diff --git a/frontend/src/lib/shared/discussion/MentionDropdown.svelte b/frontend/src/lib/shared/discussion/MentionDropdown.svelte
index 9732df50..0cc4378c 100644
--- a/frontend/src/lib/shared/discussion/MentionDropdown.svelte
+++ b/frontend/src/lib/shared/discussion/MentionDropdown.svelte
@@ -4,17 +4,18 @@ import type { components } from '$lib/generated/api';
 import { formatLifeDateRange } from '$lib/person/personLifeDates';
 import { untrack } from 'svelte';
 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;
+// 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'];
 
 // The dropdown receives a single reactive state object. PersonMentionEditor
 // mutates fields on this object (model.items = ..., etc.) and Svelte's $state
diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
index 6512f2fd..8714a574 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte
@@ -9,9 +9,7 @@ import type { PersonMention } from '$lib/shared/types';
 import { deserialize, serialize } from '$lib/shared/discussion/mentionSerializer';
 import { debounce } from '$lib/shared/utils/debounce';
 import MentionDropdown from './MentionDropdown.svelte';
-
-const SEARCH_DEBOUNCE_MS = 150;
-const SEARCH_RESULT_LIMIT = 5;
+import { SEARCH_DEBOUNCE_MS, SEARCH_RESULT_LIMIT } from './mentionConstants';
 
 type Person = components['schemas']['Person'];
 
diff --git a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
index e21eb412..591a7ef8 100644
--- a/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
+++ b/frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts
@@ -11,14 +11,14 @@ import { page, userEvent } from 'vitest/browser';
 import PersonMentionEditorHost from './PersonMentionEditor.test-host.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'];
 
-// Mirror of the debounce in PersonMentionEditor.svelte. Naming the magic and
-// using a generous slack (SEARCH_DEBOUNCE_MS + 350 = 500 ms) kills CI-jitter
-// flakiness Sara raised on PR #629.
-const SEARCH_DEBOUNCE_MS = 150;
+// Slack on top of the debounce to absorb CI jitter (total 500 ms is generous).
 const POST_DEBOUNCE_SLACK_MS = 350;
 
 const AUGUSTE: Person = {
diff --git a/frontend/src/lib/shared/discussion/mentionConstants.ts b/frontend/src/lib/shared/discussion/mentionConstants.ts
new file mode 100644
index 00000000..3b946345
--- /dev/null
+++ b/frontend/src/lib/shared/discussion/mentionConstants.ts
@@ -0,0 +1,6 @@
+/** 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;
+export const SEARCH_RESULT_LIMIT = 5;