diff --git a/frontend/src/contexts/FeatureFlagsContext.tsx b/frontend/src/contexts/FeatureFlagsContext.tsx
index 80a53af2..4283f970 100644
--- a/frontend/src/contexts/FeatureFlagsContext.tsx
+++ b/frontend/src/contexts/FeatureFlagsContext.tsx
@@ -195,7 +195,19 @@ export const FeatureFlagsProvider: React.FC<ProviderProps> = ({ children }) => {
 
   const setFlag = useCallback((key: FeatureKey, value: boolean) => {
     if (key === 'galleries') return; // locked
-    setStaged((prev) => applyDependencyRules({ ...prev, [key]: value }));
+    setStaged((prev) => {
+      const next = { ...prev, [key]: value };
+      // Reverse the bills→accounting force-enable. applyDependencyRules is a
+      // pure invariant over one state — it can't tell "Accounting is on
+      // because the admin wants it" from "…because Invoices forced it on", so
+      // turning Invoices off used to leave the Accounting master (and its
+      // sidebar entry) silently on and freshly unlocked (QA S9). The
+      // reversal has to live here, at the toggle, where the transition is
+      // known; the admin sees the switch flip in the same staged state and
+      // can turn Accounting back on before saving if they want it standalone.
+      if (key === 'bills' && !value && prev.bills) next.accounting = false;
+      return applyDependencyRules(next);
+    });
   }, []);
 
   const reset = useCallback(() => {
diff --git a/frontend/src/contexts/__tests__/featureFlagsAccountingCascade.test.tsx b/frontend/src/contexts/__tests__/featureFlagsAccountingCascade.test.tsx
new file mode 100644
index 00000000..641f01c8
--- /dev/null
+++ b/frontend/src/contexts/__tests__/featureFlagsAccountingCascade.test.tsx
@@ -0,0 +1,78 @@
+/**
+ * Turning Invoices (`bills`) on force-enables the Accounting master, but
+ * turning it back off used to leave Accounting silently on and freshly
+ * unlocked — an orphaned parent area after a trial toggle (QA S9).
+ */
+import React from 'react';
+import { describe, it, expect, vi } from 'vitest';
+import { renderHook, act, waitFor } from '@testing-library/react';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+
+const serverFlags: Record<string, boolean> = {};
+vi.mock('../../services/featureFlags.service', () => ({
+  featureFlagsService: {
+    get: vi.fn(async () => ({ ...serverFlags })),
+    update: vi.fn(async (f: Record<string, boolean>) => f),
+  },
+}));
+
+import { FeatureFlagsProvider, useFeatureFlags, DEFAULT_FLAGS } from '../FeatureFlagsContext';
+
+function renderFlags(overrides: Record<string, boolean>) {
+  Object.assign(serverFlags, DEFAULT_FLAGS, overrides);
+  const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  const wrapper = ({ children }: { children: React.ReactNode }) => (
+    <QueryClientProvider client={qc}>
+      <FeatureFlagsProvider>{children}</FeatureFlagsProvider>
+    </QueryClientProvider>
+  );
+  return renderHook(() => useFeatureFlags(), { wrapper });
+}
+
+describe('feature-flag parent/child cascade (QA S9)', () => {
+  it('turns Accounting (and its sub-features) off when Invoices is turned off', async () => {
+    const { result } = renderFlags({
+      quotes: true, bills: true, accounting: true, expenses: true, taxReport: true,
+    });
+    await waitFor(() => expect(result.current.staged.bills).toBe(true));
+
+    act(() => result.current.setFlag('bills', false));
+
+    expect(result.current.staged.bills).toBe(false);
+    expect(result.current.staged.accounting).toBe(false);
+    expect(result.current.staged.expenses).toBe(false);
+    expect(result.current.staged.taxReport).toBe(false);
+  });
+
+  it('leaves a standalone Accounting alone when an unrelated flag is toggled', async () => {
+    const { result } = renderFlags({ accounting: true, expenses: true });
+    await waitFor(() => expect(result.current.staged.accounting).toBe(true));
+
+    act(() => result.current.setFlag('workflows', true));
+
+    expect(result.current.staged.accounting).toBe(true);
+    expect(result.current.staged.expenses).toBe(true);
+  });
+
+  it('lets the admin keep Accounting standalone by re-enabling it before saving', async () => {
+    const { result } = renderFlags({ quotes: true, bills: true, accounting: true });
+    await waitFor(() => expect(result.current.staged.bills).toBe(true));
+
+    act(() => result.current.setFlag('bills', false));
+    act(() => result.current.setFlag('accounting', true));
+
+    expect(result.current.staged.bills).toBe(false);
+    expect(result.current.staged.accounting).toBe(true);
+  });
+
+  it('does not resurrect sub-features when Invoices is turned back on', async () => {
+    const { result } = renderFlags({ quotes: true, bills: true, accounting: true, expenses: true });
+    await waitFor(() => expect(result.current.staged.expenses).toBe(true));
+
+    act(() => result.current.setFlag('bills', false));
+    act(() => result.current.setFlag('bills', true));
+
+    expect(result.current.staged.accounting).toBe(true); // forced back on by bills
+    expect(result.current.staged.expenses).toBe(false);  // stays off until re-enabled
+  });
+});