diff --git a/frontend/src/routes/themen/+page.server.ts b/frontend/src/routes/themen/+page.server.ts
new file mode 100644
index 00000000..d5d3891c
--- /dev/null
+++ b/frontend/src/routes/themen/+page.server.ts
@@ -0,0 +1,12 @@
+import { error } from '@sveltejs/kit';
+import { createApiClient } from '$lib/shared/api.server';
+import type { components } from '$lib/generated/api';
+
+type TagTreeNodeDTO = components['schemas']['TagTreeNodeDTO'];
+
+export async function load({ fetch }: Parameters<import('./$types').PageServerLoad>[0]) {
+	const api = createApiClient(fetch);
+	const result = await api.GET('/api/tags/tree');
+	if (!result.response.ok) throw error(500, 'Themen konnten nicht geladen werden.');
+	return { tree: (result.data ?? []) as TagTreeNodeDTO[] };
+}
diff --git a/frontend/src/routes/themen/page.server.spec.ts b/frontend/src/routes/themen/page.server.spec.ts
new file mode 100644
index 00000000..338c4b40
--- /dev/null
+++ b/frontend/src/routes/themen/page.server.spec.ts
@@ -0,0 +1,60 @@
+import { describe, expect, it, vi, beforeEach } from 'vitest';
+
+vi.mock('$lib/shared/api.server', () => ({
+	createApiClient: vi.fn(),
+	extractErrorCode: (e: unknown) => (e as { code?: string } | undefined)?.code
+}));
+
+import { createApiClient } from '$lib/shared/api.server';
+
+beforeEach(() => vi.clearAllMocks());
+
+function mockApiGet(ok: boolean, data: unknown) {
+	vi.mocked(createApiClient).mockReturnValue({
+		GET: vi.fn().mockResolvedValue({ response: { ok }, data })
+	} as ReturnType<typeof createApiClient>);
+}
+
+const makeTag = (name: string, documentCount = 0) => ({
+	id: 'id-' + name,
+	name,
+	documentCount,
+	children: []
+});
+
+describe('/themen +page.server load', () => {
+	function makeLoadEvent() {
+		return {
+			fetch: vi.fn() as unknown as typeof fetch,
+			request: new Request('http://localhost/themen'),
+			url: new URL('http://localhost/themen')
+		};
+	}
+
+	it('returns tag tree when API succeeds', async () => {
+		const tree = [makeTag('Briefe', 5), makeTag('Fotos', 3)];
+		mockApiGet(true, tree);
+
+		const { load } = await import('./+page.server');
+		const result = await load(makeLoadEvent());
+
+		expect(result.tree).toEqual(tree);
+	});
+
+	it('returns empty array when API returns empty list', async () => {
+		mockApiGet(true, []);
+
+		const { load } = await import('./+page.server');
+		const result = await load(makeLoadEvent());
+
+		expect(result.tree).toEqual([]);
+	});
+
+	it('throws 500 when API call fails', async () => {
+		mockApiGet(false, null);
+
+		const { load } = await import('./+page.server');
+
+		await expect(load(makeLoadEvent())).rejects.toMatchObject({ status: 500 });
+	});
+});