diff --git a/frontend/src/lib/utils/commentDeepLink.spec.ts b/frontend/src/lib/utils/commentDeepLink.spec.ts
new file mode 100644
index 00000000..15c7f103
--- /dev/null
+++ b/frontend/src/lib/utils/commentDeepLink.spec.ts
@@ -0,0 +1,14 @@
+import { describe, it, expect } from 'vitest';
+import { buildCommentHref } from './commentDeepLink';
+
+describe('buildCommentHref', () => {
+	it('includes both commentId and annotationId when annotationId is present', () => {
+		const href = buildCommentHref('doc-1', 'comment-2', 'annot-3');
+		expect(href).toBe('/documents/doc-1?commentId=comment-2&annotationId=annot-3');
+	});
+
+	it('omits annotationId when null', () => {
+		const href = buildCommentHref('doc-1', 'comment-2', null);
+		expect(href).toBe('/documents/doc-1?commentId=comment-2');
+	});
+});
diff --git a/frontend/src/lib/utils/commentDeepLink.ts b/frontend/src/lib/utils/commentDeepLink.ts
new file mode 100644
index 00000000..95b5ef1d
--- /dev/null
+++ b/frontend/src/lib/utils/commentDeepLink.ts
@@ -0,0 +1,8 @@
+export function buildCommentHref(
+	documentId: string,
+	commentId: string,
+	annotationId: string | null
+): string {
+	const base = `/documents/${documentId}?commentId=${commentId}`;
+	return annotationId ? `${base}&annotationId=${annotationId}` : base;
+}