diff --git a/frontend/eslint.config.js b/frontend/eslint.config.js
index 5ef17b59..037353b8 100644
--- a/frontend/eslint.config.js
+++ b/frontend/eslint.config.js
@@ -106,6 +106,31 @@ export default defineConfig(
 			]
 		}
 	},
+	{
+		// Forbid test fixtures (*.test-fixture.svelte) from being imported by
+		// production code. Tree-shaking keeps them out of the production bundle
+		// today (no route reaches them), but a lint rule makes the boundary
+		// explicit so an accidental autocomplete import in a route or component
+		// fails fast. Test files (*.spec.ts / *.test.ts) and the fixtures
+		// themselves are exempt — see the next block. Nora #2 on PR #629
+		// round 3.
+		files: ['**/*.svelte', '**/*.svelte.ts', '**/*.svelte.js', '**/*.ts'],
+		ignores: ['**/*.spec.ts', '**/*.test.ts', '**/*.test-fixture.svelte'],
+		rules: {
+			'no-restricted-imports': [
+				'error',
+				{
+					patterns: [
+						{
+							group: ['**/*.test-fixture.svelte'],
+							message:
+								'Test fixtures (*.test-fixture.svelte) are test-only — do not import from production code. Tracked by #637.'
+						}
+					]
+				}
+			]
+		}
+	},
 	{
 		plugins: { boundaries },
 		settings: {
diff --git a/frontend/messages/de.json b/frontend/messages/de.json
index 37e25574..bdc89a7e 100644
--- a/frontend/messages/de.json
+++ b/frontend/messages/de.json
@@ -445,8 +445,12 @@
 	"person_mention_load_error": "Person konnte nicht geladen werden.",
 	"person_mention_loading": "Lade Person…",
 	"person_mention_popup_empty": "Keine Personen gefunden",
+	"person_mention_search_label": "Person suchen",
+	"person_mention_search_prompt": "Namen eingeben…",
 	"person_mention_btn_label": "Person verlinken",
 	"person_mention_create_new": "Neue Person anlegen",
+	"person_mention_results_count_singular": "1 Person gefunden",
+	"person_mention_results_count_plural": "{count} Personen gefunden",
 	"transcription_editor_aria_label": "Transkriptionstext",
 	"person_born_name_prefix": "geb.",
 	"page_title_home": "Archiv",
diff --git a/frontend/messages/en.json b/frontend/messages/en.json
index b4b675c5..0c2ad4f1 100644
--- a/frontend/messages/en.json
+++ b/frontend/messages/en.json
@@ -445,8 +445,12 @@
 	"person_mention_load_error": "Could not load person.",
 	"person_mention_loading": "Loading person…",
 	"person_mention_popup_empty": "No persons found",
+	"person_mention_search_label": "Search for a person",
+	"person_mention_search_prompt": "Enter a name…",
 	"person_mention_btn_label": "Link person",
 	"person_mention_create_new": "Create new person",
+	"person_mention_results_count_singular": "1 person found",
+	"person_mention_results_count_plural": "{count} persons found",
 	"transcription_editor_aria_label": "Transcription text",
 	"person_born_name_prefix": "née",
 	"page_title_home": "Archive",
diff --git a/frontend/messages/es.json b/frontend/messages/es.json
index 898b3e85..7631e348 100644
--- a/frontend/messages/es.json
+++ b/frontend/messages/es.json
@@ -445,8 +445,12 @@
 	"person_mention_load_error": "No se pudo cargar la persona.",
 	"person_mention_loading": "Cargando persona…",
 	"person_mention_popup_empty": "No se encontraron personas",
+	"person_mention_search_label": "Buscar persona",
+	"person_mention_search_prompt": "Escribe un nombre…",
 	"person_mention_btn_label": "Vincular persona",
 	"person_mention_create_new": "Crear nueva persona",
+	"person_mention_results_count_singular": "1 persona encontrada",
+	"person_mention_results_count_plural": "{count} personas encontradas",
 	"transcription_editor_aria_label": "Texto de transcripción",
 	"person_born_name_prefix": "n.",
 	"page_title_home": "Archivo",
diff --git a/frontend/src/lib/document/transcription/TranscriptionBlock.svelte.spec.ts b/frontend/src/lib/document/transcription/TranscriptionBlock.svelte.spec.ts
index c5562630..a635238d 100644
--- a/frontend/src/lib/document/transcription/TranscriptionBlock.svelte.spec.ts
+++ b/frontend/src/lib/document/transcription/TranscriptionBlock.svelte.spec.ts
@@ -1,7 +1,7 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
-import TranscriptionBlockHost from './TranscriptionBlock.test-host.svelte';
+import TranscriptionBlockHost from './TranscriptionBlock.test-fixture.svelte';
 import type { ConfirmService } from '$lib/shared/services/confirm.svelte.js';
 
 afterEach(cleanup);
diff --git a/frontend/src/lib/document/transcription/TranscriptionBlock.test-host.svelte b/frontend/src/lib/document/transcription/TranscriptionBlock.test-fixture.svelte
similarity index 100%
rename from frontend/src/lib/document/transcription/TranscriptionBlock.test-host.svelte
rename to frontend/src/lib/document/transcription/TranscriptionBlock.test-fixture.svelte
diff --git a/frontend/src/lib/shared/discussion/MentionDropdown.svelte b/frontend/src/lib/shared/discussion/MentionDropdown.svelte
index 4932eda4..c647f434 100644
--- a/frontend/src/lib/shared/discussion/MentionDropdown.svelte
+++ b/frontend/src/lib/shared/discussion/MentionDropdown.svelte
@@ -2,7 +2,18 @@
 import type { components } from '$lib/generated/api';
 // eslint-disable-next-line boundaries/dependencies -- mention dropdown needs person date formatting; extract to shared if it becomes reusable
 import { formatLifeDateRange } from '$lib/person/personLifeDates';
+import { untrack } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
+// Layered defence cap on the @mention search query length (CWE-400
+// amplification). The <input maxlength> attribute below caps direct
+// user edits, but the editor-mirror path (Tiptap contenteditable -> mirror
+// $effect -> searchQuery) is not covered by `maxlength` since the
+// contenteditable has no such enforcement. Clipping at the mirror keeps
+// the cap honest from both paths. Tracked server-side separately.
+// Nora #1 on PR #629. Hoisted to mentionConstants.ts so the host editor
+// (PersonMentionEditor) can clip the inserted displayName to the same cap
+// — see Felix #3 on PR #629.
+import { MAX_QUERY_LENGTH } from './mentionConstants';
 
 type Person = components['schemas']['Person'];
 
@@ -17,7 +28,46 @@ type DropdownState = {
 	clientRect: (() => DOMRect | null) | null;
 };
 
