diff --git a/backend/tests/test_operations.py b/backend/tests/test_operations.py
new file mode 100644
index 00000000..8fb85392
--- /dev/null
+++ b/backend/tests/test_operations.py
@@ -0,0 +1,189 @@
+import pytest
+from fastapi import status
+from datetime import datetime
+
+
+class TestStockOperations:
+    """Test check-in and check-out operations."""
+
+    def test_check_in_item(self, test_client, test_db):
+        """Test checking in inventory (increasing quantity)."""
+        from backend.models import Item
+
+        item = Item(
+            name="Test Item",
+            category="Electronics",
+            item_type="Component",
+            quantity=10,
+            barcode="123456789",
+            part_number="PN-12345"
+        )
+        test_db.add(item)
+        test_db.commit()
+
+        response = test_client.post(
+            "/api/operations/check-in",
+            json={"item_id": item.id, "quantity": 5}
+        )
+        assert response.status_code == status.HTTP_200_OK
+        data = response.json()
+        assert data["new_quantity"] == 15
+
+    def test_check_out_item(self, test_client, test_db):
+        """Test checking out inventory (decreasing quantity)."""
+        from backend.models import Item
+
+        item = Item(
+            name="Test Item",
+            category="Electronics",
+            item_type="Component",
+            quantity=10,
+            barcode="123456789",
+            part_number="PN-12345"
+        )
+        test_db.add(item)
+        test_db.commit()
+
+        response = test_client.post(
+            "/api/operations/check-out",
+            json={"item_id": item.id, "quantity": 3}
+        )
+        assert response.status_code == status.HTTP_200_OK
+        data = response.json()
+        assert data["new_quantity"] == 7
+
+    def test_check_out_insufficient_quantity(self, test_client, test_db):
+        """Test that check-out fails if quantity insufficient."""
+        from backend.models import Item
+
+        item = Item(
+            name="Test Item",
+            category="Electronics",
+            item_type="Component",
+            quantity=5,
+            barcode="123456789",
+            part_number="PN-12345"
+        )
+        test_db.add(item)
+        test_db.commit()
+
+        response = test_client.post(
+            "/api/operations/check-out",
+            json={"item_id": item.id, "quantity": 10}
+        )
+        assert response.status_code == status.HTTP_400_BAD_REQUEST
+
+    def test_audit_log_created(self, test_client, test_db):
+        """Test that audit log entry created for each operation."""
+        from backend.models import Item
+
+        item = Item(
+            name="Test Item",
+            category="Electronics",
+            item_type="Component",
+            quantity=10,
+            barcode="123456789",
+            part_number="PN-12345"
+        )
+        test_db.add(item)
+        test_db.commit()
+
+        # Perform operation
+        response = test_client.post(
+            "/api/operations/check-in",
+            json={"item_id": item.id, "quantity": 5}
+        )
+        assert response.status_code == status.HTTP_200_OK
+
+        # Verify audit log
+        response = test_client.get(f"/api/audit-logs?item_id={item.id}")
+        assert response.status_code == status.HTTP_200_OK
+        logs = response.json()
+        assert len(logs) > 0
+        assert logs[-1]["operation"] == "CHECK_IN"
+        assert logs[-1]["quantity_change"] == 5
+
+
+class TestBulkSync:
+    """Test offline sync with UUID idempotency."""
+
+    def test_bulk_sync_offline_operations(self, test_client, test_db):
+        """Test syncing multiple offline-generated operations."""
+        from backend.models import Item
+
+        item = Item(
+            name="Test Item",
+            category="Electronics",
+            item_type="Component",
+            quantity=10,
+            barcode="123456789",
+            part_number="PN-12345"
+        )
+        test_db.add(item)
+        test_db.commit()
+
+        # Simulate offline operations with UUIDs
+        response = test_client.post(
+            "/api/bulk-sync",
+            json={
+                "operations": [
+                    {
+                        "id": "uuid-1",
+                        "type": "CHECK_IN",
+                        "item_id": item.id,
+                        "quantity": 5
+                    },
+                    {
+                        "id": "uuid-2",
+                        "type": "CHECK_IN",
+                        "item_id": item.id,
+                        "quantity": 3
+                    }
+                ]
+            }
+        )
+        assert response.status_code == status.HTTP_200_OK
+        data = response.json()
+        assert data["synced"] == 2
+        assert data["final_quantity"] == 18
+
+    def test_bulk_sync_idempotent(self, test_client, test_db):
+        """Test that syncing same UUID twice doesn't duplicate."""
+        from backend.models import Item
+
+        item = Item(
+            name="Test Item",
+            category="Electronics",
+            item_type="Component",
+            quantity=10,
+            barcode="123456789",
+            part_number="PN-12345"
+        )
+        test_db.add(item)
+        test_db.commit()
+
+        operation = {
+            "id": "uuid-1",
+            "type": "CHECK_IN",
+            "item_id": item.id,
+            "quantity": 5
+        }
+
+        # Sync once
+        response1 = test_client.post(
+            "/api/bulk-sync",
+            json={"operations": [operation]}
+        )
+        assert response1.status_code == status.HTTP_200_OK
+        q1 = response1.json()["final_quantity"]
+
+        # Sync again (same UUID)
+        response2 = test_client.post(
+            "/api/bulk-sync",
+            json={"operations": [operation]}
+        )
+        assert response2.status_code == status.HTTP_200_OK
+        q2 = response2.json()["final_quantity"]
+
+        # Quantity should not increase twice
+        assert q1 == q2 == 15