Google swaps in Gemini 4 Hydrogen as the default model for AI Mode
Google has made Gemini 4 Hydrogen the default model in AI Mode and the model behind its hardest AI Overviews questions, rolling it out globally to all Search users. Nick Fox called it the biggest model improvement to Search so far, and Robby Stein described it as the largest model upgrade to Search yet.
Google says Hydrogen handles multi-part questions, hard-to-read typos and highly specific requests more accurately, and follows instructions and calls tools more precisely. Its query fan-out now runs simultaneous searches across subtopics, which Google says surfaces links to sites users might not otherwise have found. Responses also lean more on images, comparison tables, structured layouts and custom tools.
The release landed during a major core update, though the changes appear to sit in AI Overviews and AI Mode rather than core rankings. It also came days after Gemini 4 Carbon went out to paid AI Mode subscribers.
For brands chasing citations in AI answers, the fan-out change is the one to watch: broader subtopic querying means more pages can be pulled into a response, so coverage of adjacent questions matters more than a single head term.
Reported by Search Engine Roundtable ↗
Topics: AI Mode, AI Overviews, Google Search