-let { model }: { model: DropdownState } = $props();
+let {
+	model,
+	editorQuery = '',
+	onSearch = () => {}
+}: {
+	model: DropdownState;
+	/** Text typed after `@` in the host editor. Mirrors into the search input
+	 *  until the user takes manual ownership by typing into the input itself. */
+	editorQuery?: string;
+	onSearch?: (query: string) => void;
+} = $props();
+
+let searchQuery = $state(untrack(() => editorQuery.slice(0, MAX_QUERY_LENGTH)));
+let userHasEdited = $state(false);
+
+// Intent-revealing alias used by both the persistent aria-live announcer and
+// the visible empty-state copy. Folding the duplicated rule into one $derived
+// keeps the two branches in lockstep. Felix #3 on PR #629 round 4.
+const isQueryEmpty = $derived(searchQuery.trim() === '');
+
+// Mirror the editor's typed text until the user takes ownership.
+//
+// Why `$state + $effect` (not `$derived`): `searchQuery` is also written by
+// `bind:value` on the <input> below, so it needs to be a mutable `$state`.
+// A `$derived` would be read-only and would clobber direct user edits on
+// every editor keystroke. The `userHasEdited` latch pins ownership once the
+// user types into the input. Felix #1 on PR #629.
+$effect(() => {
+	if (!userHasEdited) {
+		searchQuery = editorQuery.slice(0, MAX_QUERY_LENGTH);
+	}
+});
+
+// Fire onSearch whenever the effective query changes — covers both the
+// editor mirror and direct input edits. This is the only place onSearch
+// fires; when the dropdown is unmounted, the effect is disposed and no
+// further fetches occur.
+$effect(() => {
+	onSearch(searchQuery);
+});
 
 // highlightedIndex must be both writable (keyboard handler mutates it) and
 // reset when `items` changes (so it never points past the end of a new list).
@@ -112,16 +162,70 @@ function selectItem(item: Person) {
 	unauthenticated users.
 -->
 <div
-	class="fixed z-50 w-72 overflow-hidden rounded-sm border border-line bg-surface shadow-lg"
+	class="fixed z-50 w-72 max-w-[calc(100vw-1rem)] overflow-hidden rounded-sm border border-line bg-surface shadow-lg"
 	role="listbox"
 	aria-label={m.person_mention_btn_label()}
 	style:top={position.top}
 	style:bottom={position.bottom}
 	style:left={position.left}
 >
+	<div class="border-b border-line px-3 py-2">
+		<label class="sr-only" for="mention-search">{m.person_mention_search_label()}</label>
+		<div class="flex items-center gap-2">
+			<svg
+				aria-hidden="true"
+				viewBox="0 0 24 24"
+				fill="none"
+				stroke="currentColor"
+				stroke-width="2"
+				class="h-5 w-5 shrink-0 text-ink-2"
+			>
+				<circle cx="11" cy="11" r="7" />
+				<path d="m20 20-3.5-3.5" stroke-linecap="round" />
+			</svg>
+			<input
+				id="mention-search"
+				type="search"
+				data-test-search-input
+				maxlength={MAX_QUERY_LENGTH}
+				class="min-h-[44px] w-full bg-transparent font-sans text-base text-ink placeholder:text-ink-3 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy focus-visible:ring-inset"
+				placeholder={m.person_mention_search_prompt()}
+				bind:value={searchQuery}
+				oninput={() => {
+					userHasEdited = true;
+				}}
+				onmousedown={(e) => e.stopPropagation()}
+			/>
+		</div>
+	</div>
+	<!--
+		Persistent aria-live region — lives ABOVE the conditional branches so the
+		element never unmounts when items transition between empty and populated.
+		VoiceOver in particular swallows announcements from freshly-mounted live
+		regions, and the previous (conditional-inside) markup silently dropped
+		the "N persons found" announcement when results populated. Leonie #3 on
+		PR #629 round 3.
+	-->
+	<p class="sr-only" aria-live="polite">
+		{#if model.items.length === 0}
+			{isQueryEmpty ? m.person_mention_search_prompt() : m.person_mention_popup_empty()}
+		{:else if model.items.length === 1}
+			{m.person_mention_results_count_singular()}
+		{:else}
+			{m.person_mention_results_count_plural({ count: model.items.length })}
+		{/if}
+	</p>
 	{#if model.items.length === 0}
-		<p class="px-3 py-2.5 font-sans text-sm text-ink-3">
-			{m.person_mention_popup_empty()}
+		<!--
+			Visible empty-state copy — visual-only. The persistent sr-only <p>
+			above is the sole AT announcer; this one is hidden from screen readers
+			via aria-hidden="true" so VoiceOver does not double-announce
+			(NVDA de-dups, VoiceOver does not). Leonie S-2 on PR #629 round 4.
+			Do NOT add an aria-live attribute here — that would re-introduce
+			the duplicate announcement.
+		-->
+		<p aria-hidden="true" class="px-3 py-2.5 font-sans text-sm text-ink-3">
+			{isQueryEmpty ? m.person_mention_search_prompt() : m.person_mention_popup_empty()}
 		</p>
 		<!--
 			Empty-state escape hatch — without it the transcriber has to close
@@ -132,7 +236,7 @@ function selectItem(item: Person) {
 		<a
 			href="/persons/new"
 			target="_blank"
-			rel="noopener"
+			rel="noopener noreferrer"
 			class="flex min-h-[44px] items-center gap-2 border-t border-line px-3 py-2.5 font-sans text-sm font-medium text-brand-navy hover:bg-canvas focus:bg-canvas focus:outline-none"
 			onmousedown={(e) => e.preventDefault()}
 		>
diff --git a/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts b/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
index 46e2da86..fcab66b6 100644
--- a/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
+++ b/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
@@ -1,22 +1,37 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
-import { page } from 'vitest/browser';
+import { page, userEvent } from 'vitest/browser';
+import { flushSync, mount, tick, unmount } from 'svelte';
 import MentionDropdown from './MentionDropdown.svelte';
+import MentionDropdownFixture from './MentionDropdown.test-fixture.svelte';
+import { m } from '$lib/paraglide/messages.js';
+import type { components } from '$lib/generated/api';
+
+type Person = components['schemas']['Person'];
 
 afterEach(cleanup);
 
-const makePerson = (id: string, name: string, overrides: Record<string, unknown> = {}) => ({
-	id,
-	firstName: name.split(' ')[0] ?? null,
-	lastName: name.split(' ').slice(1).join(' ') || name,
-	displayName: name,
-	birthYear: null as number | null,
-	deathYear: null as number | null,
-	...overrides
-});
+const makePerson = (id: string, name: string, overrides: Partial<Person> = {}): Person => {
+	const parts = name.split(' ');
+	return {
+		id,
+		firstName: parts[0],
+		lastName: parts.slice(1).join(' ') || name,
+		displayName: name,
+		personType: 'PERSON',
+		familyMember: false,
+		...overrides
+	};
+};
 
