According to XDA-Developers, Google’s NotebookLM has introduced a new feature in 2025 called Custom Mode, which functions like a system prompt but applies only to individual notebooks. This mode allows users to configure the AI’s behavior by specifying their skill level on a topic, desired response formats like bullet points or tables, and even tone and grammar preferences. The author used it to create an “AI version” of themselves and forced the AI to forgo all capital letters, demonstrating its precise control. You access the feature via a slider icon in the chat panel’s top-right corner, under “Define your conversational goal, style, or role.” This tweak is designed to stop the AI from making incorrect assumptions about user knowledge and to deliver more personalized, consistent outputs without needing repetitive instructions in every prompt.
Why This Is A Big Deal
Here’s the thing: most AI tools are frustratingly one-size-fits-all. You tell ChatGPT or Claude you’re a beginner, and three responses later it’s back to dropping jargon like you have a PhD. NotebookLM‘s Custom Mode finally tackles that context amnesia problem head-on, but in a smart, compartmentalized way. You’re not setting a global rule for everything you do. Instead, you’re creating a tailored environment for each project. A notebook for learning quantum physics can have a “explain like I’m 15” setting, while your creative writing notebook can be tuned for lyrical prose. That separation of contexts is huge. It turns NotebookLM from a generic chat-into-your-PDFs tool into a specialized workspace. Basically, it’s the difference between a Swiss Army knife and a properly organized toolbox.
The Real Use Case Is Consistency
So what’s the killer app here? It’s not just about better answers. It’s about predictable, scannable, and reusable outputs. The article highlights this perfectly with the research notebook example. Asking for bullet-point summaries and clear source attribution every single time you chat is a pain. Set it once in Custom Mode, and every answer comes pre-formatted for easy dumping into Obsidian or Notion. That’s a massive workflow win. For technical learning, telling the AI to skip fundamentals and focus on practical trade-offs cuts through the fluffy, generic advice that plagues so many AI explanations. I think this is where Custom Mode shines brightest: it reduces prompt engineering overhead for the tasks you do repeatedly. You’re not just chatting; you’re building a custom analyst or tutor for that specific topic.
When To Skip The Hassle
But let’s be real, not every notebook needs this. The author is right to point out that for a quick, one-off summary of a document or a simple lookup, fiddling with Custom Mode is overkill. The default behavior is already pretty good for that. The power—and the time investment—is in the setup. This feature is clearly aimed at power users who have dedicated, long-term notebooks for deep dives. If you’re just kicking the tires on a topic or doing a five-minute fact check, you’d spend more time configuring the mode than you’d save. It’s a tool for specialization, not spontaneity. Knowing when not to use it is just as important as knowing how to use it well.
A Nudge Toward Specialization
This move by Google is interesting. It feels like a quiet acknowledgment that general-purpose AI chat is getting commoditized. The value is shifting to AI that can adapt to your specific, repeatable workflows. Custom Mode is a step in that direction, making NotebookLM stickier for serious research and learning. It creates a moat. Why switch to another tool if you’ve spent time perfectly tuning your NotebookLM environments? For businesses or researchers, this kind of consistent, tailored output is crucial. It hints at a future where our AI tools are less like oracles and more like a team of dedicated, configurable assistants, each with a defined role and style. That’s a much more powerful—and useful—paradigm.
