users on the web, with mobile and broader plan availability to follow. Memory sources are available across all consumer plans.
What to Watch
The 52% hallucination reduction figure will face scrutiny. OpenAI's internal benchmarks have historically been optimistic, and independent evaluators will want to replicate the results on their own test sets. The memory transparency feature is more immediately verifiable — and if it works as described, it sets a new standard for how AI companies should handle personalisation data.