diff --git a/frontend/src/lib/generated/api.ts b/frontend/src/lib/generated/api.ts
index 5b059dd5..8f9e303e 100644
--- a/frontend/src/lib/generated/api.ts
+++ b/frontend/src/lib/generated/api.ts
@@ -2230,6 +2230,11 @@ export interface components {
             color?: string;
             /** Format: int32 */
             documentCount: number;
+            /**
+             * Format: int32
+             * @description Distinct documents tagged with this tag or any descendant tag (subtree rollup)
+             */
+            subtreeDocumentCount: number;
             children?: components["schemas"]["TagTreeNodeDTO"][];
             /**
              * Format: uuid
diff --git a/frontend/src/lib/shared/utils/tagUtils.test.ts b/frontend/src/lib/shared/utils/tagUtils.test.ts
index 5fe4a874..87b33763 100644
--- a/frontend/src/lib/shared/utils/tagUtils.test.ts
+++ b/frontend/src/lib/shared/utils/tagUtils.test.ts
@@ -4,26 +4,29 @@ import type { components } from '$lib/generated/api';
 
 type TagTreeNodeDTO = components['schemas']['TagTreeNodeDTO'];
 
-function makeNode(documentCount: number, children: TagTreeNodeDTO[] = []): TagTreeNodeDTO {
-	return { id: 'id', name: 'name', documentCount, children };
+function makeNode(
+	documentCount: number,
+	subtreeDocumentCount: number,
+	children: TagTreeNodeDTO[] = []
+): TagTreeNodeDTO {
+	return { id: 'id', name: 'name', documentCount, subtreeDocumentCount, children };
 }
 
 describe('hasAnyDocuments', () => {
-	it('returns false for a leaf node with documentCount=0', () => {
-		expect(hasAnyDocuments(makeNode(0))).toBe(false);
+	it('returns false for a node whose subtree holds no documents', () => {
+		expect(hasAnyDocuments(makeNode(0, 0))).toBe(false);
 	});
 
-	it('returns true for a leaf node with documentCount=3', () => {
-		expect(hasAnyDocuments(makeNode(3))).toBe(true);
+	it('returns true for a node whose subtree holds documents', () => {
+		expect(hasAnyDocuments(makeNode(3, 3))).toBe(true);
 	});
 
-	it('returns true for a root with documentCount=0 but a child with documentCount=5', () => {
-		const node = makeNode(0, [makeNode(5)]);
-		expect(hasAnyDocuments(node)).toBe(true);
+	it('keys on the subtree rollup, not direct documentCount: 0 direct but rollup 5 → true', () => {
+		// The rollup already includes descendants — a single field read, no recursion over children.
+		expect(hasAnyDocuments(makeNode(0, 5))).toBe(true);
 	});
 
-	it('returns false for a root with documentCount=0 and all children also 0', () => {
-		const node = makeNode(0, [makeNode(0), makeNode(0)]);
-		expect(hasAnyDocuments(node)).toBe(false);
+	it('keys on the subtree rollup, not direct documentCount: 5 direct but rollup 0 → false', () => {
+		expect(hasAnyDocuments(makeNode(5, 0))).toBe(false);
 	});
 });
diff --git a/frontend/src/lib/shared/utils/tagUtils.ts b/frontend/src/lib/shared/utils/tagUtils.ts
index a7fe138d..89826e68 100644
--- a/frontend/src/lib/shared/utils/tagUtils.ts
+++ b/frontend/src/lib/shared/utils/tagUtils.ts
@@ -2,6 +2,12 @@ import type { components } from '$lib/generated/api';
 
 type TagTreeNodeDTO = components['schemas']['TagTreeNodeDTO'];
 
+/**
+ * Whether a tag's whole subtree holds any documents — keyed on the subtree rollup
+ * (`subtreeDocumentCount`), which the backend already computes across all descendants.
+ * Used by the reader surfaces (/themen page, dashboard ThemenWidget) to hide empty topics.
+ * A single field read: no recursion needed, the rollup is authoritative.
+ */
 export function hasAnyDocuments(node: TagTreeNodeDTO): boolean {
-	return (node.documentCount ?? 0) > 0 || (node.children ?? []).some(hasAnyDocuments);
+	return (node.subtreeDocumentCount ?? 0) > 0;
 }