On Tuesday, 5th May, 2026, OpenAI took decisive action by unveiling ChatGPT, a new model that will serve as the default model for any user of ChatGPT. This new model will replace GPT-5.3, which was the previous default. And will have significantly more powerful features and improved reliability to address common deficiencies in previous versions of GPT. Most notably the occurrence of hallucinations in high-request situations.
This article will provide an overview of this latest update, highlight differences compared to prior versions and evaluate whether or not the hallucination saga is coming to an end.
What is GPT-5.5 Instant and why does it matter right now?
The new model of GPT-5.5 isn’t a completely new build off prior versions. OpenAI released the broader family of GPT-5.5 last month. Making this model an advance from previous models and a step in the direction of coding, knowledge work, and multimodal reasoning. GPT-5.5 Instant is the speed-optimized version of that family and is built to deliver fast and low latency responses for everyday ChatGPT users.
The other major difference between GPT-5.5 and GPT-5.3 Instant is that the latter is now the default model. Which means that if you open ChatGPT today and do not manually select a specific model, you will be talking with GPT-5.5 Instant and that will affect the user experience.
GPT-5.3 vs GPT-5.5 Instant: what actually changed between the two models?
The performance gap between GPT-5.3 and GPT-5.5 Instant is measurable, not just marketing. Here is a direct comparison on two key benchmarks:
| Benchmark | GPT-5.3 Instant | GPT-5.5 Instant |
|---|---|---|
| AIME 2025 (Math) | 65.4 | 81.2 |
| MMMU-Pro (Multimodal Reasoning) | 69.2 | 76.0 |
GPT-5.5 Instant pulled additional points out of each section. Of particular note, those who have used ChatGPT to assist with quantitative tasks, such as analytics or academic work, may rejoice in this 15-point sample for Math (from 65.4 to 81.2), reported TechCrunch.
In addition to the raw score increases, there are several practical ways in which you will see these changes through:
Increased manageability of context: Previously, ChatGPT only had access to the input of the current conversation. However, now it is able to review previous conversations that you have had with it, any files that you have uploaded and the contents of your Gmail account as part of the process of creating a more personalized and relevant answer. The deployment of this function will be first available to web users subscribing to either Plus or Pro. With a forthcoming rollout on Mobile and the Free tiers.
Visibility of memory source: ChatGPT will give you a source for where it got the answer it provided to you. Additionally, you will have the ability to delete outdated or incorrect sources. Other users will not see where you have accessed memory sources in any shared conversations. This is a step forward, especially since ChatGPT has traditionally had a fragmented memory system. Something highlighted in this guide on importing your ChatGPT history to Claude.
How does GPT-5.5 actually work and what makes it faster?
The name uses “Instant” to signify that its primary design goal is low latency- i.e., OpenAI created this version to provide fast responses without making users wait through a long chain of reasoning to receive answers right away. Although this variant is not as robust as o3 and does not provide the same depth of reasoning as that model. It has very good performance at high speeds on tasks such as writing, summarisation, coding assistance and Q & A.
This includes a new context management feature that adjusts the way this model customises responses. Rather than treating each conversation as a stand-alone event, GPT-5.5 Instant can now leverage a wider range of personal data (stored from previous chats, files & connected Gmail) in order to provide you with answers more appropriate for your unique circumstances instead of generic ones.
OpenAI and hallucinations: A pattern that has not fully gone away
Hallucination holds the most significant criticism received by OpenAI in the last few years, this is also a problem seen across all AI models. But what makes OpenAI unique from other AI models is not only how often the models hallucinate. But also what patterns they use to arrive at their hallucinations which is something worth understanding prior to trusting them. With respect to performing any kind of meaningful or important task work through a machine generated solution using GPT-5.5 Instant. If you want a broader perspective, our detailed comparison of hallucination resistance between Claude, Gemini, and ChatGPT
OpenAI’s models performed at about 0.8%-2.0% on Vectara’s summarization benchmark where Google’s Gemini models performed at 0.7%-0.8%. That sounds close, but the gap widens in more complex tasks.
The difference in performance between OpenAI and Google were much more significant and noticeable on PersonQA- OpenAI’s o3 reasoning model hallucinated 33% of the time which was 2x more than o1. And o4-mini hallucinated even more frequently than o3 which was 48%. So it appears from these models the technology itself may present structural weaknesses because when models have been trained using deep reasoning to optimise performance. They often substitute a confident sounding guess for a fact to fill an information-gaps to avoid having to respond “I don’t know.”
Why newer models don’t always mean fewer hallucinations
Hallucination benchmarks indicate significant differences between the performance of GPT-5.4 nano and GPT-4.1, with more recent OpenAI variants often hallucinate more than older base models.
The company confirms that GPT-5 has fewer hallucinations overall than the previous version but that hallucinations remain an ongoing challenge for all LLMs. Hallucinations will continue to be an aspect of the field of LLMs until further improvements are made to the models and their ability to reason.
As such, OpenAI discusses reducing hallucination risks within particular sensitive domains for the most recent GPT-5.5 Instant model. This is a targeted statement relating to specific legal, medical, and financial areas rather than a blanket statement regarding the entire GPT-5.5 model. The use of the visible memory sourcing feature in ChatGPT is significant in this regard. If a user can utilize the source documents shown in ChatGPT’s answer to perform a fact-check prior to acting on it. They reduce the likelihood of being affected by hallucination issues.
Who gets access to GPT-5.5 Instant and when?
The rollout follows a tiered schedule:
- Plus/Pro: Rollout of web access to new Context-Management capabilities is the first phase – now available.
- Mobile: Rollout to mobile will happen shortly thereafter. Web-based features of this new capability have yet to be implemented on mobile devices.
- Free & Go Business/Enterprise: Expect an extended period of time (officially less than 6 weeks) to implement all web features for all users.
- For developers, the new GPT-5.5 is available for access via an API call within the chat-latest alias. GPT-5.3 is available for paid API users as an alternative for three months before being retired.
What happens to users who got attached to older models?
OpenAI has experienced this type of situation before, when it deprecated GPT-4o in February of 2026. There was a huge backlash from users who had formed a connection to the model’s personality. Some people even referred to GPT-4o as their “best friend” or “a mirror” in the context of their lives. In the end, the company did decide to deprecate the model in February despite all of that feedback from users.
It seems as if GPT-5.3 Instant will receive the same treatment as GPT-4o did. The model will be available via an API for paid users. And will only be available for an initial period of 3 months before going away. If you use GPT-5.3 Instant for any workflow now would be a good time to begin testing the latest version of the model.
Key takeaways
- GPT-5.5 Instant has been switched to be the standard default model. And it outperforms GPT-5.3 Instant in mathematical and multimodal reasoning capabilities.
- Contextualized personalization and improved transparency through better memory sources have been implemented.
- OpenAI is reporting that hallucinations have been substantially reduced but are still present. The company is trying to make real progress in legal and medical domains as well as finance. Particularly with their recent use of “thinking mode” effective on clinical benchmarks.
- The complete transition to GPT-5.3 will be simple, and the majority of other users will automatically see the upgrade. Developers will have a transition period in place from GPT-5.3 in which they may adjust to the newer model.
