diff --git a/lib/services/autotask-client.test.ts b/lib/services/autotask-client.test.ts
new file mode 100644
index 0000000..cc50a0d
--- /dev/null
+++ b/lib/services/autotask-client.test.ts
@@ -0,0 +1,73 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { AutotaskClient } from './autotask-client';
+import type { AutotaskConfig } from '@/lib/types/autotask';
+
+const FIXTURE_CONFIG: AutotaskConfig = {
+  apiUrl: 'https://webservices.autotask.net/atservicesrest/v1.0',
+  username: 'fixture@example.com',
+  password: 'fixture-secret',
+  apiIntegrationCode: 'FIXTURE-CODE',
+};
+
+function jsonResponse(body: unknown, ok = true, status = 200): Response {
+  return {
+    ok,
+    status,
+    statusText: ok ? 'OK' : 'Error',
+    text: async () => JSON.stringify(body),
+  } as unknown as Response;
+}
+
+describe('AutotaskClient.getAttachmentContent', () => {
+  const originalFetch = global.fetch;
+
+  beforeEach(() => {
+    global.fetch = vi.fn();
+  });
+
+  afterEach(() => {
+    global.fetch = originalFetch;
+    vi.restoreAllMocks();
+  });
+
+  it('returns items[0] with populated base64 data when the API returns an items-shaped envelope', async () => {
+    const attachment = { id: 555, fullPath: 'rfc.eml', title: 'rfc.eml', data: 'YmFzZTY0LWNvbnRlbnQ=' };
+    (global.fetch as unknown as ReturnType<typeof vi.fn>).mockResolvedValue(
+      jsonResponse({ items: [attachment] })
+    );
+
+    const client = new AutotaskClient(FIXTURE_CONFIG);
+    const result = await client.getAttachmentContent('Tickets', 12345, 555);
+
+    expect(result).toEqual(attachment);
+    expect(global.fetch).toHaveBeenCalledWith(
+      `${FIXTURE_CONFIG.apiUrl}/Tickets/12345/Attachments/555`,
+      expect.objectContaining({ method: 'GET' })
+    );
+  });
+
+  it('returns null when the API returns an empty items array', async () => {
+    (global.fetch as unknown as ReturnType<typeof vi.fn>).mockResolvedValue(
+      jsonResponse({ items: [] })
+    );
+
+    const client = new AutotaskClient(FIXTURE_CONFIG);
+    const result = await client.getAttachmentContent('Tickets', 12345, 999);
+
+    expect(result).toBeNull();
+  });
+
+  it('returns null (not the attachment) when the API returns an {item:...}-shaped response', async () => {
+    // Guards against a future refactor copying uploadAttachment's `.item`
+    // convention onto this method — the live shape is `.items`.
+    const attachment = { id: 555, fullPath: 'rfc.eml', title: 'rfc.eml', data: 'YmFzZTY0LWNvbnRlbnQ=' };
+    (global.fetch as unknown as ReturnType<typeof vi.fn>).mockResolvedValue(
+      jsonResponse({ item: attachment })
+    );
+
+    const client = new AutotaskClient(FIXTURE_CONFIG);
+    const result = await client.getAttachmentContent('Tickets', 12345, 555);
+
+    expect(result).toBeNull();
+  });
+});