Google has made Gemini 4 Hydrogen the default model for its AI mode for all users worldwide, starting November 12. The change comes as the November core update is still rolling out, and it also applies, according to Robby Stein, to the most difficult questions in AI Overviews.
In a thread posted on X, Robby Stein, Vice President of Product for Google Search, states that Hydrogen is “used for your hardest questions in AI Overviews.” The Google post dedicated to Search specifies that the model is available globally for all Search users.
What Google Says About the Changes
According to this post, Hydrogen handles multi-part queries better, those containing typos or very specific requests, and follows instructions more thoroughly than previous versions. The model also improves “query fan-out,” meaning the simultaneous execution of multiple queries covering each sub-topic of a question. Google claims it enables “connecting you to a wider range of helpful links than before,” including to sites users might never have found on their own.
Neither the post nor Stein’s thread provides figures on the number of links or sites involved. Google also states that responses will include more images, structured answers, comparison tables, and personalized tools, and will be more nuanced in all languages supported by AI mode.
The previous default model change dated back to the I/O conference in May, when Gemini 3.5 Flash became the default model for AI mode worldwide. Neither the Search post nor Stein’s message specifies which model Hydrogen replaces.
A Place Among the Gemini 4 Models
Two other Gemini 4 models had already been added to AI mode, as options reserved for paying subscribers. Gemini 4 Argon has been available for Google AI Pro and Ultra subscribers since October 13, while Gemini 4 Carbon has been rolling out to Ultra subscribers in AI mode since November 10. According to Google’s post dedicated to the model, Hydrogen is designed for speed and efficiency, while Carbon and Argon are intended for deeper, longer reasoning.
Reading Google’s help page on AI mode, accessed at 1:55 p.m. ET, Carbon is listed as an option for Ultra subscribers aged 18 or older, with daily usage limits. The page describes Hydrogen as “useful for quick, versatile assistance on most questions,” with no mention of Argon.
AI Overviews and the November Update
Stein’s thread places Hydrogen on the most complex questions in AI Overviews, without specifying which ones fall into this category or which responses are handled by other models. The Search post does not mention AI Overviews. In January, Google had made Gemini 3 the default model for AI Overviews worldwide.
The November core update began on November 10 and was still rolling out at 2:05 p.m. ET on November 12, according to the Search status dashboard cited by the site. Citation data collected for AI mode or AI Overviews this month therefore covers two distinct dates: the start of the update, then the model change on November 12. Google has not yet indicated which AI Overviews questions will be classified as the most difficult.
