diff --git a/hooks/useLocalStorage.massive-scaling.test.ts b/hooks/useLocalStorage.massive-scaling.test.ts
new file mode 100644
index 000000000..b61d2bc5a
--- /dev/null
+++ b/hooks/useLocalStorage.massive-scaling.test.ts
@@ -0,0 +1,204 @@
+/* eslint-disable @typescript-eslint/no-explicit-any */
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import React from 'react';
+import { render, screen, act, waitFor } from '@testing-library/react';
+import { useLocalStorage } from './useLocalStorage';
+
+// A mock visualizer component to test integration of useLocalStorage with SVG grid rendering
+function ActivityGridVisualizer({ storageKey }: { storageKey: string }) {
+  const [activities] = useLocalStorage<any[]>(storageKey, []);
+
+  return React.createElement(
+    'div',
+    {
+      'data-testid': 'visualizer-container',
+      style: { width: '800px', height: '600px', overflow: 'hidden' },
+    },
+    React.createElement(
+      'svg',
+      {
+        'data-testid': 'visualizer-svg',
+        width: '100%',
+        height: '100%',
+        viewBox: '0 0 800 600',
+      },
+      activities.map((actItem, idx) => {
+        // SVG isometric grid layout coordinates calculation
+        const col = idx % 50;
+        const row = Math.floor(idx / 50);
+        const x = col * 15 + 10;
+        const y = row * 15 + 10;
+
+        return React.createElement('rect', {
+          key: actItem.id,
+          'data-testid': `activity-rect-${actItem.id}`,
+          x,
+          y,
+          width: 12,
+          height: 12,
+          fill: '#10b981',
+        });
+      })
+    )
+  );
+}
+
+describe('useLocalStorage - Massive Data Sets & High Bounds Scaling', () => {
+  const TEST_KEY = 'cp:scaling-activities';
+
+  beforeEach(() => {
+    localStorage.clear();
+    vi.restoreAllMocks();
+  });
+
+  describe('Massive Dataset Layout Integration & Serialization', () => {
+    it('hydrates a massive activities dataset and scales grid coordinates cleanly without crashing', async () => {
+      const massiveData: any[] = [];
+      for (let i = 0; i < 2000; i++) {
+        massiveData.push({
+          id: i,
+          type: 'push',
+          timestamp: '2026-06-12T12:00:00Z',
+        });
+      }
+
+      localStorage.setItem(TEST_KEY, JSON.stringify(massiveData));
+
+      render(React.createElement(ActivityGridVisualizer, { storageKey: TEST_KEY }));
+
+      // Wait for hook state to hydrate and React to render visual components
+      await waitFor(() => {
+        const svg = screen.getByTestId('visualizer-svg');
+        expect(svg).toBeInTheDocument();
+      });
+
+      // Sample first and last grid elements to verify coordinate mapping safety (non-NaN check)
+      const rect0 = screen.getByTestId('activity-rect-0');
+      const rect1999 = screen.getByTestId('activity-rect-1999');
+
+      expect(rect0).toBeInTheDocument();
+      expect(rect1999).toBeInTheDocument();
+
+      const x0 = Number(rect0.getAttribute('x'));
+      const y0 = Number(rect0.getAttribute('y'));
+      const x1999 = Number(rect1999.getAttribute('x'));
+      const y1999 = Number(rect1999.getAttribute('y'));
+
+      expect(isNaN(x0)).toBe(false);
+      expect(isNaN(y0)).toBe(false);
+      expect(isNaN(x1999)).toBe(false);
+      expect(isNaN(y1999)).toBe(false);
+
+      expect(x0).toBeGreaterThanOrEqual(0);
+      expect(y0).toBeGreaterThanOrEqual(0);
+      expect(x1999).toBeGreaterThan(x0);
+      expect(y1999).toBeGreaterThan(y0);
+    });
+
+    it('measures execution rendering performance stays below the latency threshold', () => {
+      const massiveData: any[] = [];
+      for (let i = 0; i < 1500; i++) {
+        massiveData.push({
+          id: i,
+          type: 'push',
+          timestamp: '2026-06-12T12:00:00Z',
+        });
+      }
+
+      localStorage.setItem(TEST_KEY, JSON.stringify(massiveData));
+
+      const startTime = performance.now();
+
+      render(React.createElement(ActivityGridVisualizer, { storageKey: TEST_KEY }));
+
+      const endTime = performance.now();
+      const elapsed = endTime - startTime;
+
+      // Layout should compile and render visual representation under 500ms
+      expect(elapsed).toBeLessThan(500);
+    });
+  });
+
+  describe('Quota Limit Resilience & State Safety', () => {
+    it('gracefully handles QuotaExceededError and updates memory React state cleanly', () => {
+      const largePayload = Array.from({ length: 5000 }).map((_, i) => ({
+        id: i,
+        type: 'commit',
+      }));
+
+      // Emulate browser quota exceeded limit error when setting localStorage
+      vi.spyOn(window.localStorage, 'setItem').mockImplementation(() => {
+        throw new DOMException('The quota has been exceeded.', 'QuotaExceededError');
+      });
+
+      // Render hook wrapper
+      let hookResult: any;
+      const TestComponent = () => {
+        const [value, setValue] = useLocalStorage<any[]>(TEST_KEY, []);
+        React.useEffect(() => {
+          hookResult = { value, setValue };
+        }, [value, setValue]);
+        return React.createElement('div', null);
+      };
+
+      render(React.createElement(TestComponent));
+
+      // Attempt to save massive payload under simulated quota limit
+      expect(() => {
+        act(() => {
+          hookResult.setValue(largePayload);
+        });
+      }).not.toThrow();
+
+      // Hook state should remain updated in memory even if persistent write fails
+      expect(hookResult.value).toEqual(largePayload);
+    });
+  });
+
+  describe('Boundary Keys and High-Frequency Scaling', () => {
+    it('supports keys with extreme lengths up to 10,000 characters', () => {
+      const longKey = 'k'.repeat(10000);
+      const testVal = { ok: true };
+
+      let hookResult: any;
+      const TestComponent = () => {
+        const [value, setValue] = useLocalStorage<any>(longKey, null);
+        React.useEffect(() => {
+          hookResult = { value, setValue };
+        }, [value, setValue]);
+        return React.createElement('div', null);
+      };
+
+      render(React.createElement(TestComponent));
+
+      act(() => {
+        hookResult.setValue(testVal);
+      });
+
+      expect(hookResult.value).toEqual(testVal);
+      expect(JSON.parse(localStorage.getItem(longKey)!)).toEqual(testVal);
+    });
+
+    it('resolves 1,000 high-frequency sequential updates cleanly to the final value', () => {
+      let hookResult: any;
+      const TestComponent = () => {
+        const [value, setValue] = useLocalStorage<number>(TEST_KEY, 0);
+        React.useEffect(() => {
+          hookResult = { value, setValue };
+        }, [value, setValue]);
+        return React.createElement('div', null);
+      };
+
+      render(React.createElement(TestComponent));
+
+      act(() => {
+        for (let i = 1; i <= 1000; i++) {
+          hookResult.setValue(i);
+        }
+      });
+
+      expect(hookResult.value).toBe(1000);
+      expect(localStorage.getItem(TEST_KEY)).toBe('1000');
+    });
+  });
+});