diff --git a/packages/core/tsdown.config.ts b/packages/core/tsdown.config.ts
index e5fd07a3..dd23d7a3 100644
--- a/packages/core/tsdown.config.ts
+++ b/packages/core/tsdown.config.ts
@@ -23,9 +23,6 @@ const createConfig = (mode: BuildMode): UserConfig => ({
   clean: true,
   hash: false,
   unbundle: true,
-  alias: {
-    '@': new URL('./src/core', import.meta.url).pathname,
-  },
   outDir: `dist/${mode}`,
   define: {
     __DEV__: mode === 'dev' ? 'true' : 'false',
diff --git a/packages/store/src/core/combine.ts b/packages/store/src/core/combine.ts
index 32e19efd..2f0d622d 100644
--- a/packages/store/src/core/combine.ts
+++ b/packages/store/src/core/combine.ts
@@ -12,6 +12,19 @@ export function combine<Target, const Slices extends Slice<Target, any>[]>(
   return {
     state: (ctx: StateContext<Target>) => {
       const states = slices.map((slice) => slice.state(ctx));
+
+      if (__DEV__) {
+        const seen = new Set<string>();
+        for (const state of states) {
+          for (const key of Object.keys(state as object)) {
+            if (seen.has(key)) {
+              console.warn(`[vjs-store] combine(): duplicate state key "${key}" — later slice overwrites earlier one`);
+            }
+            seen.add(key);
+          }
+        }
+      }
+
       return Object.assign({}, ...states) as UnionSliceState<Slices>;
     },
 
diff --git a/packages/store/src/core/globals.d.ts b/packages/store/src/core/globals.d.ts
new file mode 100644
index 00000000..b867229b
--- /dev/null
+++ b/packages/store/src/core/globals.d.ts
@@ -0,0 +1 @@
+declare const __DEV__: boolean;
diff --git a/packages/store/src/core/tests/combine.test.ts b/packages/store/src/core/tests/combine.test.ts
new file mode 100644
index 00000000..82cefcac
--- /dev/null
+++ b/packages/store/src/core/tests/combine.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, it, vi } from 'vitest';
+
+import { combine } from '../combine';
+import { defineSlice } from '../slice';
+import { createStore } from '../store';
+
+class MockTarget extends EventTarget {
+  value = 0;
+}
+
+const slice = defineSlice<MockTarget>();
+
+describe('combine', () => {
+  it('merges state from multiple slices', () => {
+    const a = slice({ state: () => ({ count: 0 }) });
+    const b = slice({ state: () => ({ label: 'hello' }) });
+
+    const store = createStore<MockTarget>()(combine(a, b));
+
+    expect(store.state).toMatchObject({ count: 0, label: 'hello' });
+  });
+
+  it('calls attach for each slice', () => {
+    const attachA = vi.fn();
+    const attachB = vi.fn();
+
+    const a = slice({ state: () => ({ count: 0 }), attach: attachA });
+    const b = slice({ state: () => ({ label: '' }), attach: attachB });
+
+    const store = createStore<MockTarget>()(combine(a, b));
+    store.attach(new MockTarget());
+
+    expect(attachA).toHaveBeenCalledOnce();
+    expect(attachB).toHaveBeenCalledOnce();
+  });
+
+  it('catches and reports attach errors via onError callback', () => {
+    const error = new Error('attach failed');
+    const onError = vi.fn();
+
+    const a = slice({
+      state: () => ({ count: 0 }),
+      attach: () => {
+        throw error;
+      },
+    });
+    const b = slice({ state: () => ({ label: '' }) });
+
+    const store = createStore<MockTarget>()(combine(a, b), { onError });
+    store.attach(new MockTarget());
+
+    expect(onError).toHaveBeenCalled();
+  });
+
+  it('warns on duplicate state keys in __DEV__ mode', () => {
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+    const a = slice({ state: () => ({ count: 0 }) });
+    const b = slice({ state: () => ({ count: 1 }) });
+
+    createStore<MockTarget>()(combine(a, b));
+
+    expect(warn).toHaveBeenCalledWith(expect.stringContaining('duplicate state key "count"'));
+
+    warn.mockRestore();
+  });
+});
diff --git a/packages/store/tsdown.config.ts b/packages/store/tsdown.config.ts
index 542162f2..f693dd8f 100644
--- a/packages/store/tsdown.config.ts
+++ b/packages/store/tsdown.config.ts
@@ -17,9 +17,6 @@ const createConfig = (mode: BuildMode): UserConfig => ({
   clean: true,
   hash: false,
   unbundle: true,
-  alias: {
-    '@': new URL('./src/core', import.meta.url).pathname,
-  },
   outDir: `dist/${mode}`,
   define: {
     __DEV__: mode === 'dev' ? 'true' : 'false',
diff --git a/packages/utils/tsdown.config.ts b/packages/utils/tsdown.config.ts
index bd2679f5..e441c6e5 100644
--- a/packages/utils/tsdown.config.ts
+++ b/packages/utils/tsdown.config.ts
@@ -20,8 +20,5 @@ export default defineConfig({
   clean: true,
   hash: false,
   unbundle: true,
-  alias: {
-    '@': new URL('./src', import.meta.url).pathname,
-  },
   dts: true,
 });