diff --git a/packages/core/tests/config.test.ts b/packages/core/tests/config.test.ts
index 3a94feeb..16cc5e67 100644
--- a/packages/core/tests/config.test.ts
+++ b/packages/core/tests/config.test.ts
@@ -5,7 +5,7 @@ import type { RslibConfig } from '../src/types/config';
 
 describe('Should load config file correctly', () => {
   test('Load config.js in cjs project', async () => {
-    const fixtureDir = join(__dirname, 'config/cjs');
+    const fixtureDir = join(__dirname, 'fixtures/config/cjs');
     const configDir = join(fixtureDir, 'rslib.config.js');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
@@ -22,7 +22,7 @@ describe('Should load config file correctly', () => {
   });
 
   test('Load config.mjs in cjs project', async () => {
-    const fixtureDir = join(__dirname, 'config/cjs');
+    const fixtureDir = join(__dirname, 'fixtures/config/cjs');
     const configDir = join(fixtureDir, 'rslib.config.mjs');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
@@ -39,7 +39,7 @@ describe('Should load config file correctly', () => {
   });
 
   test('Load config.ts in cjs project', async () => {
-    const fixtureDir = join(__dirname, 'config/cjs');
+    const fixtureDir = join(__dirname, 'fixtures/config/cjs');
     const configDir = join(fixtureDir, 'rslib.config.ts');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
@@ -56,7 +56,7 @@ describe('Should load config file correctly', () => {
   });
 
   test('Load config.cjs with defineConfig in cjs project', async () => {
-    const fixtureDir = join(__dirname, 'config/cjs');
+    const fixtureDir = join(__dirname, 'fixtures/config/cjs');
     const configDir = join(fixtureDir, 'rslib.config.cjs');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
@@ -73,7 +73,7 @@ describe('Should load config file correctly', () => {
   });
 
   test('Load config.js in esm project', async () => {
-    const fixtureDir = join(__dirname, 'config/esm');
+    const fixtureDir = join(__dirname, 'fixtures/config/esm');
     const configDir = join(fixtureDir, 'rslib.config.js');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
@@ -90,7 +90,7 @@ describe('Should load config file correctly', () => {
   });
 
   test('Load config.cjs in esm project', async () => {
-    const fixtureDir = join(__dirname, 'config/esm');
+    const fixtureDir = join(__dirname, 'fixtures/config/esm');
     const configDir = join(fixtureDir, 'rslib.config.cjs');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
@@ -107,7 +107,7 @@ describe('Should load config file correctly', () => {
   });
 
   test('Load config.ts in esm project', async () => {
-    const fixtureDir = join(__dirname, 'config/esm');
+    const fixtureDir = join(__dirname, 'fixtures/config/esm');
     const configDir = join(fixtureDir, 'rslib.config.ts');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
@@ -124,7 +124,7 @@ describe('Should load config file correctly', () => {
   });
 
   test('Load config.mjs with defineConfig in esm project', async () => {
-    const fixtureDir = join(__dirname, 'config/esm');
+    const fixtureDir = join(__dirname, 'fixtures/config/esm');
     const configDir = join(fixtureDir, 'rslib.config.mjs');
     const config = await loadConfig(configDir);
     expect(config).toEqual({
diff --git a/packages/core/tests/core.test.ts b/packages/core/tests/core.test.ts
deleted file mode 100644
index cc15a93c..00000000
--- a/packages/core/tests/core.test.ts
+++ /dev/null
@@ -1,7 +0,0 @@
-import { describe, expect, it } from 'vitest';
-
-describe('core', () => {
-  it('should work', () => {
-    expect(1).toBe(1);
-  });
-});
diff --git a/packages/core/tests/extension.test.ts b/packages/core/tests/extension.test.ts
new file mode 100644
index 00000000..eedadcf6
--- /dev/null
+++ b/packages/core/tests/extension.test.ts
@@ -0,0 +1,99 @@
+import { join } from 'node:path';
+import { describe, expect, it } from 'vitest';
+import { getDefaultExtension } from '../src/utils/extension';
+
+describe('should get extension correctly', () => {
+  it('autoExtension is false', () => {
+    const options = {
+      format: 'cjs',
+      root: '/path/to/root',
+      autoExtension: false,
+    };
+
+    const result = getDefaultExtension(options);
+
+    expect(result).toEqual({
+      jsExtension: '.js',
+      dtsExtension: '.d.ts',
+    });
+  });
+
+  it('package.json is broken', () => {
+    const options = {
+      format: 'cjs',
+      root: '/path/to/root',
+      autoExtension: true,
+    };
+
+    const result = getDefaultExtension(options);
+
+    expect(result).toEqual({
+      jsExtension: '.js',
+      dtsExtension: '.d.ts',
+    });
+  });
+
+  it('format is cjs and type is module in package.json', () => {
+    const options = {
+      format: 'cjs',
+      root: join(__dirname, 'fixtures/extension/type-module'),
+      autoExtension: true,
+    };
+
+    const result = getDefaultExtension(options);
+
+    expect(result).toEqual({
+      jsExtension: '.cjs',
+      dtsExtension: '.d.cts',
+      isModule: true,
+    });
+  });
+
+  it('format is cjs and type is commonjs in package.json', () => {
+    const options = {
+      format: 'cjs',
+      root: join(__dirname, 'fixtures/extension/type-commonjs'),
+      autoExtension: true,
+    };
+
+    const result = getDefaultExtension(options);
+
+    expect(result).toEqual({
+      jsExtension: '.js',
+      dtsExtension: '.d.ts',
+      isModule: false,
+    });
+  });
+
+  it('format is esm and type is commonjs in package.json', () => {
+    const options = {
+      format: 'esm',
+      root: join(__dirname, 'fixtures/extension/type-commonjs'),
+      autoExtension: true,
+    };
+
+    const result = getDefaultExtension(options);
+
+    expect(result).toEqual({
+      jsExtension: '.mjs',
+      dtsExtension: '.d.mts',
+      isModule: false,
+    });
+  });
+
+  it('format is esm and type is module in package.json', () => {
+    const options = {
+      format: 'esm',
+      root: join(__dirname, 'fixtures/extension/type-module'),
+      autoExtension: true,
+    };
+
+    const result = getDefaultExtension(options);
+
+    expect(result).toEqual({
+      jsExtension: '.js',
+      dtsExtension: '.d.ts',
+      isModule: true,
+    });
+  });
+});
diff --git a/packages/core/tests/config/cjs/package.json b/packages/core/tests/fixtures/config/cjs/package.json
similarity index 100%
rename from packages/core/tests/config/cjs/package.json
rename to packages/core/tests/fixtures/config/cjs/package.json
diff --git a/packages/core/tests/config/cjs/rslib.config.cjs b/packages/core/tests/fixtures/config/cjs/rslib.config.cjs
similarity index 65%
rename from packages/core/tests/config/cjs/rslib.config.cjs
rename to packages/core/tests/fixtures/config/cjs/rslib.config.cjs
index aa2b913a..0b9702ca 100644
--- a/packages/core/tests/config/cjs/rslib.config.cjs
+++ b/packages/core/tests/fixtures/config/cjs/rslib.config.cjs
@@ -1,4 +1,4 @@
-const { defineConfig } = require('../../../../core/src/config');
+const { defineConfig } = require('../../../../../core/src/config');
 
