diff --git a/.github/workflows/php-checks.yml b/.github/workflows/php-checks.yml
index bbfea1ad..b74a7223 100644
--- a/.github/workflows/php-checks.yml
+++ b/.github/workflows/php-checks.yml
@@ -6,6 +6,58 @@ on:
   merge_group:
 
 jobs:
+  diff-check:
+    name: Diff check
+    runs-on: ubuntu-latest
+
+    steps:
+      # Setup
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+      - name: Update submodules
+        run: git submodule update --remote --recursive
+      - uses: actions/setup-node@v4
+        id: setup_node_id
+        with:
+          node-version: 18
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.2
+
+      # Install openapi-generator-cli
+      - run: echo "OPENAPI_GENERATOR_VERSION=6.6.0" >> $GITHUB_ENV
+      - uses: actions/cache@v4
+        id: openapi-generator-cache
+        env:
+          cache-name: openapi-generator-cache
+        with:
+          path: ~/bin/openapitools
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ env.OPENAPI_GENERATOR_VERSION }}
+      - if: steps.openapi-generator-cache.outputs.cache-hit != 'true'
+        run: |
+          mkdir -p ~/bin/openapitools
+          curl https://raw.githubusercontent.com/OpenAPITools/openapi-generator/master/bin/utils/openapi-generator-cli.sh > ~/bin/openapitools/openapi-generator-cli
+          chmod u+x ~/bin/openapitools/openapi-generator-cli
+          export PATH=$PATH:~/bin/openapitools/
+          OPENAPI_GENERATOR_VERSION=${{ env.OPENAPI_GENERATOR_VERSION }} openapi-generator-cli version
+
+      - name: Generate codes
+        run: |
+          export PATH=$PATH:~/bin/openapitools/
+          bash tools/gen-oas-client.sh
+      - name: Update document
+        run: |
+          wget https://github.com/phpDocumentor/phpDocumentor/releases/download/v3.3.1/phpDocumentor.phar
+          php phpDocumentor.phar run -d src -t docs
+      - run: |
+          diff=$(git --no-pager diff --name-only HEAD)
+          echo "DIFF_IS_EMPTY=$([[ -z "$diff" ]] && echo 'true' || echo 'false')" >> $GITHUB_ENV
+          echo "CURRENT_DATETIME=$(date +'%Y%m%d%H%M%S')" >> $GITHUB_ENV
+      - if: ${{ env.DIFF_IS_EMPTY != 'true' }}
+        run: |
+          echo "There are changes from the auto-generated files by OAS. Please run diff-check.yml and merge the PR generated by it first."
+          exit 1
   tests:
     name: Run checks on PHP ${{ matrix.php }}
     runs-on: ubuntu-latest
@@ -86,4 +138,4 @@ jobs:
 
       - name: Run unit tests
         if: matrix.analysis
-        run: ./vendor/bin/phpunit --test-suffix=Test.php
+        run: ./vendor/bin/phpunit --test-suffix=Test.php --testdox
diff --git a/HACKING.md b/HACKING.md
index 13fe36da..9060f1d3 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -8,9 +8,9 @@ Run `make` or `make check` to check comprehensively. Following tests will run.
 
 - [PHPUnit](https://github.com/sebastianbergmann/phpunit) `make test`
 - Copyright check `make copyright`
-- [PHP_CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer) `make cs`
-- [PHPMD](https://phpmd.org/) `make md`
-- [PHPStan](https://phpstan.org/) `make stan`
+- [PHP_CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer) `make phpcs`
+- [PHPMD](https://phpmd.org/) `make phpmd`
+- [PHPStan](https://phpstan.org/) `make phpstan`
 
 Pull request policy
 --
diff --git a/phpunit.xml b/phpunit.xml
index 1f61c451..f8beef29 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -11,5 +11,8 @@
         <testsuite name="Parser">
             <directory suffix="Test.php">./src/parser/</directory>
         </testsuite>
+        <testsuite name="Test">
+            <directory suffix="Test.php">./test/</directory>
+        </testsuite>
     </testsuites>
 </phpunit>
diff --git a/test/clients/messaging-api/Api/MessagingApiApiTest.php b/test/clients/messaging-api/Api/MessagingApiApiTest.php
new file mode 100644
index 00000000..435d6864
--- /dev/null
+++ b/test/clients/messaging-api/Api/MessagingApiApiTest.php
@@ -0,0 +1,57 @@
+<?php
+/**
+ * Copyright 2024 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+namespace LINE\Tests\Clients\MessagingApi\Api;
+
+use GuzzleHttp\ClientInterface;
+use GuzzleHttp\Psr7\Request;
+use GuzzleHttp\Psr7\Response;
+use LINE\Clients\MessagingApi\Api\MessagingApiApi;
+use Mockery;
+use PHPUnit\Framework\TestCase;
+
+class MessagingApiApiTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+    }
+
+    public function testGetFollowers(): void
+    {
+        $client = Mockery::mock(ClientInterface::class);
+        $client->shouldReceive('send')
+            ->with(
+                Mockery::on(function (Request $request) {
+                    $this->assertEquals('GET', $request->getMethod());
+                    $this->assertEquals('https://api.line.me/v2/bot/followers/ids?limit=99', (string)$request->getUri());
+                    return true;
+                }),
+                []
+            )
+            ->once()
+            ->andReturn(new Response(
+                status: 200,
+                headers: [],
+                body: json_encode(['userIds' => ["Uaaaaaaaa...", "Ubbbbbbbb...", "Ucccccccc..."], 'next' => "yANU9IA.."]),
+            ));
+        $api = new MessagingApiApi($client);
+        $followers = $api->getFollowers(limit:  99);
+        $this->assertEquals(["Uaaaaaaaa...", "Ubbbbbbbb...", "Ucccccccc..."], $followers->getUserIds());
+        $this->assertEquals("yANU9IA..", $followers->getNext());
+    }
+}