-const baseModel = (overrides: Record<string, unknown> = {}) => ({
-	items: [] as ReturnType<typeof makePerson>[],
+type DropdownState = {
+	items: Person[];
+	command: (item: Person) => void;
+	clientRect: (() => DOMRect | null) | null;
+};
+
+const baseModel = (overrides: Partial<DropdownState> = {}): DropdownState => ({
+	items: [],
 	command: vi.fn(),
 	clientRect: () => new DOMRect(100, 100, 0, 24),
 	...overrides
@@ -29,14 +44,32 @@ describe('MentionDropdown', () => {
 		await expect.element(page.getByRole('listbox', { name: /person verlinken/i })).toBeVisible();
 	});
 
-	it('renders the empty placeholder when items is empty', async () => {
+	it('shows the "enter a name" prompt when the search field is empty', async () => {
 		render(MentionDropdown, { props: { model: baseModel() } });
 
-		await expect.element(page.getByText('Keine Personen gefunden')).toBeVisible();
+		// Scope to the visible empty-state <p> (text-ink-3) — the persistent
+		// sr-only aria-live region above also contains the same prompt copy.
+		const visibleEmptyP = document.querySelector(
+			'[role="listbox"] p.text-ink-3'
+		) as HTMLElement | null;
+		expect(visibleEmptyP).not.toBeNull();
+		expect(visibleEmptyP!.textContent ?? '').toContain(m.person_mention_search_prompt());
+		expect(visibleEmptyP!.textContent ?? '').not.toContain(m.person_mention_popup_empty());
+	});
+
+	it('shows "no persons found" when the search has a query but the list is empty', async () => {
+		render(MentionDropdown, { props: { model: baseModel(), editorQuery: 'WdG' } });
+
+		const visibleEmptyP = document.querySelector(
+			'[role="listbox"] p.text-ink-3'
+		) as HTMLElement | null;
+		expect(visibleEmptyP).not.toBeNull();
+		expect(visibleEmptyP!.textContent ?? '').toContain(m.person_mention_popup_empty());
+		expect(visibleEmptyP!.textContent ?? '').not.toContain(m.person_mention_search_prompt());
 	});
 
 	it('shows the create-new escape hatch link in the empty state', async () => {
-		render(MentionDropdown, { props: { model: baseModel() } });
+		render(MentionDropdown, { props: { model: baseModel(), editorQuery: 'unknown' } });
 
 		const link = (await page
 			.getByRole('link', { name: /neue person anlegen/i })
@@ -44,6 +77,7 @@ describe('MentionDropdown', () => {
 		expect(link.href).toContain('/persons/new');
 		expect(link.target).toBe('_blank');
 		expect(link.rel).toContain('noopener');
+		expect(link.rel).toContain('noreferrer');
 	});
 