 module.exports = defineConfig((args) => ({
   lib: [],
diff --git a/packages/core/tests/config/cjs/rslib.config.js b/packages/core/tests/fixtures/config/cjs/rslib.config.js
similarity index 100%
rename from packages/core/tests/config/cjs/rslib.config.js
rename to packages/core/tests/fixtures/config/cjs/rslib.config.js
diff --git a/packages/core/tests/config/cjs/rslib.config.mjs b/packages/core/tests/fixtures/config/cjs/rslib.config.mjs
similarity index 100%
rename from packages/core/tests/config/cjs/rslib.config.mjs
rename to packages/core/tests/fixtures/config/cjs/rslib.config.mjs
diff --git a/packages/core/tests/config/cjs/rslib.config.ts b/packages/core/tests/fixtures/config/cjs/rslib.config.ts
similarity index 100%
rename from packages/core/tests/config/cjs/rslib.config.ts
rename to packages/core/tests/fixtures/config/cjs/rslib.config.ts
diff --git a/packages/core/tests/config/esm/package.json b/packages/core/tests/fixtures/config/esm/package.json
similarity index 100%
rename from packages/core/tests/config/esm/package.json
rename to packages/core/tests/fixtures/config/esm/package.json
diff --git a/packages/core/tests/config/esm/rslib.config.cjs b/packages/core/tests/fixtures/config/esm/rslib.config.cjs
similarity index 100%
rename from packages/core/tests/config/esm/rslib.config.cjs
rename to packages/core/tests/fixtures/config/esm/rslib.config.cjs
diff --git a/packages/core/tests/config/esm/rslib.config.js b/packages/core/tests/fixtures/config/esm/rslib.config.js
similarity index 100%
rename from packages/core/tests/config/esm/rslib.config.js
rename to packages/core/tests/fixtures/config/esm/rslib.config.js
diff --git a/packages/core/tests/config/esm/rslib.config.mjs b/packages/core/tests/fixtures/config/esm/rslib.config.mjs
similarity index 66%
rename from packages/core/tests/config/esm/rslib.config.mjs
rename to packages/core/tests/fixtures/config/esm/rslib.config.mjs
index facaa7ee..f94b552b 100644
--- a/packages/core/tests/config/esm/rslib.config.mjs
+++ b/packages/core/tests/fixtures/config/esm/rslib.config.mjs
@@ -1,4 +1,4 @@
-import { defineConfig } from '../../../../core/src/config';
+import { defineConfig } from '../../../../../core/src/config';
 
 export default defineConfig((args) => ({
   lib: [],
diff --git a/packages/core/tests/config/esm/rslib.config.ts b/packages/core/tests/fixtures/config/esm/rslib.config.ts
similarity index 100%
rename from packages/core/tests/config/esm/rslib.config.ts
rename to packages/core/tests/fixtures/config/esm/rslib.config.ts
diff --git a/packages/core/tests/fixtures/extension/type-commonjs/package.json b/packages/core/tests/fixtures/extension/type-commonjs/package.json
new file mode 100644
index 00000000..5bbefffb
--- /dev/null
+++ b/packages/core/tests/fixtures/extension/type-commonjs/package.json
@@ -0,0 +1,3 @@
+{
+  "type": "commonjs"
+}
diff --git a/packages/core/tests/fixtures/extension/type-module/package.json b/packages/core/tests/fixtures/extension/type-module/package.json
new file mode 100644
index 00000000..3dbc1ca5
--- /dev/null
+++ b/packages/core/tests/fixtures/extension/type-module/package.json
@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}