diff --git a/apps/web/src/lib/utils.test.mjs b/apps/web/src/lib/utils.test.mjs
new file mode 100644
index 00000000..bea81c7e
--- /dev/null
+++ b/apps/web/src/lib/utils.test.mjs
@@ -0,0 +1,56 @@
+import { describe, expect, test } from "bun:test";
+import { buildNotebookTree } from "./utils.ts";
+
+const notebook = (id, parentId, memoCount, lastMemoUpdatedAt = null) => ({
+  id,
+  parentId,
+  name: id,
+  slug: null,
+  icon: null,
+  color: null,
+  sortOrder: 0,
+  memoCount,
+  lastMemoUpdatedAt,
+  createdAt: "2026-01-01T00:00:00.000Z",
+  updatedAt: "2026-01-01T00:00:00.000Z",
+});
+
+describe("buildNotebookTree", () => {
+  test("rolls descendant memo counts and update times into every ancestor", () => {
+    const [root] = buildNotebookTree([
+      notebook("root", null, 2, "2026-01-02T00:00:00.000Z"),
+      notebook("child", "root", 3, "2026-01-03T00:00:00.000Z"),
+      notebook("grandchild", "child", 5, "2026-01-04T00:00:00.000Z"),
+    ]);
+
+    expect(root.memoCount).toBe(10);
+    expect(root.lastMemoUpdatedAt).toBe("2026-01-04T00:00:00.000Z");
+    expect(root.children[0].memoCount).toBe(8);
+    expect(root.children[0].lastMemoUpdatedAt).toBe("2026-01-04T00:00:00.000Z");
+    expect(root.children[0].children[0].memoCount).toBe(5);
+  });
+
+  test("sorts siblings after descendant counts have been rolled up", () => {
+    const roots = buildNotebookTree(
+      [
+        notebook("larger-subtree", null, 0),
+        notebook("child", "larger-subtree", 4),
+        notebook("smaller-subtree", null, 3),
+      ],
+      (first, second) => second.memoCount - first.memoCount,
+    );
+
+    expect(roots.map((node) => node.id)).toEqual(["larger-subtree", "smaller-subtree"]);
+    expect(roots.map((node) => node.memoCount)).toEqual([4, 3]);
+  });
+
+  test("does not mutate the notebook records returned by the repository", () => {
+    const root = notebook("root", null, 1);
+    const child = notebook("child", "root", 2);
+
+    buildNotebookTree([root, child]);
+
+    expect(root.memoCount).toBe(1);
+    expect(root).not.toHaveProperty("children");
+  });
+});
diff --git a/apps/web/src/lib/utils.ts b/apps/web/src/lib/utils.ts
index a3ef6ba2..c5eb8d9d 100644
--- a/apps/web/src/lib/utils.ts
+++ b/apps/web/src/lib/utils.ts
@@ -29,12 +29,23 @@ export const buildNotebookTree = (notebooks: Notebook[], compareNodes: NotebookN
     }
   }
 
-  const sortNodes = (items: NotebookNode[]) => {
+  const rollUpAndSortNodes = (items: NotebookNode[]) => {
+    for (const item of items) {
+      rollUpAndSortNodes(item.children);
+
+      for (const child of item.children) {
+        item.memoCount += child.memoCount;
+
+        if (child.lastMemoUpdatedAt && (!item.lastMemoUpdatedAt || child.lastMemoUpdatedAt > item.lastMemoUpdatedAt)) {
+          item.lastMemoUpdatedAt = child.lastMemoUpdatedAt;
+        }
+      }
+    }
+
     items.sort(compareNodes);
-    items.forEach((item) => sortNodes(item.children));
   };
 
-  sortNodes(roots);
+  rollUpAndSortNodes(roots);
   return roots;
 };