 	it('renders one option per item when populated', async () => {
@@ -104,3 +138,315 @@ describe('MentionDropdown', () => {
 		expect(dropdown.style.left).toBe('123px');
 	});
 });
+
+// ─── Search input — Issue #380 ────────────────────────────────────────────────
+
+describe('MentionDropdown — search input', () => {
+	it('renders a search input pre-filled with the editorQuery prop', async () => {
+		render(MentionDropdown, {
+			props: { model: baseModel(), editorQuery: 'WdG' }
+		});
+
+		await expect.element(page.getByRole('searchbox')).toHaveValue('WdG');
+	});
+
+	it('exposes a data-test-search-input attribute for E2E selectors', async () => {
+		render(MentionDropdown, { props: { model: baseModel() } });
+
+		const input = document.querySelector('[data-test-search-input]');
+		expect(input).not.toBeNull();
+		expect((input as HTMLInputElement).type).toBe('search');
+	});
+
+	it('search input wrapper meets the 44px touch target (WCAG 2.2 AA)', async () => {
+		render(MentionDropdown, { props: { model: baseModel() } });
+
+		const input = document.querySelector('[data-test-search-input]') as HTMLElement;
+		expect(input).not.toBeNull();
+		expect(input.className).toContain('min-h-[44px]');
+	});
+
+	it('renders a persistent aria-live="polite" region (does not remount on items transition; Leonie #3 on PR #629)', async () => {
+		render(MentionDropdown, { props: { model: baseModel() } });
+
+		const listbox = document.querySelector('[role="listbox"]');
+		expect(listbox).not.toBeNull();
+		const live = listbox!.querySelector('p[aria-live="polite"]');
+		expect(live).not.toBeNull();
+		// Empty + empty-query → "Namen eingeben…" prompt
+		expect(live!.textContent ?? '').toContain(m.person_mention_search_prompt());
+	});
+
+	it('announces the result count in the persistent live region when items populate (Leonie #3 on PR #629)', async () => {
+		render(MentionDropdown, {
+			props: {
+				model: baseModel({
+					items: [
+						makePerson('p1', 'Anna Schmidt'),
+						makePerson('p2', 'Bert Meier'),
+						makePerson('p3', 'Carl Vogel')
+					]
+				})
+			}
+		});
+
+		const listbox = document.querySelector('[role="listbox"]');
+		expect(listbox).not.toBeNull();
+		const live = listbox!.querySelector('p[aria-live="polite"]');
+		expect(live).not.toBeNull();
+		// Populated → "3 Personen gefunden" (plural)
+		expect(live!.textContent ?? '').toContain('3');
+	});
+
+	it('announces the singular form when exactly one item is present (Sara #4 on PR #629)', async () => {
+		render(MentionDropdown, {
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt')]
+				})
+			}
+		});
+
+		const listbox = document.querySelector('[role="listbox"]');
+		expect(listbox).not.toBeNull();
+		const live = listbox!.querySelector('p[aria-live="polite"]');
+		expect(live).not.toBeNull();
+		// Singular branch — "1 Person gefunden" / "1 person found" / "1 persona encontrada"
+		// (locale-dependent; resolved via the Paraglide message helper).
+		expect(live!.textContent ?? '').toContain(m.person_mention_results_count_singular());
+	});
+
+	it('keeps the visible empty-state copy without its own aria-live and hides it from AT (Leonie #3 on PR #629 round 3; Leonie S-2 round 4)', async () => {
+		render(MentionDropdown, { props: { model: baseModel(), editorQuery: 'WdG' } });
+
+		// Visible empty-state <p> exists with the empty-result copy ...
+		const empty = document.querySelector('p.text-ink-3') as HTMLElement | null;
+		expect(empty).not.toBeNull();
+		expect(empty!.textContent ?? '').toContain(m.person_mention_popup_empty());
+		// ... but it must NOT carry its own aria-live (the persistent sr-only
+		// region above the conditional is the announcer now).
+		expect(empty!.hasAttribute('aria-live')).toBe(false);
+		// ... and it MUST be hidden from screen readers via aria-hidden="true"
+		// so VoiceOver does not double-announce (the persistent sr-only region
+		// is the sole AT source of truth). Leonie S-2 on PR #629 round 4.
+		expect(empty!.getAttribute('aria-hidden')).toBe('true');
+	});
+
+	it('renders the magnifier icon at h-5 w-5 with text-ink-2 (Leonie BLOCKER on PR #629)', async () => {
+		render(MentionDropdown, { props: { model: baseModel() } });
+
+		const icon = document.querySelector('[data-test-search-input]')
+			?.previousElementSibling as SVGElement | null;
+		expect(icon).not.toBeNull();
+		expect(icon!.tagName.toLowerCase()).toBe('svg');
+		expect(icon!.getAttribute('class') ?? '').toContain('h-5');
+		expect(icon!.getAttribute('class') ?? '').toContain('w-5');
+		expect(icon!.getAttribute('class') ?? '').toContain('text-ink-2');
+	});
+
+	it('caps the search input at maxlength=100 (CWE-400 amplification — Nora on PR #629)', async () => {
+		render(MentionDropdown, { props: { model: baseModel() } });
+
+		const input = document.querySelector('[data-test-search-input]') as HTMLInputElement;
+		expect(input).not.toBeNull();
+		expect(input.maxLength).toBe(100);
+	});
+
+	it('clips a long editorQuery mirror to 100 chars (CWE-400 layered — Nora #1 on PR #629)', async () => {
+		const longQuery = 'A'.repeat(200);
+		render(MentionDropdown, { props: { model: baseModel(), editorQuery: longQuery } });
+
+		const input = document.querySelector('[data-test-search-input]') as HTMLInputElement;
+		expect(input).not.toBeNull();
+		expect(input.value.length).toBe(100);
+		expect(input.value).toBe('A'.repeat(100));
+	});
+
+	it('caps the listbox width to the viewport (320 px reflow guard — Leonie FINDING-MENTION-005)', async () => {
+		render(MentionDropdown, { props: { model: baseModel() } });
+
+		const listbox = document.querySelector('[role="listbox"]') as HTMLElement;
+		expect(listbox).not.toBeNull();
+		expect(listbox.className).toContain('max-w-[calc(100vw-1rem)]');
+	});
+
+	it('renders the @mention search input at text-base (16 px senior-audience floor — Leonie FINDING-MENTION-006)', async () => {
+		render(MentionDropdown, { props: { model: baseModel() } });
+
+		const input = document.querySelector('[data-test-search-input]') as HTMLInputElement;
+		expect(input).not.toBeNull();
+		expect(input.className).toContain('text-base');
+		expect(input.className).not.toContain('text-sm');
+	});
+
+	it('invokes onSearch with the current value whenever the user types', async () => {
+		const onSearch = vi.fn();
+		render(MentionDropdown, { props: { model: baseModel(), onSearch } });
+
+		await userEvent.type(page.getByRole('searchbox'), 'Walter');
+
+		await vi.waitFor(() => {
+			expect(onSearch).toHaveBeenCalled();
+			expect(onSearch).toHaveBeenLastCalledWith('Walter');
+		});
+	});
+
+	it('keeps the user-edited search value when editorQuery changes after the takeover (Felix on PR #629)', async () => {
+		let setEditorQuery!: (q: string) => void;
+		render(MentionDropdownFixture, {
+			model: baseModel(),
+			initialEditorQuery: 'WdG',
+			onReady: (s: (q: string) => void) => {
+				setEditorQuery = s;
+			}
+		});
+
+		await expect.element(page.getByRole('searchbox')).toHaveValue('WdG');
+
+		await page.getByRole('searchbox').fill('Walter');
+		await expect.element(page.getByRole('searchbox')).toHaveValue('Walter');
+
+		setEditorQuery('WdGruyter');
+		// Flush pending Svelte reactivity so any (non-)update from the mirror
+		// $effect has landed before we assert. expect.element already polls, so
+		// no fixed-timeout fallback is needed. Sara on PR #629 round 3.
+		await tick();
+
+		await expect.element(page.getByRole('searchbox')).toHaveValue('Walter');
+	});
+});
+
+// ─── ArrowDown via exported onKeyDown (Sara #3 on PR #629) ──────────────────
+//
+// In production, Tiptap intercepts ArrowDown/ArrowUp/Enter at the editor level
+// and forwards them to the dropdown via its exported onKeyDown(event) function
+// — the dropdown itself has no DOM keydown listener. This test exercises the
+// same export so a regression in highlightedIndex/selection logic is caught
+// at the unit level. The full E2E focus-chain test is deferred to a separate
+// issue (Playwright).
+//
+// These unit tests directly invoke the exported `onKeyDown` to pin its
+// behaviour in isolation. They do NOT exercise the Tiptap forwarding
+// chain (PersonMentionEditor.suggestion.render() returning { onKeyDown })
+// — that integration is covered by the 'ArrowDown moves the highlight'
+// test in PersonMentionEditor.svelte.spec.ts. Sara on PR #629 round 3.
+
+describe('MentionDropdown — onKeyDown forwarding', () => {
+	// flushSync ensures Svelte reactivity propagation completes before
+	// asserting (uniform across all four key tests so the next reader
+	// doesn't have to figure out why some are wrapped and others aren't).
+	// Felix #1 suggestion on PR #629 round 3.
+
+	it('ArrowDown advances aria-selected to the next option in the listbox', async () => {
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt'), makePerson('p2', 'Bert Meier')]
+				})
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+
+			// First option starts highlighted.
+			const first = container.querySelector('[data-test-person-id="p1"]') as HTMLElement;
+			const second = container.querySelector('[data-test-person-id="p2"]') as HTMLElement;
+			expect(first.getAttribute('aria-selected')).toBe('true');
+			expect(second.getAttribute('aria-selected')).toBe('false');
+
+			let consumed = false;
+			flushSync(() => {
+				consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'ArrowDown' }));
+			});
+			expect(consumed).toBe(true);
+
+			expect(first.getAttribute('aria-selected')).toBe('false');
+			expect(second.getAttribute('aria-selected')).toBe('true');
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+
+	it('ArrowUp wraps from the first option to the last', async () => {
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt'), makePerson('p2', 'Bert Meier')]
+				})
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+
+			let consumed = false;
+			flushSync(() => {
+				consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'ArrowUp' }));
+			});
+			expect(consumed).toBe(true);
+
+			const second = container.querySelector('[data-test-person-id="p2"]') as HTMLElement;
+			expect(second.getAttribute('aria-selected')).toBe('true');
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+
+	it('Enter invokes model.command with the currently highlighted item', async () => {
+		const command = vi.fn();
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt'), makePerson('p2', 'Bert Meier')],
+					command
+				})
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+
+			let consumed = false;
+			flushSync(() => {
+				consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'Enter' }));
+			});
+			expect(consumed).toBe(true);
+			expect(command).toHaveBeenCalledTimes(1);
+			expect(command.mock.calls[0][0].id).toBe('p1');
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+
+	it('Escape returns false so the suggestion plugin can handle it', async () => {
+		const container = document.createElement('div');
+		document.body.appendChild(container);
+		const instance = mount(MentionDropdown, {
+			target: container,
+			props: {
+				model: baseModel({ items: [makePerson('p1', 'Anna Schmidt')] })
+			}
+		});
+		try {
+			const exports = instance as unknown as { onKeyDown: (e: KeyboardEvent) => boolean };
+			let consumed = true;
+			flushSync(() => {
+				consumed = exports.onKeyDown(new KeyboardEvent('keydown', { key: 'Escape' }));
+			});
+			expect(consumed).toBe(false);
+		} finally {
+			unmount(instance);
+			container.remove();
+		}
+	});
+});
diff --git a/frontend/src/lib/shared/discussion/MentionDropdown.test-fixture.svelte b/frontend/src/lib/shared/discussion/MentionDropdown.test-fixture.svelte
new file mode 100644
index 00000000..4a69d9e5
--- /dev/null
+++ b/frontend/src/lib/shared/discussion/MentionDropdown.test-fixture.svelte
@@ -0,0 +1,32 @@
+<script lang="ts">
+import { untrack } from 'svelte';
+import MentionDropdown from './MentionDropdown.svelte';
+import type { components } from '$lib/generated/api';
+
+type Person = components['schemas']['Person'];
+type DropdownState = {
+	items: Person[];
+	command: (item: Person) => void;
+	clientRect: (() => DOMRect | null) | null;
+};
+
+type Props = {
+	model: DropdownState;
+	initialEditorQuery: string;
+	/** Test hook: receives a setter for editorQuery so the test can mutate it. */
+	onReady?: (setEditorQuery: (q: string) => void) => void;
+	onSearch?: (q: string) => void;
+};
+
+let { model, initialEditorQuery, onReady, onSearch = () => {} }: Props = $props();
+
+let editorQuery = $state(untrack(() => initialEditorQuery));
+
+$effect(() => {
+	onReady?.((q) => {
+		editorQuery = q;
+	});
+});
+</script>
+
+<MentionDropdown model={model} editorQuery={editorQuery} onSearch={onSearch} />
diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
index 6c7e0941..466c00ed 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
@@ -7,7 +7,9 @@ import { m } from '$lib/paraglide/messages.js';
 import type { components } from '$lib/generated/api';
 import type { PersonMention } from '$lib/shared/types';
 import { deserialize, serialize } from '$lib/shared/discussion/mentionSerializer';
+import { debounce } from '$lib/shared/utils/debounce';
 import MentionDropdown from './MentionDropdown.svelte';
+import { MAX_QUERY_LENGTH, SEARCH_DEBOUNCE_MS, SEARCH_RESULT_LIMIT } from './mentionConstants';
 
