diff --git a/src/modules/inventory/inventory.service.test.ts b/src/modules/inventory/inventory.service.test.ts
new file mode 100644
index 0000000..3fe8901
--- /dev/null
+++ b/src/modules/inventory/inventory.service.test.ts
@@ -0,0 +1,242 @@
+import { describe, it, expect, mock, beforeEach } from "bun:test";
+import { inventoryService } from "./inventory.service";
+import { inventory, items, userTimers } from "@/db/schema";
+import { app } from "@/index"; // We don't need app here, removed.
+
+// Mock dependencies
+const mockFindFirst = mock();
+const mockFindMany = mock();
+const mockInsert = mock();
+const mockUpdate = mock();
+const mockDelete = mock();
+const mockValues = mock();
+const mockReturning = mock();
+const mockSet = mock();
+const mockWhere = mock();
+const mockSelect = mock();
+const mockFrom = mock();
+const mockOnConflictDoUpdate = mock();
+
+// Chain setup
+mockInsert.mockReturnValue({ values: mockValues });
+mockValues.mockReturnValue({
+    returning: mockReturning,
+    onConflictDoUpdate: mockOnConflictDoUpdate
+});
+mockOnConflictDoUpdate.mockResolvedValue({});
+
+mockUpdate.mockReturnValue({ set: mockSet });
+mockSet.mockReturnValue({ where: mockWhere });
+mockWhere.mockReturnValue({ returning: mockReturning });
+
+mockDelete.mockReturnValue({ where: mockWhere });
+
+mockSelect.mockReturnValue({ from: mockFrom });
+mockFrom.mockReturnValue({ where: mockWhere });
+
+// Mock DrizzleClient
+mock.module("@/lib/DrizzleClient", () => {
+    const createMockTx = () => ({
+        query: {
+            inventory: { findFirst: mockFindFirst, findMany: mockFindMany },
+            items: { findFirst: mockFindFirst },
+            userTimers: { findFirst: mockFindFirst },
+        },
+        insert: mockInsert,
+        update: mockUpdate,
+        delete: mockDelete,
+        select: mockSelect,
+    });
+
+    return {
+        DrizzleClient: {
+            ...createMockTx(),
+            transaction: async (cb: any) => cb(createMockTx()),
+        }
+    };
+});
+
+const mockModifyUserBalance = mock();
+mock.module("@/modules/economy/economy.service", () => ({
+    economyService: {
+        modifyUserBalance: mockModifyUserBalance
+    }
+}));
+
+const mockAddXp = mock();
+mock.module("@/modules/leveling/leveling.service", () => ({
+    levelingService: {
+        addXp: mockAddXp
+    }
+}));
+
+mock.module("@/lib/config", () => ({
+    config: {
+        inventory: {
+            maxStackSize: 100n,
+            maxSlots: 10
+        }
+    }
+}));
+
+describe("inventoryService", () => {
+    beforeEach(() => {
+        mockFindFirst.mockReset();
+        mockFindMany.mockReset();
+        mockInsert.mockClear();
+        mockUpdate.mockClear();
+        mockDelete.mockClear();
+        mockValues.mockClear();
+        mockReturning.mockClear();
+        mockSet.mockClear();
+        mockWhere.mockClear();
+        mockSelect.mockClear();
+        mockFrom.mockClear();
+        mockModifyUserBalance.mockClear();
+        mockAddXp.mockClear();
+        mockOnConflictDoUpdate.mockClear();
+    });
+
+    describe("addItem", () => {
+        it("should add new item if slot available", async () => {
+            // Check existing (none) -> Check count (0) -> Insert
+            mockFindFirst.mockResolvedValue(null);
+            const mockCountResult = mock().mockResolvedValue([{ count: 0 }]);
+            mockFrom.mockReturnValue({ where: mockCountResult });
+
+            mockReturning.mockResolvedValue([{ itemId: 1, quantity: 5n }]);
+
+            const result = await inventoryService.addItem("1", 1, 5n);
+
+            expect(result).toEqual({ itemId: 1, quantity: 5n } as any);
+            expect(mockInsert).toHaveBeenCalledWith(inventory);
+            expect(mockValues).toHaveBeenCalledWith({
+                userId: 1n,
+                itemId: 1,
+                quantity: 5n
+            });
+        });
+
+        it("should stack existing item up to limit", async () => {
+            // Check existing (found with 10)
+            mockFindFirst.mockResolvedValue({ quantity: 10n });
+            mockReturning.mockResolvedValue([{ itemId: 1, quantity: 15n }]);
+
+            const result = await inventoryService.addItem("1", 1, 5n);
+
+            expect(result.quantity).toBe(15n);
+            expect(mockUpdate).toHaveBeenCalledWith(inventory);
+            expect(mockSet).toHaveBeenCalledWith({ quantity: 15n });
+        });
+
+        it("should throw if max stack exceeded", async () => {
+            mockFindFirst.mockResolvedValue({ quantity: 99n });
