diff --git a/application/Espo/Core/Api/ActionProcessor.php b/application/Espo/Core/Api/ActionProcessor.php
index 68d1f3aef3..b32e234ddf 100644
--- a/application/Espo/Core/Api/ActionProcessor.php
+++ b/application/Espo/Core/Api/ActionProcessor.php
@@ -48,9 +48,9 @@ use stdClass;
  */
 class ActionProcessor
 {
-    private $injectableFactory;
+    private InjectableFactory $injectableFactory;
 
-    private $classFinder;
+    private ClassFinder $classFinder;
 
     public function __construct(InjectableFactory $injectableFactory, ClassFinder $classFinder)
     {
@@ -130,6 +130,9 @@ class ActionProcessor
         return;
     }
 
+    /**
+     * @param mixed $result
+     */
     private function handleResult(Response $response, $result): void
     {
         $responseContents = $result;
@@ -183,6 +186,10 @@ class ActionProcessor
         return false;
     }
 
+    /**
+     * @return class-string
+     * @throws NotFound
+     */
     private function getControllerClassName(string $name): string
     {
         $className = $this->classFinder->find('Controllers', $name);
diff --git a/application/Espo/Core/Api/Auth.php b/application/Espo/Core/Api/Auth.php
index c2c1df3d6e..77b75bf882 100644
--- a/application/Espo/Core/Api/Auth.php
+++ b/application/Espo/Core/Api/Auth.php
@@ -171,6 +171,10 @@ class Auth
         return AuthResult::createNotResolved();
     }
 
+    /**
+     * @return array{string,string}
+     * @throws BadRequest
+     */
     protected function decodeAuthorizationString(string $string): array
     {
         $stringDecoded = base64_decode($string);
@@ -260,6 +264,9 @@ class Auth
         return null;
     }
 
+    /**
+     * @return array{?string,?string}
+     */
     protected function obtainUsernamePasswordFromRequest(Request $request): array
     {
         if ($request->hasHeader('Espo-Authorization')) {
diff --git a/application/Espo/Core/Api/AuthBuilder.php b/application/Espo/Core/Api/AuthBuilder.php
index 0945df3723..39e89e9556 100644
--- a/application/Espo/Core/Api/AuthBuilder.php
+++ b/application/Espo/Core/Api/AuthBuilder.php
@@ -40,13 +40,13 @@ use Espo\Core\{
  */
 class AuthBuilder
 {
-    private $authRequired = false;
+    private bool $authRequired = false;
 
-    private $isEntryPoint = false;
+    private bool $isEntryPoint = false;
 
-    private $authentication = null;
+    private ?Authentication $authentication = null;
 
-    private $log;
+    private Log $log;
 
     public function __construct(Log $log)
     {
diff --git a/application/Espo/Core/Api/AuthResult.php b/application/Espo/Core/Api/AuthResult.php
index e2becd9d6f..8c99ba17d7 100644
--- a/application/Espo/Core/Api/AuthResult.php
+++ b/application/Espo/Core/Api/AuthResult.php
@@ -34,9 +34,9 @@ namespace Espo\Core\Api;
  */
 class AuthResult
 {
-    private $isResolved = false;
+    private bool $isResolved = false;
 
-    private $isResolvedUseNoAuth = false;
+    private bool $isResolvedUseNoAuth = false;
 
     public static function createResolved(): self
     {
diff --git a/application/Espo/Core/Api/ErrorOutput.php b/application/Espo/Core/Api/ErrorOutput.php
index 65183c1ce0..6f38c00e16 100644
--- a/application/Espo/Core/Api/ErrorOutput.php
+++ b/application/Espo/Core/Api/ErrorOutput.php
@@ -45,6 +45,9 @@ use Throwable;
  */
 class ErrorOutput
 {
+    /**
+     * @var array<int,string>
+     */
     private $errorDescriptions = [
         400 => 'Bad Request',
         401 => 'Unauthorized',
@@ -55,6 +58,9 @@ class ErrorOutput
         503 => 'Service Unavailable',
     ];
 
+    /**
+     * @var int[]
+     */
     private $allowedStatusCodeList = [
         200,
         201,
@@ -67,13 +73,16 @@ class ErrorOutput
         503,
     ];
 
+    /**
+     * @var class-string[]
+     */
     private $ignorePrintXStatusReasonExceptionClassNameList = [
         'PDOException',
     ];
 
-    private $log;
+    private Log $log;
 
-    private $config;
+    private Config $config;
 
     public function __construct(Log $log, Config $config)
     {
diff --git a/application/Espo/Core/Api/Request.php b/application/Espo/Core/Api/Request.php
index 8659c2db86..d40773d2db 100644
--- a/application/Espo/Core/Api/Request.php
+++ b/application/Espo/Core/Api/Request.php
@@ -46,12 +46,14 @@ interface Request
     /**
      * Get a query parameter.
      *
-     * @return string|array|null
+     * @return string|string[]|null
      */
     public function getQueryParam(string $name);
 
     /**
      * Get all query parameters.
+     *
+     * @return array<string,string|string[]>
      */
     public function getQueryParams(): array;
 
@@ -67,6 +69,8 @@ interface Request
 
     /**
      * Get all route parameters.
+     *
+     * @return array<string,string>
      */
     public function getRouteParams(): array;
 
diff --git a/application/Espo/Core/Api/RequestNull.php b/application/Espo/Core/Api/RequestNull.php
index fce049fc02..e424039372 100644
--- a/application/Espo/Core/Api/RequestNull.php
+++ b/application/Espo/Core/Api/RequestNull.php
@@ -48,13 +48,16 @@ class RequestNull implements ApiRequest
     }
 
