diff --git a/server/tests/unit/services/airtrailWriteGate.test.ts b/server/tests/unit/services/airtrailWriteGate.test.ts
new file mode 100644
index 00000000..18405abd
--- /dev/null
+++ b/server/tests/unit/services/airtrailWriteGate.test.ts
@@ -0,0 +1,92 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+/**
+ * The #1240 write gate: pushReservationToAirtrail must NOT write to AirTrail unless
+ * the flight's owner has opted in (airtrail_write_enabled). Collaborators are mocked
+ * so the test exercises just the gate + payload wiring.
+ */
+
+vi.mock('../../../src/db/database', () => ({ db: { prepare: vi.fn() } }));
+vi.mock('../../../src/services/adminService', () => ({ isAddonEnabled: vi.fn(() => true) }));
+vi.mock('../../../src/services/auditLog', () => ({ logError: vi.fn(), logInfo: vi.fn() }));
+vi.mock('../../../src/websocket', () => ({ broadcast: vi.fn() }));
+vi.mock('../../../src/services/reservationService', () => ({
+  getReservation: vi.fn(),
+  getReservationWithJoins: vi.fn(),
+  updateReservation: vi.fn(),
+}));
+vi.mock('../../../src/services/airtrail/airtrailClient', () => ({
+  AirtrailAuthError: class AirtrailAuthError extends Error {},
+  getFlight: vi.fn(),
+  listFlights: vi.fn(),
+  saveFlight: vi.fn(),
+}));
+vi.mock('../../../src/services/airtrail/airtrailMapper', () => ({
+  canonicalHash: vi.fn(() => 'hash'),
+  mapFlightToReservation: vi.fn(() => ({})),
+  entityCode: (e: any) => e?.icao || e?.iata || null,
+}));
+vi.mock('../../../src/services/airtrail/airtrailService', () => ({
+  isAirtrailWriteEnabled: vi.fn(),
+  getAirtrailCredentials: vi.fn(),
+}));
+
+import { pushReservationToAirtrail } from '../../../src/services/airtrail/airtrailSync';
+import { db } from '../../../src/db/database';
+import { getReservationWithJoins } from '../../../src/services/reservationService';
+import { getFlight, saveFlight } from '../../../src/services/airtrail/airtrailClient';
+import { isAirtrailWriteEnabled, getAirtrailCredentials } from '../../../src/services/airtrail/airtrailService';
+
+const linkedRow = { id: 5, trip_id: 9, external_id: '42', external_owner_user_id: 7, sync_enabled: 1 };
+const runSpy = vi.fn();
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  // Route db reads: global sync setting + the linked reservation row.
+  (db.prepare as any).mockImplementation((sql: string) => ({
+    get: () => {
+      if (sql.includes('app_settings')) return { value: 'true' };
+      if (sql.includes('FROM reservations')) return { ...linkedRow };
+      return undefined;
+    },
+    run: (...args: any[]) => {
+      runSpy(sql, args);
+      return {};
+    },
+    all: () => [],
+  }));
+  (getAirtrailCredentials as any).mockReturnValue({ baseUrl: 'https://at.example', apiKey: 'k', allowInsecureTls: false });
+  // GET returns AirTrail-owned detail TREK doesn't model — must survive the writeback.
+  (getFlight as any).mockResolvedValue({ id: 42, from: { iata: 'JFK' }, to: { iata: 'LHR' }, seats: [], departureTerminal: '7' });
+  (saveFlight as any).mockResolvedValue({ id: 42 });
+  (getReservationWithJoins as any).mockReturnValue({
+    external_id: '42',
+    reservation_time: '2021-09-01T19:00',
+    reservation_end_time: '2021-09-02T08:00',
+    notes: 'note',
+    metadata: JSON.stringify({}),
+    endpoints: [
+      { role: 'from', code: 'JFK' },
+      { role: 'to', code: 'LHR' },
+    ],
+  });
+});
+
+describe('pushReservationToAirtrail write gate (#1240)', () => {
+  it('does nothing — and does not detach — when the owner has not opted in', async () => {
+    (isAirtrailWriteEnabled as any).mockReturnValue(false);
+    await pushReservationToAirtrail(5, 9);
+    expect(getFlight).not.toHaveBeenCalled();
+    expect(saveFlight).not.toHaveBeenCalled();
+    expect(runSpy).not.toHaveBeenCalled(); // no detach, no hash write — pure no-op
+  });
+
+  it('writes back, preserving AirTrail-owned fields, when the owner has opted in', async () => {
+    (isAirtrailWriteEnabled as any).mockReturnValue(true);
+    await pushReservationToAirtrail(5, 9);
+    expect(saveFlight).toHaveBeenCalledTimes(1);
+    const payload = (saveFlight as any).mock.calls[0][1];
+    expect(payload.departureTerminal).toBe('7'); // spread preserved the unmanaged field
+    expect(payload.from).toBe('JFK'); // TREK-managed field still applied as a code
+  });
+});
diff --git a/server/tests/unit/services/airtrailWriteback.test.ts b/server/tests/unit/services/airtrailWriteback.test.ts
new file mode 100644
index 00000000..679f8156
--- /dev/null
+++ b/server/tests/unit/services/airtrailWriteback.test.ts
@@ -0,0 +1,38 @@
+import { describe, it, expect, vi } from 'vitest';
+
+// Avoid any real DNS/network from the SSRF guard during saveSettings.
+vi.mock('../../../src/utils/ssrfGuard', () => ({
+  checkSsrf: vi.fn(async () => ({ allowed: true, isPrivate: false })),
+  safeFetch: vi.fn(),
+}));
+
+import { db } from '../../../src/db/database';
+import { createUser } from '../../helpers/factories';
+import {
+  getConnectionSettings,
+  isAirtrailWriteEnabled,
+  saveSettings,
+} from '../../../src/services/airtrail/airtrailService';
+
+describe('airtrail writeback opt-in persistence (#1240)', () => {
+  it('defaults the writeback opt-in to off for a new user', () => {
+    const { user } = createUser(db);
+    expect(isAirtrailWriteEnabled(user.id)).toBe(false);
+    expect(getConnectionSettings(user.id).writeEnabled).toBe(false);
+  });
+
+  it('persists the opt-in and lets it be toggled back off without dropping the key', async () => {
+    const { user } = createUser(db);
+
+    await saveSettings(user.id, 'https://at.example.com', 'secret-key', false, true, null);
+    expect(isAirtrailWriteEnabled(user.id)).toBe(true);
+    const on = getConnectionSettings(user.id);
+    expect(on.writeEnabled).toBe(true);
+    expect(on.connected).toBe(true); // key stored
+
+    // No key supplied keeps the stored key; only the opt-in flips back off.
+    await saveSettings(user.id, 'https://at.example.com', undefined, false, false, null);
+    expect(isAirtrailWriteEnabled(user.id)).toBe(false);
+    expect(getConnectionSettings(user.id).connected).toBe(true);
+  });
+});