diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 930038b..030e76a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-versions: ['8.1', '8.2', '8.3']
+        php-versions: ['8.3']
         phpunit-versions: [ 'latest' ]
     name: PHP ${{ matrix.php-versions }} Test
     steps:
diff --git a/composer.json b/composer.json
index e2e5fc6..693f92d 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,7 @@
     }
   ],
   "require": {
-    "php": "^8.1",
+    "php": "^8.3",
     "ext-crypto": "*",
     "koriym/http-constants": "^1.1",
     "psr/http-message": "^1.0",
diff --git a/src/Client/GuzzleClient.php b/src/Client/GuzzleClient.php
index f631a93..0d2c3dd 100644
--- a/src/Client/GuzzleClient.php
+++ b/src/Client/GuzzleClient.php
@@ -12,7 +12,7 @@
 use GuzzleHttp\RequestOptions;
 use Psr\Http\Message\ResponseInterface;
 
-final class GuzzleClient implements ClientInterface
+final readonly class GuzzleClient implements ClientInterface
 {
     public function __construct(
         private HttpClientInterface $client,
diff --git a/src/Extend/AesCmac.php b/src/Extend/AesCmac.php
index 5b4972d..30d5be0 100644
--- a/src/Extend/AesCmac.php
+++ b/src/Extend/AesCmac.php
@@ -12,7 +12,7 @@
 
 final class AesCmac
 {
-    private const ALGORITHM = 'AES-128-CBC';
+    private const string ALGORITHM = 'AES-128-CBC';
 
     /**
      * Returns a MAC in hex encoding.
diff --git a/src/Sesame.php b/src/Sesame.php
index c5ca7fe..7429868 100644
--- a/src/Sesame.php
+++ b/src/Sesame.php
@@ -15,11 +15,11 @@
 
 use function sprintf;
 
-final class Sesame implements SesameInterface
+final readonly class Sesame implements SesameInterface
 {
     public function __construct(
-        private readonly ClientInterface $client,
-        private readonly ResponseFactoryInterface $responseFactory,
+        private ClientInterface $client,
+        private ResponseFactoryInterface $responseFactory,
     ) {
     }
 
diff --git a/src/SesamiResponse.php b/src/SesamiResponse.php
index 841a958..0d248b0 100644
--- a/src/SesamiResponse.php
+++ b/src/SesamiResponse.php
@@ -8,10 +8,10 @@
 use Chanshige\SmartLock\Extend\Json;
 use Psr\Http\Message\ResponseInterface as PsrResponseInterface;
 
-final class SesamiResponse implements SesamiResponseInterface
+final readonly class SesamiResponse implements SesamiResponseInterface
 {
     public function __construct(
-        private readonly PsrResponseInterface $response,
+        private PsrResponseInterface $response,
     ) {
     }