     /**
-     * @return string|array|null
+     * @return null
      */
     public function getQueryParam(string $name)
     {
         return null;
     }
 
+    /**
+     * @return string[]
+     */
     public function getQueryParams(): array
     {
         return [];
diff --git a/application/Espo/Core/Api/RequestWrapper.php b/application/Espo/Core/Api/RequestWrapper.php
index dcf8de44a2..87d74f9812 100644
--- a/application/Espo/Core/Api/RequestWrapper.php
+++ b/application/Espo/Core/Api/RequestWrapper.php
@@ -43,14 +43,20 @@ use stdClass;
  */
 class RequestWrapper implements ApiRequest
 {
-    private $request;
+    private Psr7Request $request;
 
-    private $basePath;
+    private string $basePath;
 
-    private $parsedBody = null;
+    private ?stdClass $parsedBody = null;
 
-    private $routeParams;
+    /**
+     * @var array<string,string>
+     */
+    private array $routeParams;
 
+    /**
+     * @param array<string,string> $routeParams
+     */
     public function __construct(Psr7Request $request, string $basePath = '', array $routeParams = [])
     {
         $this->request = $request;
@@ -92,6 +98,9 @@ class RequestWrapper implements ApiRequest
         return $this->routeParams[$name] ?? null;
     }
 
+    /**
+     * @return array<string,string>
+     */
     public function getRouteParams(): array
     {
         return $this->routeParams;
@@ -103,7 +112,7 @@ class RequestWrapper implements ApiRequest
     }
 
     /**
-     * @return string|array|null
+     * @return string|string[]|null
      */
     public function getQueryParam(string $name)
     {
@@ -116,6 +125,9 @@ class RequestWrapper implements ApiRequest
         return $value;
     }
 
+    /**
+     * @return array<string,string|string[]>
+     */
     public function getQueryParams(): array
     {
         return $this->request->getQueryParams();
@@ -184,7 +196,7 @@ class RequestWrapper implements ApiRequest
         return Util::cloneObject($this->parsedBody);
     }
 
-    private function initParsedBody()
+    private function initParsedBody(): void
     {
         $contents = $this->getBodyContents();
 
diff --git a/application/Espo/Core/Api/Route.php b/application/Espo/Core/Api/Route.php
index bd0ce283cc..b6fb11c0a6 100644
--- a/application/Espo/Core/Api/Route.php
+++ b/application/Espo/Core/Api/Route.php
@@ -31,14 +31,20 @@ namespace Espo\Core\Api;
 
 class Route
 {
-    private $method;
+    private string $method;
 
-    private $route;
+    private string $route;
 
-    private $params;
+    /**
+     * @var array<string,string>
+     */
+    private array $params;
 
-    private $noAuth;
+    private bool $noAuth;
 
+    /**
+     * @param array<string,string> $params
+     */
     public function __construct(
         string $method,
         string $route,
@@ -61,6 +67,9 @@ class Route
         return $this->route;
     }
 
+    /**
+     * @return array<string,string>
+     */
     public function getParams(): array
     {
         return $this->params;
diff --git a/application/Espo/Core/Api/Route/RouteParamsFetcher.php b/application/Espo/Core/Api/Route/RouteParamsFetcher.php
index 6684aafbc8..0ca1b2a58d 100644
--- a/application/Espo/Core/Api/Route/RouteParamsFetcher.php
+++ b/application/Espo/Core/Api/Route/RouteParamsFetcher.php
@@ -33,6 +33,10 @@ use Espo\Core\Api\Route;
 
 class RouteParamsFetcher
 {
+    /**
+     * @param array<string,mixed> $args
+     * @return array<string,mixed>
+     */
     public function fetch(Route $item, array $args): array
     {
         $params = [];
diff --git a/application/Espo/Core/Api/Util.php b/application/Espo/Core/Api/Util.php
index a8272d07c2..2d6360007e 100644
--- a/application/Espo/Core/Api/Util.php
+++ b/application/Espo/Core/Api/Util.php
@@ -29,11 +29,11 @@
 
 namespace Espo\Core\Api;
 
-use StdClass;
+use stdClass;
 
 class Util
 {
-    public static function cloneObject(StdClass $source): StdClass
+    public static function cloneObject(stdClass $source): stdClass
     {
         $cloned = (object) [];
 
@@ -44,6 +44,10 @@ class Util
         return $cloned;
     }
 
+    /**
+     * @param mixed $item
+     * @return mixed
+     */
     private static function cloneObjectItem($item)
     {
         if (is_array($item)) {
@@ -56,7 +60,7 @@ class Util
             return $cloned;
         }
 
-        if ($item instanceof StdClass) {
+        if ($item instanceof stdClass) {
             return self::cloneObject($item);
         }
 
diff --git a/application/Espo/Core/Utils/ClassFinder.php b/application/Espo/Core/Utils/ClassFinder.php
index 3b097e673c..585a68ab78 100644
--- a/application/Espo/Core/Utils/ClassFinder.php
+++ b/application/Espo/Core/Utils/ClassFinder.php
@@ -49,6 +49,8 @@ class ClassFinder
 
     /**
      * Find class name by a category and name.
+     *
+     * @return ?class-string
      */
     public function find(string $category, string $name, bool $subDirs = false): ?string
     {
@@ -62,7 +64,7 @@ class ClassFinder
     /**
      * Get a name => class name map.
      *
-     * @return array<string, string>
+     * @return array<string,class-string>
      */
     public function getMap(string $category, bool $subDirs = false): array
     {