diff --git a/frontend/src/lib/utils/mention.spec.ts b/frontend/src/lib/utils/mention.spec.ts
index 4f15bc10..04b659b1 100644
--- a/frontend/src/lib/utils/mention.spec.ts
+++ b/frontend/src/lib/utils/mention.spec.ts
@@ -24,6 +24,17 @@ describe('escapeHtml', () => {
 	it('escapes ampersand before other entities to avoid double-encoding', () => {
 		expect(escapeHtml('a&<b')).toBe('a&amp;&lt;b');
 	});
+
+	it('escapes apostrophe to &#39;', () => {
+		expect(escapeHtml("d'Artagnan")).toBe('d&#39;Artagnan');
+	});
+
+	it('does not collapse already-encoded entities (re-escapes the &)', () => {
+		// escapeHtml is idempotent by composition: the second pass re-escapes
+		// the & that was added by the first. Pin the property so the helper
+		// can't be "cleverly" optimised to skip it.
+		expect(escapeHtml('&amp;')).toBe('&amp;amp;');
+	});
 });
 
 // ─── detectMention ────────────────────────────────────────────────────────────
diff --git a/frontend/src/lib/utils/mention.ts b/frontend/src/lib/utils/mention.ts
index c0e0f11b..8b91bc1c 100644
--- a/frontend/src/lib/utils/mention.ts
+++ b/frontend/src/lib/utils/mention.ts
@@ -45,15 +45,21 @@ export function extractContent(
 }
 
 /**
- * Escapes the four HTML-special characters that can break out of text content
+ * Escapes the five HTML-special characters that can break out of text content
  * or attribute values. & must be escaped first to avoid double-encoding.
+ *
+ * Includes the apostrophe so the helper is safe in single-quoted attribute
+ * values too — the renderTranscriptionBody anchor template in PR-B2 uses
+ * double quotes today, but a future template change shouldn't open a
+ * stored-XSS hole (Sina #5505 action item).
  */
 export function escapeHtml(str: string): string {
 	return str
 		.replaceAll('&', '&amp;')
 		.replaceAll('<', '&lt;')
 		.replaceAll('>', '&gt;')
-		.replaceAll('"', '&quot;');
+		.replaceAll('"', '&quot;')
+		.replaceAll("'", '&#39;');
 }
 
 /**