What the mind map feature does
NotebookLM's mind map generates a branching outline from the sources inside one notebook — not from your chat history, and not from a prompt you type afterwards. Click the Mind map chip and the product collapses every loaded source into a hierarchy: source titles become first-level branches, sections become second-level, and each node expands into the underlying points when you click it. The result is a structural summary you navigate rather than read.
Two consequences follow from how it is built, and neither is obvious until you have produced a few bad maps. First, the map is only as good as the sources it can see — a clean, well-structured source produces a clean map, and a rambling transcript produces a rambling one. Second, because the map reads sources rather than chat, asking the model to "redo the map but shorter" does nothing at all. You change the map by changing the sources, or by expanding and collapsing different branches before you export.
What to feed it
- Tightly scoped sources. One topic per notebook beats a mixed pile every time. A notebook holding a syllabus and three lecture PDFs maps cleanly; the same PDFs plus unrelated meeting notes produce a branch salad.
- Structured text over transcripts. Headings, lists and captions give the map its branch shape. Raw video transcripts produce one long stem with everything hung off it.
- Fewer, denser sources. Three good sources beat fifteen thin ones — every source becomes a top-level branch whether it deserves to be or not, so a stray appendix can crowd out a real document.
Getting a clean map
Generate the map, then prune before you export. Expand only the branches you actually need — collapsed branches stay collapsed in the exported PNG, which is the difference between a shareable artifact and a wall of text. For anything meant to be presented, splitting a broad notebook into two narrower ones and mapping each separately beats shipping one unreadable chart.
If the first-level branches are wrong, the cause is almost always a source that should not be in the notebook — an appendix, a changelog, a table-of-contents page. Remove it and regenerate rather than working around it.
Limits and quirks
The map regenerates from scratch each time — there is no pinning nodes and no manual editing. Export is PNG only; for an editable diagram, rebuild the outline in a dedicated tool (entry coverage lands as verified tools appear). And very long sources can push real content down to third-level branches — expand before concluding a point was dropped.