Google makes Gemini 4 Hydrogen the default model in AI Mode worldwide, right in the middle of a core update

Google has made Gemini 4 Hydrogen the default model in AI Mode for all users worldwide, effective November 12. In a series of posts on X, Robby Stein, VP of Product for Google Search, said Hydrogen is also "being used for your hardest questions in AI Overviews." Google Search's official post says the model is available globally in Search for all users.

According to Search Engine Journal, in a story published November 12, the switch came two days after the November core update began rolling out, and the update was still in progress as of 2:05 p.m. Eastern that day. If you track how often AI Mode cites your site, there is now a new model to factor in.

What Google says is changing

According to the Search post, Hydrogen handles multi-part questions, questions with typos and highly specific questions better than previous versions, and it follows instructions more closely.

The model also improves query fan-out, the process in which Search runs several queries at once on each subtopic of a question. Google says the result is "connecting you to a wider range of helpful links than before," including sites users might never have found on their own. Stein wrote that these improvements lead to showing more links to the web, but neither he nor the post gives any figures on links or sites.

The post also points to more visual elements in responses, such as images, structured answers, comparison tables and custom tools, plus more nuanced answers in every language AI Mode supports.

Where Hydrogen comes from

The previous default switch happened at I/O in May, when Gemini 3.5 Flash became the global default in AI Mode. Neither the Search post nor Stein said which model Hydrogen replaced.

Two Gemini 4 models had already reached AI Mode as paid options. Gemini 4 Argon became available to Google AI Pro and Ultra subscribers on October 13, and Gemini 4 Carbon began rolling out to Ultra subscribers on November 10. According to Google, Hydrogen is built for speed and efficiency, while Carbon and Argon are meant for longer reasoning.

The AI Mode help page lists Carbon as an option for Ultra subscribers aged 18 and over, with a daily usage limit, describes Hydrogen as "helpful for fast, thorough assistance on most questions," and does not mention Argon. Stein did not explain which questions count as the hardest in AI Overviews or which model answers the rest. In January, Google had made Gemini 3 the global default for AI Overviews.

What this means for brand teams

This month's citation data from AI Mode and AI Overviews spans a model switch on November 12, inside the window of the core update that started November 10. Logging both dates lets you compare before and after each change instead of reading the month as a single block. Since Google gave no figures on the additional links, the way forward is to check citations and referral traffic in your own reports and watch whether Google spells out which AI Overviews questions it treats as the hardest.