+            // Max is 100
+            expect(inventoryService.addItem("1", 1, 5n)).rejects.toThrow("Cannot exceed max stack size");
+        });
+
+        it("should throw if inventory full", async () => {
+            mockFindFirst.mockResolvedValue(null);
+
+            const mockCountResult = mock().mockResolvedValue([{ count: 10 }]); // Max slots 10
+            mockFrom.mockReturnValue({ where: mockCountResult });
+
+            expect(inventoryService.addItem("1", 1, 1n)).rejects.toThrow("Inventory full");
+        });
+    });
+
+    describe("removeItem", () => {
+        it("should decrease quantity if enough", async () => {
+            mockFindFirst.mockResolvedValue({ quantity: 10n });
+            mockReturning.mockResolvedValue([{ quantity: 5n }]);
+
+            await inventoryService.removeItem("1", 1, 5n);
+
+            expect(mockUpdate).toHaveBeenCalledWith(inventory);
+            // mockSet uses sql template, hard to check exact value, checking call presence
+            expect(mockSet).toHaveBeenCalled();
+        });
+
+        it("should delete item if quantity becomes 0", async () => {
+            mockFindFirst.mockResolvedValue({ quantity: 5n });
+
+            const result = await inventoryService.removeItem("1", 1, 5n);
+
+            expect(mockDelete).toHaveBeenCalledWith(inventory);
+            expect(result.quantity).toBe(0n);
+        });
+
+        it("should throw if insufficient quantity", async () => {
+            mockFindFirst.mockResolvedValue({ quantity: 2n });
+            expect(inventoryService.removeItem("1", 1, 5n)).rejects.toThrow("Insufficient item quantity");
+        });
+    });
+
+    describe("buyItem", () => {
+        it("should buy item successfully", async () => {
+            const mockItem = { id: 1, name: "Potion", price: 100n };
+            mockFindFirst.mockResolvedValue(mockItem);
+
+            // For addItem internal call, we need to mock findFirst again or ensure it works.
+            // DrizzleClient.transaction calls callback.
+            // buyItem calls findFirst for item.
+            // buyItem calls modifyUserBalance.
+            // buyItem calls addItem.
+
+            // addItem calls findFirst for inventory.
+
+            // So mockFindFirst needs to return specific values in sequence.
+            mockFindFirst
+                .mockResolvedValueOnce(mockItem) // Item check
+                .mockResolvedValueOnce(null);    // addItem -> existing check (null = new)
+
+            // addItem -> count check
+            const mockCountResult = mock().mockResolvedValue([{ count: 0 }]);
+            mockFrom.mockReturnValue({ where: mockCountResult });
+
+            mockReturning.mockResolvedValue([{}]);
+
+            const result = await inventoryService.buyItem("1", 1, 2n);
+
+            expect(result.success).toBe(true);
+            expect(mockModifyUserBalance).toHaveBeenCalledWith("1", -200n, 'PURCHASE', expect.stringContaining("Bought 2x"), null, expect.anything());
+            expect(mockInsert).toHaveBeenCalledWith(inventory); // from addItem
+        });
+    });
+
+    describe("useItem", () => {
+        it("should apply effects and consume item", async () => {
+            const mockItem = {
+                id: 1,
+                name: "XP Potion",
+                usageData: {
+                    consume: true,
+                    effects: [
+                        { type: "ADD_XP", amount: 100 },
+                        { type: "XP_BOOST", durationMinutes: 60, multiplier: 2.0 }
+                    ]
+                }
+            };
+
+            // inventory entry
+            mockFindFirst.mockResolvedValue({ quantity: 1n, item: mockItem });
+
+            // For removeItem:
+            // removeItem calls findFirst (inventory).
+            // So sequence: 
+            // 1. useItem -> findFirst (inventory + item)
+            // 2. removeItem -> findFirst (inventory)
+
+            mockFindFirst
+                .mockResolvedValueOnce({ quantity: 1n, item: mockItem }) // useItem check
+                .mockResolvedValueOnce({ quantity: 1n }); // removeItem check
+
+            const result = await inventoryService.useItem("1", 1);
+
+            expect(result.success).toBe(true);
+            expect(mockAddXp).toHaveBeenCalledWith("1", 100n, expect.anything());
+            expect(mockInsert).toHaveBeenCalledWith(userTimers); // XP Boost
+            expect(mockDelete).toHaveBeenCalledWith(inventory); // Consume
+        });
+    });
+});