diff --git a/backend/tests/test_quantity_patch.py b/backend/tests/test_quantity_patch.py
new file mode 100644
index 00000000..bda790d2
--- /dev/null
+++ b/backend/tests/test_quantity_patch.py
@@ -0,0 +1,165 @@
+"""
+Test PATCH /items/{item_id} endpoint for quantity adjustment
+Ensures audit logging and validation work correctly
+"""
+
+import pytest
+import json
+from fastapi.testclient import TestClient
+from sqlalchemy.orm import Session
+from ..models import Item, Category, AuditLog
+from ..schemas import Item as ItemSchema
+from ..database import Base, engine
+
+
+@pytest.fixture
+def test_item(db: Session):
+    """Create a test item"""
+    category = Category(name="Test Category")
+    db.add(category)
+    db.commit()
+
+    item = Item(
+        name="Test Item",
+        category="Test Category",
+        type="Test Type",
+        quantity=10,
+        barcode="TEST123",
+        part_number="PN-TEST-001"
+    )
+    db.add(item)
+    db.commit()
+    db.refresh(item)
+    return item
+
+
+def test_patch_quantity_success(client: TestClient, test_item: Item, db: Session, auth_token: str):
+    """Test successful quantity update with audit logging"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 25},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 200
+    data = response.json()
+    assert data["quantity"] == 25
+    assert data["id"] == test_item.id
+
+    # Verify audit log was created
+    audit_entries = db.query(AuditLog).filter(
+        AuditLog.target_item_id == test_item.id,
+        AuditLog.action == "UPDATE_QUANTITY"
+    ).all()
+
+    assert len(audit_entries) == 1
+    audit = audit_entries[0]
+    assert audit.quantity_change == 15  # 25 - 10
+    assert audit.target_item_name == "Test Item"
+    assert audit.target_item_pn == "PN-TEST-001"
+    assert "10 → 25" in audit.details
+
+
+def test_patch_quantity_to_zero(client: TestClient, test_item: Item, db: Session, auth_token: str):
+    """Test setting quantity to zero"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 0},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 200
+    data = response.json()
+    assert data["quantity"] == 0
+
+    # Verify audit log has correct delta
+    audit = db.query(AuditLog).filter(
+        AuditLog.target_item_id == test_item.id,
+        AuditLog.action == "UPDATE_QUANTITY"
+    ).first()
+
+    assert audit.quantity_change == -10  # 0 - 10
+
+
+def test_patch_quantity_negative_fails(client: TestClient, test_item: Item, auth_token: str):
+    """Test that negative quantity is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": -5},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 400
+    assert "non-negative" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_missing_field(client: TestClient, test_item: Item, auth_token: str):
+    """Test that missing quantity field is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 400
+    assert "quantity" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_invalid_type(client: TestClient, test_item: Item, auth_token: str):
+    """Test that non-integer quantity is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": "not a number"},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 400
+    assert "integer" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_not_found(client: TestClient, auth_token: str):
+    """Test that updating non-existent item returns 404"""
+    response = client.patch(
+        "/items/99999",
+        json={"quantity": 10},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 404
+    assert "not found" in response.json()["detail"].lower()
+
+
+def test_patch_quantity_no_auth(client: TestClient, test_item: Item):
+    """Test that unauthenticated request is rejected"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 25}
+    )
+
+    assert response.status_code == 401
+
+
+def test_patch_quantity_audit_fields(client: TestClient, test_item: Item, db: Session, auth_token: str, current_user_id: str):
+    """Test that audit log contains all required fields"""
+    response = client.patch(
+        f"/items/{test_item.id}",
+        json={"quantity": 30},
+        headers={"Authorization": f"Bearer {auth_token}"}
+    )
+
+    assert response.status_code == 200
+
+    audit = db.query(AuditLog).filter(
+        AuditLog.target_item_id == test_item.id,
+        AuditLog.action == "UPDATE_QUANTITY"
+    ).first()
+
+    assert audit is not None
+    assert audit.user_id == current_user_id
+    assert audit.action == "UPDATE_QUANTITY"
+    assert audit.target_item_id == test_item.id
+    assert audit.target_item_name == "Test Item"
+    assert audit.target_item_pn == "PN-TEST-001"
+    assert audit.target_item_barcode == "TEST123"
+    assert audit.quantity_change == 20  # 30 - 10
+    assert "→" in audit.details  # Direction indicator in details
diff --git a/frontend/tests/inventory/quick-adjust.test.ts b/frontend/tests/inventory/quick-adjust.test.ts
new file mode 100644
index 00000000..d9ea8644
--- /dev/null
+++ b/frontend/tests/inventory/quick-adjust.test.ts
@@ -0,0 +1,202 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { renderHook, act, waitFor } from '@testing-library/react';
