- GPT-5.5 Instant is now the default model for all free ChatGPT users, rolling out globally starting today.
- The new model produced 52.5% fewer hallucinated claims on high-stakes medical, legal, and financial prompts in OpenAI tests.
- GPT-5.5 Instant is the first “Instant” model classified as “High Capability” in Cybersecurity and biological domains, triggering additional safety guardrails.
- It scores significantly higher on factual accuracy benchmarks, including a jump to 38.4 points on the clinical-use HealthBench Professional test.
OpenAI has silently upgraded the engine powering its flagship chatbot, making GPT-5.5 Instant the default model for hundreds of millions of ChatGPT users worldwide starting today. This core upgrade, detailed in a system card, focuses on drastically reducing the AI’s tendency to fabricate information rather than introducing flashy new features.
The new model demonstrated a 52.5% reduction in hallucinated claims during internal testing on critical prompts from medicine, law, and finance. Consequently, its performance on HealthBench Professional, a benchmark for clinical questions, improved from 32.9 to 38.4 points.
Meanwhile, on conversations previously flagged by real users for factual errors, inaccurate claims dropped by 37.3%. This represents a significant step in addressing the most persistent flaw plaguing large language models since their inception.
The model also enhances personalization by more actively using a user’s past chats and connected data like Gmail. However, OpenAI emphasizes user control, now showing what context is used and allowing its deletion.
Notably, GPT-5.5 Instant is the first model in its speed-optimized “Instant” tier to be deemed “High Capability” in both cybersecurity and biological domains. This classification mandates the implementation of automated safeguards previously reserved for more powerful, analytical models.
Paid subscribers who prefer the previous default, GPT-5.3 Instant, have a three-month grace period before it is retired. The enhanced personalization features will first roll out to Plus and Pro users on the web.
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