diff --git a/.devcontainer/devcontainer.json b/.devcontainer/devcontainer.json
new file mode 100644
index 0000000..3c59058
--- /dev/null
+++ b/.devcontainer/devcontainer.json
@@ -0,0 +1,17 @@
+{
+  "name": "microcms-php-sdk",
+  "image": "mcr.microsoft.com/devcontainers/php:8.5",
+  "features": {
+    "ghcr.io/devcontainers/features/github-cli:1": {}
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "xdebug.php-debug",
+        "bmewburn.vscode-intelephense-client",
+        "junstyle.php-cs-fixer"
+      ]
+    }
+  },
+  "postCreateCommand": "composer install"
+}
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6b5d349..f21ac85 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -6,50 +6,61 @@ on:
   pull_request:
     branches: [ main ]
 
-jobs:
-  tests:
+env:
+  MAIN_PHP_VERSION: '8.5'
 
+jobs:
+  lint:
     runs-on: ubuntu-latest
-
-    strategy:
-      matrix:
-        php-versions: ['7.3', '7.4', '8.0']
-
+    timeout-minutes: 5
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      with:
+        persist-credentials: false
 
     - name: Setup PHP
-      uses: shivammathur/setup-php@master
+      uses: step-security/setup-php@0b4c3f57a1fb50d5cc8172a9700f35b97b3b5d97 # v2.37.0
       with:
-        php-version: ${{ matrix.php-versions }}
+        php-version: ${{ env.MAIN_PHP_VERSION }}
+        tools: composer, php-cs-fixer
+      env:
+        fail-fast: true
 
     - name: Validate composer.json and composer.lock
       run: composer validate --strict
 
-    - name: Cache Composer packages
-      id: composer-cache
-      uses: actions/cache@v2
-      with:
-        path: vendor
-        key: ${{ runner.os }}-php-${{ hashFiles('**/composer.lock') }}
-        restore-keys: |
-          ${{ runner.os }}-php-
-
     - name: Install dependencies
       run: composer install --prefer-dist --no-progress
 
-    - name: Run formatter (src/)
-      uses: docker://oskarstark/php-cs-fixer-ga
+    - name: Run formatter
+      run: php-cs-fixer fix --diff --dry-run
+
+    - name: Run linter
+      run: vendor/bin/phpstan analyse
+
+  tests:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    strategy:
+      fail-fast: true
+      matrix:
+        php-versions: ['7.4', '8.5']
+
+    steps:
+    - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
       with:
-        args: --diff --dry-run src/
+        persist-credentials: false
 
-    - name: Run formatter (tests/)
-      uses: docker://oskarstark/php-cs-fixer-ga
+    - name: Setup PHP
+      uses: step-security/setup-php@0b4c3f57a1fb50d5cc8172a9700f35b97b3b5d97 # v2.37.0
       with:
-        args: --diff --dry-run tests/
+        php-version: ${{ matrix.php-versions }}
+        tools: composer
+      env:
+        fail-fast: true
 
-    - name: Run linter
-      run: vendor/bin/phpstan analyse --level 5 src/ tests/
+    - name: Install dependencies
+      run: composer install --prefer-dist --no-progress
 
     - name: Run test suite
       run: vendor/bin/phpunit tests/
diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
new file mode 100644
index 0000000..08469fd
--- /dev/null
+++ b/.php-cs-fixer.dist.php
@@ -0,0 +1,12 @@
+<?php
+
+$finder = (new PhpCsFixer\Finder())
+    ->in([__DIR__ . '/src', __DIR__ . '/tests']);
+
+return (new PhpCsFixer\Config())
+    ->setRules([
+        '@PER-CS' => true,
+        'trailing_comma_in_multiline' => false,
+        'operator_linebreak' => ['position' => 'end'],
+    ])
+    ->setFinder($finder);
diff --git a/composer.json b/composer.json
index 99740a4..e958a62 100644
--- a/composer.json
+++ b/composer.json
@@ -25,7 +25,7 @@
     ],
     "require-dev": {
         "psy/psysh": "@stable",
-        "phpstan/phpstan": "^0.12.99",
+        "phpstan/phpstan": "^2.0",
         "phpunit/phpunit": "^9.5"
     }
 }
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..1cd333b
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,5 @@
+parameters:
+    level: 8
+    paths:
+        - src
+        - tests
diff --git a/src/Client.php b/src/Client.php
index 3e8929b..5161781 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -4,12 +4,12 @@
 
 class Client
 {
-    private $serviceDomain;
-    private $apiKey;
+    private string $serviceDomain;
+    private string $apiKey;
 