 type Person = components['schemas']['Person'];
 
@@ -33,6 +35,13 @@ let {
 
 let editorEl: HTMLDivElement;
 let editor: Editor | null = null;
+// Hoisted so onDestroy can guarantee the imperatively-mounted dropdown is
+// torn down even if Tiptap's suggestion plugin onExit didn't fire (e.g. when
+// the host component is unmounted while the dropdown is still open).
+let mountedDropdown: object | null = null;
+// Hoisted so onDestroy can cancel any pending fetch — otherwise a trailing
+// debounced search can fire after the editor is gone and pollute later tests.
+let cancelPendingSearch: (() => void) | null = null;
 
 // Single reactive state object shared with MentionDropdown. Mutating these
 // fields propagates to the mounted dropdown via Svelte's $state proxy —
@@ -42,10 +51,12 @@ let dropdownState = $state<{
 	items: Person[];
 	command: (item: Person) => void;
 	clientRect: (() => DOMRect | null) | null;
+	editorQuery: string;
 }>({
 	items: [],
 	command: () => {},
-	clientRect: null
+	clientRect: null,
+	editorQuery: ''
 });
 
 type DropdownExports = {
@@ -138,16 +149,13 @@ onMount(() => {
 					// Nora #5618 #3 — separate issue tracks the GET /api/persons
 					// response-shape audit (PersonSummaryDTO leaks `notes`).
 					// ─────────────────────────────────────────────────────────────
-					items: async ({ query }: { query: string }) => {
-						if (!query) return [];
-						try {
-							const res = await fetch(`/api/persons?q=${encodeURIComponent(query)}`);
-							if (!res.ok) return [];
-							return ((await res.json()) as Person[]).slice(0, 5);
-						} catch {
-							return [];
-						}
-					},
+					// Tiptap's suggestion plugin requires an `items()` callback to keep
+					// the dropdown alive, but the actual fetch is owned by `runSearch`
+					// below — routed through the dropdown's search input via the
+					// debounced `onSearch` channel. Returning `[]` here keeps Tiptap
+					// happy without firing a duplicate per-keystroke fetch.
+					// Markus #5616 / Felix / Nora / Sara on PR #629.
+					items: async () => [],
 					// AC-1 fix: insert the typed query as displayName, not person.displayName.
 					command({ editor: ed, range, props }) {
 						const p = props as unknown as { personId: string; displayName: string };
@@ -165,7 +173,6 @@ onMount(() => {
 							.run();
 					},
 					render() {
-						let component: object | null = null;
 						let exports: DropdownExports | null = null;
 
 						// Tiptap's SuggestionProps types `command` against the default
@@ -178,25 +185,84 @@ onMount(() => {
 							clientRect?: (() => DOMRect | null) | null;
 						};
 
+						// Request-token guard: every onSearch invocation bumps `requestId`;
+						// runSearch captures the id active when its fetch starts and discards
+						// the response if a newer onSearch has fired since. Without this, a
+						// late response can repopulate the dropdown after the user cleared
+						// the search input. Sara on PR #629.
+						let requestId = 0;
+						const runSearch = async (query: string) => {
+							const id = requestId;
+							try {
+								// Defensive client-side cap — server-side enforcement is tracked
+								// separately. Markus on PR #629.
+								const res = await fetch(
+									`/api/persons?q=${encodeURIComponent(query)}&limit=${SEARCH_RESULT_LIMIT}`
+								);
+								if (id !== requestId) return;
+								if (!res.ok) {
+									dropdownState.items = [];
+									return;
+								}
+								const data = (await res.json()) as Person[];
+								if (id !== requestId) return;
+								dropdownState.items = data.slice(0, SEARCH_RESULT_LIMIT);
+							} catch {
+								if (id !== requestId) return;
+								dropdownState.items = [];
+							}
+						};
+						const debouncedSearch = debounce(runSearch, SEARCH_DEBOUNCE_MS);
+						cancelPendingSearch = () => debouncedSearch.cancel();
+						const onSearch = (query: string) => {
+							requestId++;
+							if (query.trim() === '') {
+								debouncedSearch.cancel();
+								dropdownState.items = [];
+								return;
+							}
+							debouncedSearch(query);
+						};
+
 						const updateState = (renderProps: LooseRenderProps) => {
-							dropdownState.items = renderProps.items as Person[];
+							// Clip once here so both the inserted displayName and the
+							// dropdown's editor-mirror see the same value. The dropdown
+							// already clips the mirror (Nora #1 CWE-400), but without
+							// clipping at the command boundary an unclipped query would
+							// still flow through as the inserted displayName — visible
+							// UI divergence between "what I searched" and "what was
+							// inserted". Felix #3 on PR #629.
+							const clippedQuery = renderProps.query.slice(0, MAX_QUERY_LENGTH);
 							// AC-1: pass typed query as displayName, not person.displayName
 							dropdownState.command = (item: Person) =>
 								renderProps.command({
 									personId: item.id,
-									displayName: renderProps.query
+									displayName: clippedQuery
 								});
 							dropdownState.clientRect = renderProps.clientRect ?? null;
+							dropdownState.editorQuery = clippedQuery;
 						};
 
