diff --git a/lib/services/route53-dns-delegation.test.ts b/lib/services/route53-dns-delegation.test.ts
new file mode 100644
index 0000000..74d8f26
--- /dev/null
+++ b/lib/services/route53-dns-delegation.test.ts
@@ -0,0 +1,87 @@
+import { describe, it, expect } from 'vitest';
+import { normalizeNsList, compareNsDelegation } from './route53-dns-delegation';
+
+describe('normalizeNsList', () => {
+  it('lowercases and strips trailing dots', () => {
+    expect(normalizeNsList(['NS-123.AWSDNS-45.com.', 'ns-999.awsdns-01.org'])).toEqual([
+      'ns-123.awsdns-45.com',
+      'ns-999.awsdns-01.org',
+    ]);
+  });
+
+  it('returns [] for null', () => {
+    expect(normalizeNsList(null)).toEqual([]);
+  });
+
+  it('returns [] for undefined', () => {
+    expect(normalizeNsList(undefined)).toEqual([]);
+  });
+
+  it('returns [] for a non-array input', () => {
+    expect(normalizeNsList('not-an-array')).toEqual([]);
+    expect(normalizeNsList(42)).toEqual([]);
+    expect(normalizeNsList({})).toEqual([]);
+  });
+
+  it('de-duplicates and sorts so ordering differences never register as a mismatch', () => {
+    expect(normalizeNsList(['ns-2.awsdns.com.', 'ns-1.awsdns.com', 'ns-1.awsdns.com.'])).toEqual([
+      'ns-1.awsdns.com',
+      'ns-2.awsdns.com',
+    ]);
+  });
+
+  it('drops empty strings after trimming', () => {
+    expect(normalizeNsList(['  ', 'ns-1.awsdns.com', ''])).toEqual(['ns-1.awsdns.com']);
+  });
+});
+
+describe('compareNsDelegation', () => {
+  it('returns mismatch: false with empty diffs when sets are identical', () => {
+    const result = compareNsDelegation(
+      ['ns-1.awsdns.com', 'ns-2.awsdns.com'],
+      ['NS-1.AWSDNS.com.', 'ns-2.awsdns.com.'],
+    );
+    expect(result.mismatch).toBe(false);
+    expect(result.missingFromLive).toEqual([]);
+    expect(result.extraInLive).toEqual([]);
+  });
+
+  it('returns mismatch: true with populated missingFromLive when an authoritative NS is absent from the live answer', () => {
+    const result = compareNsDelegation(
+      ['ns-1.awsdns.com', 'ns-2.awsdns.com'],
+      ['ns-1.awsdns.com'],
+    );
+    expect(result.mismatch).toBe(true);
+    expect(result.missingFromLive).toEqual(['ns-2.awsdns.com']);
+    expect(result.extraInLive).toEqual([]);
+  });
+
+  it('returns mismatch: true with populated extraInLive when the live answer contains an NS Route 53 does not consider authoritative', () => {
+    const result = compareNsDelegation(
+      ['ns-1.awsdns.com'],
+      ['ns-1.awsdns.com', 'ns-rogue.example.com'],
+    );
+    expect(result.mismatch).toBe(true);
+    expect(result.extraInLive).toEqual(['ns-rogue.example.com']);
+    expect(result.missingFromLive).toEqual([]);
+  });
+
+  it('returns mismatch: true when live has no answer but authoritative is non-empty (delegation problem, not a pass)', () => {
+    const result = compareNsDelegation(['ns1.example.com'], []);
+    expect(result.mismatch).toBe(true);
+  });
+
+  it('returns mismatch: false when authoritative is empty (unjudgeable, must not false-alarm)', () => {
+    const result = compareNsDelegation([], ['ns1.example.com']);
+    expect(result.mismatch).toBe(false);
+  });
+
+  it('is case-insensitive and trailing-dot-insensitive on both sides', () => {
+    const result = compareNsDelegation(['NS-1.AWSDNS.COM.'], ['ns-1.awsdns.com']);
+    expect(result.mismatch).toBe(false);
+  });
+
+  it('handles null/undefined inputs on both sides without throwing', () => {
+    expect(compareNsDelegation(null, undefined)).toMatchObject({ mismatch: false });
+  });
+});