diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
index 4262f3bc..e21eb412 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
@@ -319,6 +319,37 @@ describe('PersonMentionEditor — stale-response race', () => {
 	});
 });
 
+// ─── Server failure characterization (Sara #2 on PR #629) ───────────────────
+
+describe('PersonMentionEditor — server failure', () => {
+	it('on 500 response keeps the dropdown open with the empty-state copy (silent failure pinned; distinct error UX tracked separately)', async () => {
+		const fetchMock = vi
+			.fn()
+			.mockResolvedValue({ ok: false, status: 500, json: vi.fn().mockResolvedValue({}) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+
+		// Pins current silent-failure behaviour. The day someone implements a
+		// distinct error UX (toast / "Suche fehlgeschlagen" copy), this test
+		// goes red and forces them to update the assertion.
+		await expect.element(page.getByText(m.person_mention_popup_empty())).toBeInTheDocument();
+	});
+
+	it('on a fetch reject (network failure) keeps the dropdown open with the empty-state copy', async () => {
+		const fetchMock = vi.fn().mockRejectedValue(new TypeError('NetworkError'));
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+
+		await expect.element(page.getByText(m.person_mention_popup_empty())).toBeInTheDocument();
+	});
+});
+
 // ─── onExit cancels pending debounce (Felix #1 on PR #629) ───────────────────
 
 describe('PersonMentionEditor — onExit cancels pending debounce', () => {