 						return {
 							onStart(renderProps) {
-								updateState(renderProps as unknown as LooseRenderProps);
+								const loose = renderProps as unknown as LooseRenderProps;
+								updateState(loose);
+								// MentionDropdown reads `editorQuery` off the shared state
+								// proxy via its `editorQuery` prop binding below — this is
+								// the same pattern as `model.items`. We do not pass it as a
+								// separate prop because Svelte 5's mount() does not expose
+								// settable prop accessors, so we route through the proxy.
 								const mounted = mount(MentionDropdown, {
 									target: document.body,
-									props: { model: dropdownState }
+									props: {
+										model: dropdownState,
+										get editorQuery() {
+											return dropdownState.editorQuery;
+										},
+										onSearch
+									}
 								});
-								component = mounted as object;
+								mountedDropdown = mounted as object;
 								exports = mounted as unknown as DropdownExports;
 							},
 							onUpdate(renderProps) {
@@ -208,9 +274,16 @@ onMount(() => {
 								return exports?.onKeyDown(event) ?? false;
 							},
 							onExit() {
-								if (component) {
-									unmount(component);
-									component = null;
+								// Cancel any pending debounce so a closed dropdown's trailing
+								// runSearch cannot fire against the *next* dropdown's state.
+								// The hoisted `cancelPendingSearch` would be overwritten by
+								// the next render()'s onStart before the trailing call fires,
+								// so we cancel locally via the closure-scoped debouncedSearch.
+								// Felix #1 on PR #629.
+								debouncedSearch.cancel();
+								if (mountedDropdown) {
+									unmount(mountedDropdown);
+									mountedDropdown = null;
 									exports = null;
 								}
 							}
@@ -253,7 +326,15 @@ onMount(() => {
 });
 
 onDestroy(() => {
+	cancelPendingSearch?.();
 	editor?.destroy();
+	// Tiptap suggestion onExit usually unmounts the dropdown, but if the host
+	// component is destroyed while a suggestion is active the dropdown can
+	// outlive the editor — clean it up explicitly.
+	if (mountedDropdown) {
+		unmount(mountedDropdown);
+		mountedDropdown = null;
+	}
 });
 
 // Keep the data-placeholder attribute in sync with actual emptiness so the
diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
index 92faa307..3b58a62f 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
@@ -8,29 +8,45 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
-import PersonMentionEditorHost from './PersonMentionEditor.test-host.svelte';
+import { tick } from 'svelte';
+import PersonMentionEditorHost from './PersonMentionEditor.test-fixture.svelte';
 import type { components } from '$lib/generated/api';
 import { m } from '$lib/paraglide/messages.js';
+// Single source of truth for the debounce window — imported from the shared
+// module so the test cannot drift from production. Sara on PR #629 round 3.
+import { SEARCH_DEBOUNCE_MS } from './mentionConstants';
 
 type Person = components['schemas']['Person'];
 type PersonMention = components['schemas']['PersonMention'];
 
+/**
+ * Headroom above SEARCH_DEBOUNCE_MS for the debounce-window wait
+ * assertions in this file. 350 ms is calibrated against CI-runner jitter
+ * we observed pre-#629; dropping it below ~200 ms reintroduces flake.
+ * See PR #629 round-2 review comment #10935 (Sara).
+ */
+const POST_DEBOUNCE_SLACK_MS = 350;
+
 const AUGUSTE: Person = {
 	id: 'p-aug',
 	firstName: 'Auguste',
 	lastName: 'Raddatz',
 	displayName: 'Auguste Raddatz',
+	personType: 'PERSON',
+	familyMember: false,
 	birthYear: 1882,
 	deathYear: 1944
-} as unknown as Person;
+};
 
 const ANNA: Person = {
 	id: 'p-anna',
 	firstName: 'Anna',
 	lastName: 'Schmidt',
 	displayName: 'Anna Schmidt',
+	personType: 'PERSON',
+	familyMember: false,
 	birthYear: 1860
-} as unknown as Person;
+};
 
 function mockFetchWithPersons(persons: Person[] = [AUGUSTE, ANNA]) {
 	vi.stubGlobal(
@@ -125,6 +141,20 @@ describe('PersonMentionEditor — typeahead', () => {
 		});
 	});
 
+	it('appends &limit=5 to the fetch URL (defensive client-side cap, Markus on PR #629)', async () => {
+		const fetchMock = vi
+			.fn()
+			.mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([AUGUSTE]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+
+		await vi.waitFor(() => {
+			expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('limit=5'));
+		});
+	});
+
 	it('shows life dates next to the name in the dropdown', async () => {
 		mockFetchWithPersons();
 		renderHost();
@@ -142,8 +172,15 @@ describe('PersonMentionEditor — typeahead', () => {
 
 		await userEvent.type(page.getByRole('textbox'), '@xyz');
 
-		await vi.waitFor(async () => {
-			await expect.element(page.getByText('Keine Personen gefunden')).toBeInTheDocument();
+		// The visible empty-state <p> (text-ink-3) shows the copy. The persistent
+		// sr-only aria-live region also contains the same copy, so we scope to the
+		// visible element to avoid a multi-match resolution in expect.element.
+		await vi.waitFor(() => {
+			const visibleEmptyP = document.querySelector(
+				'[role="listbox"] p.text-ink-3'
+			) as HTMLElement | null;
+			expect(visibleEmptyP).not.toBeNull();
+			expect(visibleEmptyP!.textContent ?? '').toContain('Keine Personen gefunden');
 		});
 	});
 
@@ -161,6 +198,254 @@ describe('PersonMentionEditor — typeahead', () => {
 	});
 });
 