+import { useQuantityAdjustment } from '@/hooks/useQuantityAdjustment';
+import axios from 'axios';
+
+// Mock axios
+vi.mock('axios');
+const mockedAxios = axios as any;
+
+describe('Quick Quantity Adjustment - useQuantityAdjustment Hook', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockedAxios.patch.mockClear();
+  });
+
+  it('initializes with correct quantity', () => {
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    expect(result.current.quantity).toBe(10);
+    expect(result.current.isLoading).toBe(false);
+    expect(result.current.error).toBeNull();
+  });
+
+  it('updates quantity optimistically and confirms with API', async () => {
+    mockedAxios.patch.mockResolvedValueOnce({
+      data: { quantity: 15 },
+    });
+
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    await act(async () => {
+      await result.current.adjustQuantity(15);
+    });
+
+    expect(result.current.quantity).toBe(15);
+    expect(result.current.error).toBeNull();
+    expect(mockedAxios.patch).toHaveBeenCalledWith(
+      expect.stringContaining('/items/1'),
+      { quantity: 15 }
+    );
+  });
+
+  it('reverts quantity on API failure', async () => {
+    const error = new Error('Network error');
+    mockedAxios.patch.mockRejectedValueOnce(error);
+
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    await act(async () => {
+      try {
+        await result.current.adjustQuantity(15);
+      } catch {
+        // Expected to throw
+      }
+    });
+
+    await waitFor(() => {
+      expect(result.current.quantity).toBe(10);
+      expect(result.current.error).toBeTruthy();
+    });
+  });
+
+  it('validates quantity >= 0', async () => {
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    await act(async () => {
+      await result.current.adjustQuantity(-5);
+    });
+
+    expect(result.current.error).toContain('non-negative');
+    expect(mockedAxios.patch).not.toHaveBeenCalled();
+  });
+
+  it('validates integer quantities', async () => {
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    await act(async () => {
+      await result.current.adjustQuantity(10.5);
+    });
+
+    expect(result.current.error).toContain('integer');
+    expect(mockedAxios.patch).not.toHaveBeenCalled();
+  });
+
+  it('resets error on resetError call', async () => {
+    const error = new Error('Network error');
+    mockedAxios.patch.mockRejectedValueOnce(error);
+
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    await act(async () => {
+      try {
+        await result.current.adjustQuantity(15);
+      } catch {
+        // Expected to throw
+      }
+    });
+
+    expect(result.current.error).toBeTruthy();
+
+    act(() => {
+      result.current.resetError();
+    });
+
+    expect(result.current.error).toBeNull();
+  });
+
+  it('debounces rapid successive calls', async () => {
+    mockedAxios.patch.mockResolvedValue({
+      data: { quantity: 15 },
+    });
+
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    await act(async () => {
+      // Queue multiple rapid adjustments
+      const promises = [
+        result.current.adjustQuantity(12),
+        result.current.adjustQuantity(14),
+        result.current.adjustQuantity(15),
+      ];
+
+      await Promise.all(promises);
+    });
+
+    // With debouncing, only the last call should fully process
+    // (The implementation uses 100ms debounce)
+    await waitFor(() => {
+      expect(mockedAxios.patch).toHaveBeenCalled();
+    });
+  });
+
+  it('handles API error responses gracefully', async () => {
+    mockedAxios.patch.mockRejectedValueOnce({
+      response: {
+        data: {
+          detail: 'Quantity exceeds available stock',
+        },
+      },
+    });
+
+    const { result } = renderHook(() =>
+      useQuantityAdjustment('1', 10)
+    );
+
+    await act(async () => {
+      try {
+        await result.current.adjustQuantity(999);
+      } catch {
+        // Expected
+      }
+    });
+
+    await waitFor(() => {
+      expect(result.current.error).toBeTruthy();
+      expect(result.current.quantity).toBe(10); // Reverted
+    });
+  });
+});
+
+describe('Quick Quantity Adjustment - Component Integration', () => {
+  it('should support UI workflow: tap → +/- → commit', async () => {
+    // This is a placeholder for E2E/integration test that would:
+    // 1. Render QuantityDisplay
+    // 2. Click on quantity to enter edit mode
+    // 3. Click +/- buttons to adjust
+    // 4. Press Enter to commit
+    // 5. Verify API call with correct delta
+    // Implementation requires React Testing Library setup with actual component rendering
+
+    expect(true).toBe(true); // Placeholder assertion
+  });
+
+  it('should show loading state during API call', async () => {
+    // Placeholder for component loading state test
+    expect(true).toBe(true);
+  });
+
+  it('should show error toast on API failure', async () => {
+    // Placeholder for error handling test
+    expect(true).toBe(true);
+  });
+
+  it('should cancel edit on Escape key without API call', async () => {
+    // Placeholder for cancel behavior test
+    expect(true).toBe(true);
+  });
+});