diff --git a/lib/services/route53-sync-service.test.ts b/lib/services/route53-sync-service.test.ts
new file mode 100644
index 0000000..c8347fa
--- /dev/null
+++ b/lib/services/route53-sync-service.test.ts
@@ -0,0 +1,126 @@
+/**
+ * lib/services/route53-sync-service.ts unit tests.
+ *
+ * Only buildDriftHistoryRows is exercised here — a pure function taking
+ * hand-constructed NormalizedRecordSet maps, no AWS or Postgres mocking
+ * required (matches the plan's stated test scope for this file).
+ */
+
+import { describe, it, expect } from 'vitest';
+import { buildDriftHistoryRows } from './route53-sync-service';
+import type { NormalizedRecordSet } from './route53-record-key';
+
+function makeRecordSet(overrides: Partial<NormalizedRecordSet> = {}): NormalizedRecordSet {
+  return {
+    recordKey: 'Z123:www.example.com.:A:',
+    zoneId: 'Z123',
+    name: 'www.example.com.',
+    type: 'A',
+    setIdentifier: null,
+    ttl: 300,
+    resourceRecords: [{ value: '10.0.0.1' }],
+    aliasTarget: null,
+    ...overrides,
+  };
+}
+
+describe('buildDriftHistoryRows', () => {
+  it('produces one update row with non-null before/after when TTL changes', () => {
+    const key = 'Z123:www.example.com.:A:';
+    const prev = makeRecordSet({ ttl: 300 });
+    const next = makeRecordSet({ ttl: 600 });
+
+    const rows = buildDriftHistoryRows(
+      new Map([[key, prev]]),
+      new Map([[key, next]]),
+      'Z123'
+    );
+
+    expect(rows).toHaveLength(1);
+    expect(rows[0].changeAction).toBe('update');
+    expect(rows[0].zoneId).toBe('Z123');
+    expect(rows[0].recordKey).toBe(key);
+    expect(rows[0].beforeValue).not.toBeNull();
+    expect(rows[0].afterValue).not.toBeNull();
+  });
+
+  it('produces a delete row with non-null before and null after when the mirror row has no AWS match', () => {
+    const key = 'Z123:gone.example.com.:A:';
+    const prev = makeRecordSet({ recordKey: key, name: 'gone.example.com.' });
+
+    const rows = buildDriftHistoryRows(new Map([[key, prev]]), new Map(), 'Z123');
+
+    expect(rows).toHaveLength(1);
+    expect(rows[0].changeAction).toBe('delete');
+    expect(rows[0].beforeValue).not.toBeNull();
+    expect(rows[0].afterValue).toBeNull();
+  });
+
+  it('produces a create row with null before and non-null after when AWS has no matching mirror row', () => {
+    const key = 'Z123:new.example.com.:A:';
+    const next = makeRecordSet({ recordKey: key, name: 'new.example.com.' });
+
+    const rows = buildDriftHistoryRows(new Map(), new Map([[key, next]]), 'Z123');
+
+    expect(rows).toHaveLength(1);
+    expect(rows[0].changeAction).toBe('create');
+    expect(rows[0].beforeValue).toBeNull();
+    expect(rows[0].afterValue).not.toBeNull();
+  });
+
+  it('produces zero rows when mirror and AWS recordsets are identical', () => {
+    const key = 'Z123:www.example.com.:A:';
+    const prev = makeRecordSet();
+    const next = makeRecordSet();
+
+    const rows = buildDriftHistoryRows(
+      new Map([[key, prev]]),
+      new Map([[key, next]]),
+      'Z123'
+    );
+
+    expect(rows).toHaveLength(0);
+  });
+
+  it('stores the whole recordset (name, type, setIdentifier, ttl, resourceRecords, aliasTarget) in before/after, not a per-field delta', () => {
+    const key = 'Z123:www.example.com.:A:';
+    const prev = makeRecordSet({ ttl: 300 });
+    const next = makeRecordSet({ ttl: 600 });
+
+    const rows = buildDriftHistoryRows(
+      new Map([[key, prev]]),
+      new Map([[key, next]]),
+      'Z123'
+    );
+
+    expect(rows[0].afterValue).toEqual({
+      name: 'www.example.com.',
+      type: 'A',
+      setIdentifier: null,
+      ttl: 600,
+      resourceRecords: [{ value: '10.0.0.1' }],
+      aliasTarget: null,
+    });
+  });
+
+  it('a record changed by Pulse CRUD in the same window is still tagged as drift by the sync (sync cannot distinguish actor)', () => {
+    // The sync has no notion of "who changed this" — it only compares
+    // mirror vs. AWS-live state. A CRUD-originated change looks identical
+    // to an out-of-band AWS console edit from the sync's perspective, so it
+    // still produces a drift row here. The CRUD route's own pulse_crud row
+    // (written elsewhere, not by this function) is the authoritative one;
+    // both rows are expected to coexist in the ledger.
+    const key = 'Z123:www.example.com.:A:';
+    const prev = makeRecordSet({ ttl: 300 });
+    const next = makeRecordSet({ ttl: 900 }); // as if Pulse CRUD had just updated this
+
+    const rows = buildDriftHistoryRows(
+      new Map([[key, prev]]),
+      new Map([[key, next]]),
+      'Z123'
+    );
+
+    expect(rows).toHaveLength(1);
+    expect(rows[0].changeAction).toBe('update');
+  });
+});