+// ─── AC-2/3: search input drives the person fetch (debounced) ───────────────
+
+describe('PersonMentionEditor — AC-2/3: search input drives fetch', () => {
+	it('editing the search input fires a debounced fetch with the new query', async () => {
+		const fetchMock = vi
+			.fn()
+			.mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([AUGUSTE]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		// Open the dropdown so the search input is reachable.
+		await userEvent.type(page.getByRole('textbox'), '@');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toBeVisible();
+		});
+
+		const fetchesBeforeSearch = fetchMock.mock.calls.length;
+
+		// `fill` simulates a single input event with the final value — sidesteps
+		// per-keystroke timing of userEvent.type so the test can deterministically
+		// assert that one input event collapses into one debounced fetch.
+		await page.getByRole('searchbox').fill('Walter');
+
+		await vi.waitFor(
+			() => {
+				expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('q=Walter'));
+			},
+			{ timeout: 1000 }
+		);
+
+		const fetchesAfterSearch = fetchMock.mock.calls.length - fetchesBeforeSearch;
+		expect(fetchesAfterSearch).toBe(1);
+	});
+
+	it('fires exactly one /api/persons fetch when the user searches for Walter (regression guard)', async () => {
+		// Regression guard: a previous version of PersonMentionEditor had a
+		// duplicated `items()` callback in the Tiptap suggestion config that
+		// fetched per-keystroke in addition to the debounced search-input fetch
+		// (Markus & Felix round-1). To catch that regression, we must NOT
+		// subtract any baseline — every fetch from render onwards counts.
+		// Sara on PR #629 round 3.
+		const fetchMock = vi
+			.fn()
+			.mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([AUGUSTE]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		// Open the dropdown, then drive the search input via fill() — sidesteps
+		// per-keystroke timing of userEvent.type that Sara flagged round 2.
+		await userEvent.type(page.getByRole('textbox'), '@');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toBeVisible();
+		});
+		await page.getByRole('searchbox').fill('Walter');
+
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+
+		// No baseline subtraction — count ALL /api/persons fetches since render.
+		// If the legacy per-keystroke items() callback returns, typing `@` alone
+		// would already produce one fetch and `fill('Walter')` another, breaking
+		// this assertion.
+		const personsFetches = fetchMock.mock.calls.filter(
+			([url]) => typeof url === 'string' && url.startsWith('/api/persons')
+		);
+		expect(personsFetches.length).toBe(1);
+	});
+
+	it('clearing the search input clears the list without firing a fetch', async () => {
+		const fetchMock = vi
+			.fn()
+			.mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([AUGUSTE]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByText('Auguste Raddatz')).toBeInTheDocument();
+		});
+
+		const fetchesBeforeClear = fetchMock.mock.calls.length;
+
+		await userEvent.clear(page.getByRole('searchbox'));
+
+		// Negative assertion: wait past the debounce window to confirm no
+		// trailing fetch was scheduled. Removing this wait would mask a
+		// re-introduction of the keystroke-driven items() fetch.
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+
+		expect(fetchMock.mock.calls.length).toBe(fetchesBeforeClear);
+		await expect.element(page.getByText('Auguste Raddatz')).not.toBeInTheDocument();
+	});
+});
+
+// ─── Whitespace-only query (Elicit AC-4 ambiguity on PR #629) ───────────────
+
+describe('PersonMentionEditor — whitespace-only query', () => {
+	it('keeps the "Namen eingeben…" prompt and fires no fetch when @ is followed only by spaces', async () => {
+		const fetchMock = vi.fn().mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		await userEvent.type(page.getByRole('textbox'), '@   ');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toBeVisible();
+		});
+
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+
+		// Scope to the visible empty-state <p> (text-ink-3) — the persistent
+		// sr-only aria-live region above contains the same copy.
+		const visibleEmptyP = document.querySelector(
+			'[role="listbox"] p.text-ink-3'
+		) as HTMLElement | null;
+		expect(visibleEmptyP).not.toBeNull();
+		expect(visibleEmptyP!.textContent ?? '').toContain(m.person_mention_search_prompt());
+		expect(fetchMock).not.toHaveBeenCalled();
+	});
+});
+
+// ─── Stale-response race (Sara on PR #629) ───────────────────────────────────
+
+describe('PersonMentionEditor — stale-response race', () => {
+	it('discards a stale response that resolves after the search has been cleared', async () => {
+		let resolveFetch!: (v: { ok: boolean; json: () => Promise<Person[]> }) => void;
+		const pendingResponse = new Promise<{ ok: boolean; json: () => Promise<Person[]> }>((r) => {
+			resolveFetch = r;
+		});
+		const fetchMock = vi.fn().mockReturnValue(pendingResponse);
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		// Open the dropdown and let the debounce fire so a fetch is in flight.
+		await userEvent.type(page.getByRole('textbox'), '@Aug');
+		await vi.waitFor(() => {
+			expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('/api/persons?q=Aug'));
+		});
+
+		// Clear the search input *before* the fetch resolves.
+		await userEvent.clear(page.getByRole('searchbox'));
+		await expect.element(page.getByRole('searchbox')).toHaveValue('');
+
+		// The stale fetch now resolves with persons. The dropdown must stay empty.
+		resolveFetch({ ok: true, json: () => Promise.resolve([AUGUSTE]) });
+		// Flush pending Svelte reactivity so any (non-)update from the stale
+		// fetch resolution has landed before we assert. expect.element already
+		// polls, so no fixed-timeout fallback is needed. Sara on PR #629 round 4.
+		await tick();
+
+		await expect.element(page.getByText('Auguste Raddatz')).not.toBeInTheDocument();
+	});
+});
+
+// ─── 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. Scope to the
+		// visible <p> (text-ink-3) — the persistent sr-only live region
+		// above contains the same copy.
+		const visibleEmptyP = document.querySelector(
+			'[role="listbox"] p.text-ink-3'
+		) as HTMLElement | null;
+		expect(visibleEmptyP).not.toBeNull();
+		expect(visibleEmptyP!.textContent ?? '').toContain(m.person_mention_popup_empty());
+	});
+
+	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));
+
+		const visibleEmptyP = document.querySelector(
+			'[role="listbox"] p.text-ink-3'
+		) as HTMLElement | null;
+		expect(visibleEmptyP).not.toBeNull();
+		expect(visibleEmptyP!.textContent ?? '').toContain(m.person_mention_popup_empty());
+	});
+});
+
+// ─── onExit cancels pending debounce (Felix #1 on PR #629) ───────────────────
+
+describe('PersonMentionEditor — onExit cancels pending debounce', () => {
+	it('cancels the pending debounced fetch when Escape closes the dropdown before the debounce fires', async () => {
+		const fetchMock = vi.fn().mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		// Open the dropdown by typing @ + a query in the editor.
+		await userEvent.type(page.getByRole('textbox'), '@A');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toBeVisible();
+		});
+
+		// Wait for any in-flight fetch from opening the dropdown to settle.
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+		const fetchesBeforeEscape = fetchMock.mock.calls.length;
+
+		// Trigger a new debounced search (queues runSearch after 150 ms), then
+		// immediately Escape *while focus is back in the editor* so Tiptap's
+		// suggestion-plugin Escape handler fires onExit before the debounce.
+		// Without onExit cancelling the pending debounce, runSearch executes
+		// against the now-unmounted dropdown's state.
+		await page.getByRole('searchbox').fill('Walter');
+		// Focus the editor so the Escape lands on Tiptap's suggestion handler.
+		(page.getByRole('textbox').element() as HTMLElement).focus();
+		await userEvent.keyboard('{Escape}');
+
+		// Wait past the debounce window. If onExit did not cancel the pending
+		// debounce, a fetch with q=Walter would still fire here.
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+
+		const newFetches = fetchMock.mock.calls.slice(fetchesBeforeEscape);
+		const walterFetches = newFetches.filter(
+			([url]) => typeof url === 'string' && url.includes('q=Walter')
+		);
+		expect(walterFetches.length).toBe(0);
+	});
+});
+
+// ─── AC-1: search input prefilled with text typed after @ ───────────────────
+
+describe('PersonMentionEditor — AC-1: search input prefill', () => {
+	it('prefills the dropdown search input with the text typed after @', async () => {
+		mockFetchEmpty();
+		renderHost();
+
+		await userEvent.type(page.getByRole('textbox'), '@WdG');
+
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toHaveValue('WdG');
+		});
+	});
+});
+
 // ─── AC-1: typed text becomes displayName, not DB name ───────────────────────
 
