Google made Gemini 4 Hydrogen the default model in AI Mode for all users and uses it for the hardest AI Overview questions, two days into a core update.

Google made Gemini 4 Hydrogen the default model in AI Mode for all users worldwide, starting today. Sites that track how often AI Mode cites them now have a new model to account for.

In an X thread announcing the change, Robby Stein, VP of Product for Google Search, said Hydrogen is also “being used for your hardest questions in AI Overviews.” Google’s Search post says the model is available globally in Search for all users.

The switch came two days into the November core update, which was still rolling out as of 2:05 p.m. ET today.

The Search post says Hydrogen is able to handle multi-part inquiries, inquiries that contain typos, highly specific inquiries, and follow commands more thoroughly than previous versions of Search.

The post from Search says that Hydrogen also allows for better query fan-out. Query fan-out refers to how Search simultaneously executes multiple queries across each of the subtopics of an inquiry. According to the post, the result is what Google calls “connecting you to a broader range of helpful links than before,” and this includes websites which users may have never stumbled upon themselves. Stein’s thread says that the fan-out improvements result in showing more links to the web. Neither the post nor Stein’s thread provide counts for links or sites.

The post references more visual aids in answers through use of images, structured responses, comparison tables and additional customized tools available to users; additionally, the answers provided will be more nuanced in all languages supported by AI Mode.

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