diff --git a/frontend/src/lib/shared/discussion/MentionDropdown.svelte b/frontend/src/lib/shared/discussion/MentionDropdown.svelte
index 4191879d..cc89df6c 100644
--- a/frontend/src/lib/shared/discussion/MentionDropdown.svelte
+++ b/frontend/src/lib/shared/discussion/MentionDropdown.svelte
@@ -7,6 +7,15 @@ import { m } from '$lib/paraglide/messages.js';
 
 type Person = components['schemas']['Person'];
 
+// 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.
+const MAX_QUERY_LENGTH = 100;
+
 // The dropdown receives a single reactive state object. PersonMentionEditor
 // mutates fields on this object (model.items = ..., etc.) and Svelte's $state
 // proxy reactivity propagates the change here. This is the supported way to
@@ -30,7 +39,7 @@ let {
 	onSearch?: (query: string) => void;
 } = $props();
 
-let searchQuery = $state(untrack(() => editorQuery));
+let searchQuery = $state(untrack(() => editorQuery.slice(0, MAX_QUERY_LENGTH)));
 let userHasEdited = $state(false);
 
 // Mirror the editor's typed text until the user takes ownership.
@@ -42,7 +51,7 @@ let userHasEdited = $state(false);
 // user types into the input. Felix #1 on PR #629.
 $effect(() => {
 	if (!userHasEdited) {
-		searchQuery = editorQuery;
+		searchQuery = editorQuery.slice(0, MAX_QUERY_LENGTH);
 	}
 });
 
@@ -172,7 +181,7 @@ function selectItem(item: Person) {
 				id="mention-search"
 				type="search"
 				data-test-search-input
-				maxlength="100"
+				maxlength={MAX_QUERY_LENGTH}
 				class="min-h-[44px] w-full bg-transparent font-sans text-sm 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}
diff --git a/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts b/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
index 7ead48f9..47549bd5 100644
--- a/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
+++ b/frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts
@@ -186,6 +186,16 @@ describe('MentionDropdown — search input', () => {
 		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('invokes onSearch with the current value whenever the user types', async () => {
 		const onSearch = vi.fn();
 		render(MentionDropdown, { props: { model: baseModel(), onSearch } });