 describe('PersonMentionEditor — AC-1: typed text as displayName', () => {
@@ -229,6 +514,39 @@ describe('PersonMentionEditor — AC-1: typed text as displayName', () => {
 		});
 	});
 
+	it('clips the inserted displayName to MAX_QUERY_LENGTH=100 chars (Felix #3 on PR #629)', async () => {
+		// CWE-400 amplification: the dropdown clips its search input + mirror at
+		// 100 chars (Nora #1), but the host editor was passing the unclipped
+		// renderProps.query straight through to displayName — so a 105-char
+		// @-suffix in the editor could insert a 105-char displayName into the
+		// sidecar even though the dropdown only searched the first 100.
+		mockFetchWithPersons();
+		const host = renderHost();
+
+		// Type @ + 105 'A' chars in the contenteditable. The renderProps.query
+		// fed into the command callback derives from the editor text after `@`,
+		// not the dropdown's searchbox — so we must drive the editor.
+		await userEvent.type(page.getByRole('textbox'), '@' + 'A'.repeat(105));
+
+		// The mocked /api/persons returns AUGUSTE for any query — wait for it.
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('option', { name: /Auguste Raddatz/ })).toBeVisible();
+		});
+
+		const option = (await page
+			.getByRole('option', { name: /Auguste Raddatz/ })
+			.element()) as HTMLElement;
+		option.dispatchEvent(new MouseEvent('mousedown', { bubbles: true, cancelable: true }));
+
+		await vi.waitFor(() => {
+			expect(host.snapshot.mentionedPersons).toHaveLength(1);
+			// Tight assertion: input is 105 chars, cap is exactly 100. Using
+			// `toHaveLength(100)` discriminates "clip works" from "clip works
+			// AND nothing weakened it to e.g. 95". Sara on PR #629 round 4.
+			expect(host.snapshot.mentionedPersons[0].displayName).toHaveLength(100);
+		});
+	});
+
 	it('does not duplicate the sidecar entry when the same person is selected twice', async () => {
 		mockFetchWithPersons();
 		const host = renderHost({
diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.test-host.svelte b/frontend/src/lib/shared/discussion/PersonMentionEditor.test-fixture.svelte
similarity index 100%
rename from frontend/src/lib/shared/discussion/PersonMentionEditor.test-host.svelte
rename to frontend/src/lib/shared/discussion/PersonMentionEditor.test-fixture.svelte
diff --git a/frontend/src/lib/shared/discussion/mentionConstants.ts b/frontend/src/lib/shared/discussion/mentionConstants.ts
new file mode 100644
index 00000000..04721782
--- /dev/null
+++ b/frontend/src/lib/shared/discussion/mentionConstants.ts
@@ -0,0 +1,10 @@
+/** Shared knobs for the @mention typeahead. Single source of truth for
+ *  the dropdown component and the host editor — keeps the layered length
+ *  cap and the debounce window consistent across both files. */
+export const MAX_QUERY_LENGTH = 100;
+export const SEARCH_DEBOUNCE_MS = 150;
+/** Defensive client-side cap on the result list. Single consumer today
+ *  (PersonMentionEditor), kept here for symmetry with the other limit
+ *  knobs so the @mention configuration lives in one place. Felix #1 on
+ *  PR #629 round 4. */
+export const SEARCH_RESULT_LIMIT = 5;
diff --git a/frontend/src/lib/shared/services/confirm.svelte.test.ts b/frontend/src/lib/shared/services/confirm.svelte.test.ts
index a1449b96..3b9126f7 100644
--- a/frontend/src/lib/shared/services/confirm.svelte.test.ts
+++ b/frontend/src/lib/shared/services/confirm.svelte.test.ts
@@ -1,7 +1,7 @@
 import { describe, it, expect, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
-import TestHost from './confirm.test-host.svelte';
+import TestHost from './confirm.test-fixture.svelte';
 import type { ConfirmService } from './confirm.svelte.js';
 
 afterEach(cleanup);
diff --git a/frontend/src/lib/shared/services/confirm.test-host.svelte b/frontend/src/lib/shared/services/confirm.test-fixture.svelte
similarity index 100%
rename from frontend/src/lib/shared/services/confirm.test-host.svelte
rename to frontend/src/lib/shared/services/confirm.test-fixture.svelte
diff --git a/frontend/src/lib/shared/utils/debounce.ts b/frontend/src/lib/shared/utils/debounce.ts
index b8958e45..3d876de9 100644
--- a/frontend/src/lib/shared/utils/debounce.ts
+++ b/frontend/src/lib/shared/utils/debounce.ts
@@ -1,12 +1,25 @@
 /**
  * Returns a debounced version of fn that delays invocation until after
- * `delay` ms have elapsed since the last call.
+ * `delay` ms have elapsed since the last call. The returned function
+ * exposes a `cancel()` method that DROPS (does not flush) the pending
+ * trailing invocation — essential when the host context (a destroyed
+ * component, an unmounted editor) shouldn't fire the trailing call.
  */
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
-export function debounce<T extends (...args: any[]) => void>(fn: T, delay: number): T {
-	let timer: ReturnType<typeof setTimeout>;
-	return ((...args: Parameters<T>) => {
-		clearTimeout(timer);
+export function debounce<T extends (...args: any[]) => void>(
+	fn: T,
+	delay: number
+): T & { cancel: () => void } {
+	let timer: ReturnType<typeof setTimeout> | undefined;
+	const wrapped = ((...args: Parameters<T>) => {
+		if (timer !== undefined) clearTimeout(timer);
 		timer = setTimeout(() => fn(...args), delay);
-	}) as T;
+	}) as T & { cancel: () => void };
+	wrapped.cancel = () => {
+		if (timer !== undefined) {
+			clearTimeout(timer);
+			timer = undefined;
+		}
+	};
+	return wrapped;
 }