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OpenAI Just Upgraded ChatGPT's Default Model—Here's What GPT-5.5 Instant Actually Does
TL;DR
OpenAI has upgraded ChatGPT's default model to GPT-5.5 Instant, which reduces hallucinations by 52.5% compared to GPT-5.3 Instant. This model is classified as 'High Capability' in cybersecurity and biological domains.
Key points
- GPT-5.5 Instant replaces GPT-5.3 Instant as the default model
- Model reduces hallucinations by 52.5% on high-stakes prompts
- First Instant-tier model classified as 'High Capability'
- Enhanced performance in cybersecurity and biological domains
- Available for free to all users
In brief
- GPT-5.5 Instant replaces GPT-5.3 Instant as ChatGPT's default model starting today, rolling out to all users for free.
- The model produced 52.5% fewer hallucinated claims than its predecessor on high-stakes medical, legal, and financial prompts in OpenAI's internal tests.
- GPT-5.5 Instant is the first Instant-tier model OpenAI classifies as "High Capability" in both cybersecurity and biological domains, requiring additional safeguards at deployment.
OpenAI just swapped out the engine inside ChatGPT. Starting today, GPT-5.5 Instant replaces GPT-5.3 Instant as the default model used by hundreds of millions of people who open ChatGPT every day.
This isn't a flashy launch; no new mode, no jaw-dropping demo. But "small improvement" is a relative term when the upgrade cuts hallucinations by more than half.
What is GPT-5.5 Instant?
OpenAI's GPT family ships in tiers. Instant is the everyday model, built for speed and general use; Thinking is the slower, more analytical version for complex problems; and Pro is the heavyweight for maximum-intensity tasks.
GPT-5.5 Instant is the latest update to the tier that most ChatGPT users will interact with, whether they realize it or not.
According to OpenAI, the new model produced fewer hallucinated claims than GPT-5.3 Instant on high-stakes prompts in medicine, law, and finance. Hallucinations have been ChatGPT's most persistent flaw since the beginning.
OpenAI also tested against conversations real users had previously flagged for factual errors. On those, inaccurate claims dropped by 37.3%.
On HealthBench—a benchmark testing AI responses to real medical questions, scored from 0 to 100— GPT-5.5 Instant scores 51.4 points, up from 49.6. On HealthBench Professional, the clinical-use version, it jumps from 32.9 to 38.4 points.
Health questions are among the most common things people ask ChatGPT, which makes getting them right more than a benchmark exercise. These results mean GPT 5.5 Instant increased accuracy by responding correctly 38.4% of the time.
Q&A
What improvements does GPT-5.5 Instant offer over GPT-5.3 Instant?
GPT-5.5 Instant reduces hallucinations by 52.5% on high-stakes prompts compared to GPT-5.3 Instant.
What does 'High Capability' mean for GPT-5.5 Instant?
The 'High Capability' classification indicates that GPT-5.5 Instant has enhanced performance in cybersecurity and biological domains, requiring additional safeguards.
When did OpenAI launch GPT-5.5 Instant as the default model?
GPT-5.5 Instant was launched as the default model starting today.
How does GPT-5.5 Instant affect users of ChatGPT?
All users of ChatGPT will benefit from the upgrade to GPT-5.5 Instant, which aims to provide more accurate and reliable responses.
