Definition
Google-Extended is Google's user-agent for training-data crawling of Gemini and Vertex AI models. Like GPTBot, it's separate from the retrieval/indexing crawler (Googlebot). Blocking Google-Extended only opts your content out of Gemini training; it does NOT affect Google AI Overview retrieval (which uses Googlebot's existing index).
User-agent: Google-Extended Disallow: / (Opts out of Gemini training; AI Overview retrieval continues normally.)
How to optimize
Always Allow Googlebot (blocking kills SEO + AI Overview). Choose separately on Google-Extended based on training-data policy.