-    private $client;
+    private \GuzzleHttp\Client $client;
 
-    public function __construct(string $serviceDomain, string $apiKey, \GuzzleHttp\ClientInterface $client = null)
+    public function __construct(string $serviceDomain, string $apiKey, ?\GuzzleHttp\Client $client = null)
     {
         $this->serviceDomain = $serviceDomain;
         $this->apiKey = $apiKey;
@@ -21,6 +21,11 @@ public function __construct(string $serviceDomain, string $apiKey, \GuzzleHttp\C
         }
     }
 
+    /**
+     * @param array<string, mixed> $options
+     *
+     * @return mixed
+     */
     public function list(string $endpoint, array $options = [])
     {
         $path = $endpoint;
@@ -33,6 +38,11 @@ public function list(string $endpoint, array $options = [])
         return json_decode($response->getBody());
     }
 
+    /**
+     * @param array<string, mixed> $options
+     *
+     * @return mixed
+     */
     public function get(string $endpoint, string $contentId = "", array $options = [])
     {
         $path = $contentId ? implode("/", [$endpoint, $contentId]) : $endpoint;
@@ -45,6 +55,12 @@ public function get(string $endpoint, string $contentId = "", array $options = [
         return json_decode($response->getBody());
     }
 
+    /**
+     * @param array<string, mixed> $body
+     * @param array<string, mixed> $options
+     *
+     * @return mixed
+     */
     public function create(string $endpoint, array $body = [], array $options = [])
     {
         if (array_key_exists("id", $body)) {
@@ -66,6 +82,11 @@ public function create(string $endpoint, array $body = [], array $options = [])
         return json_decode($response->getBody());
     }
 
+    /**
+     * @param array<string, mixed> $body
+     *
+     * @return mixed
+     */
     public function update(string $endpoint, array $body = [])
     {
         $response = $this->client->patch(
@@ -83,13 +104,18 @@ function ($v, $k) {
         return json_decode($response->getBody());
     }
 
-    public function delete(string $endpoint, string $id)
+    public function delete(string $endpoint, string $id): void
     {
         $path = implode("/", [$endpoint, $id]);
         $this->client->delete($path, $this->buildOption());
     }
 
-    private function buildOption(array $option = [])
+    /**
+     * @param array<string, mixed> $option
+     *
+     * @return array<string, mixed>
+     */
+    private function buildOption(array $option = []): array
     {
         return array_merge(
             [
@@ -102,7 +128,12 @@ private function buildOption(array $option = [])
         );
     }
 
-    private function buildQuery(array $options)
+    /**
+     * @param array<string, mixed> $options
+     *
+     * @return array<string, mixed>
+     */
+    private function buildQuery(array $options): array
     {
         return array_filter(
             [
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index 644b4ae..9273330 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -11,19 +11,22 @@
 
 final class ClientTest extends TestCase
 {
-    private $handlerStack;
-    private $mock;
+    private HandlerStack $handlerStack;
+    private MockHandler $mock;
+
+    /** @var array<int, array<string, mixed>> */
     private $container;
 
     protected function setUp(): void
     {
         $this->handlerStack = HandlerStack::create();
         $this->container = [];
+        $container = &$this->container;
 
         $this->mock = new MockHandler([]);
         $this->handlerStack->setHandler($this->mock);
 
-        $history = Middleware::history($this->container);
+        $history = Middleware::history($container);
         $this->handlerStack->push($history);
     }