diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
index 8714a574..466c00ed 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
@@ -9,7 +9,7 @@ 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 { SEARCH_DEBOUNCE_MS, SEARCH_RESULT_LIMIT } from './mentionConstants';
+import { MAX_QUERY_LENGTH, SEARCH_DEBOUNCE_MS, SEARCH_RESULT_LIMIT } from './mentionConstants';
 
 type Person = components['schemas']['Person'];
 
@@ -225,14 +225,22 @@ onMount(() => {
 						};
 
 						const updateState = (renderProps: LooseRenderProps) => {
+							// 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 = renderProps.query;
+							dropdownState.editorQuery = clippedQuery;
 						};
 
 						return {
diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
index 591a7ef8..0620ecfb 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
@@ -473,6 +473,36 @@ 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);
+			expect(host.snapshot.mentionedPersons[0].displayName.length).toBeLessThanOrEqual(100);
+		});
+	});
+
 	it('does not duplicate the sidecar entry when the same person is selected twice', async () => {
 		mockFetchWithPersons();
 		const host = renderHost({