diff --git a/src/App/bootstrap.php b/src/App/bootstrap.php
index c249515b..708567ec 100644
--- a/src/App/bootstrap.php
+++ b/src/App/bootstrap.php
@@ -22,3 +22,5 @@ spl_autoload_register(static function (string $class): void {
         }
     }
 });
+
+require_once dirname(__DIR__) . '/ModulesCore/LegacyCompat.php';
diff --git a/src/ModulesCore/LegacyCompat.php b/src/ModulesCore/LegacyCompat.php
new file mode 100644
index 00000000..a63bb885
--- /dev/null
+++ b/src/ModulesCore/LegacyCompat.php
@@ -0,0 +1,220 @@
+<?php
+
+declare(strict_types=1);
+
+namespace ModulesCore {
+
+    use App\ConfigLoader;
+    use App\KeycloakAuth;
+    use RuntimeException;
+
+    final class LegacyCompat
+    {
+        private static ?LegacyCompatAppContext $appContext = null;
+        private static ?LegacyCompatModuleRuntime $moduleRuntime = null;
+
+        public static function app(): LegacyCompatAppContext
+        {
+            if (self::$appContext === null) {
+                self::$appContext = new LegacyCompatAppContext(self::projectRoot());
+            }
+
+            return self::$appContext;
+        }
+
+        public static function modules(): LegacyCompatModuleRuntime
+        {
+            if (self::$moduleRuntime === null) {
+                self::$moduleRuntime = new LegacyCompatModuleRuntime(self::projectRoot());
+            }
+
+            return self::$moduleRuntime;
+        }
+
+        public static function projectRoot(): string
+        {
+            return dirname(__DIR__, 2);
+        }
+    }
+
+    final class LegacyCompatAppContext
+    {
+        private ?LegacyCompatConfig $config = null;
+        private ?LegacyCompatAuthContext $auth = null;
+
+        public function __construct(
+            private readonly string $projectRoot,
+        ) {
+        }
+
+        public function config(): LegacyCompatConfig
+        {
+            if ($this->config === null) {
+                $this->config = new LegacyCompatConfig($this->projectRoot);
+            }
+
+            return $this->config;
+        }
+
+        public function auth(): LegacyCompatAuthContext
+        {
+            if ($this->auth === null) {
+                $this->auth = new LegacyCompatAuthContext($this->projectRoot);
+            }
+
+            return $this->auth;
+        }
+    }
+
+    final class LegacyCompatConfig
+    {
+        /** @var array<string, mixed> */
+        public array $dbConfig;
+
+        public function __construct(string $projectRoot)
+        {
+            $baseDb = ConfigLoader::load($projectRoot, 'base_db');
+            $db = ConfigLoader::load($projectRoot, 'db_settings_basic');
+            $baseDbEnabled = (bool) ($baseDb['enabled'] ?? false);
+            $baseDbConfig = is_array($baseDb['db'] ?? null) ? $baseDb['db'] : [];
+            $dbConfig = is_array($db) ? $db : [];
+
+            $this->dbConfig = $baseDbEnabled && $baseDbConfig !== []
+                ? $baseDbConfig
+                : $dbConfig;
+        }
+    }
+
+    final class LegacyCompatAuthContext
+    {
+        private ?KeycloakAuth $auth = null;
+
+        public function __construct(
+            private readonly string $projectRoot,
+        ) {
+        }
+
+        public function isEnabled(): bool
+        {
+            return $this->auth()->isEnabled();
+        }
+
+        /**
+         * @return array<string, mixed>|null
+         */
+        public function user(): ?array
+        {
+            $session = $_SESSION['desktop_auth']['user'] ?? null;
+
+            return is_array($session) ? $session : null;
+        }
+
+        private function auth(): KeycloakAuth
+        {
+            if ($this->auth === null) {
+                $this->auth = new KeycloakAuth(ConfigLoader::load($this->projectRoot, 'keycloak'));
+            }
+
+            return $this->auth;
+        }
+    }
+
+    final class LegacyCompatModuleRuntime
+    {
+        /** @var array<string, array<string, mixed>> */
+        private array $settingsCache = [];
+
+        public function __construct(
+            private readonly string $projectRoot,
+        ) {
+        }
+
+        public function isEnabled(string $moduleName): bool
+        {
+            return is_dir($this->modulePath($moduleName));
+        }
+
+        public function hasFunction(string $moduleName, string $functionName): bool
+        {
+            return false;
+        }
+
+        public function call(string $moduleName, string $functionName, mixed ...$args): mixed
+        {
+            throw new RuntimeException(sprintf(
+                'Modulfunktion %s::%s ist in dieser Laufzeit nicht registriert.',
+                $moduleName,
+                $functionName
+            ));
+        }
+
+        /**
+         * @return array<string, mixed>
+         */
+        public function settings(string $moduleName): array
+        {
+            if (isset($this->settingsCache[$moduleName])) {
+                return $this->settingsCache[$moduleName];
+            }
+
+            $configPath = $this->modulePath($moduleName) . '/config/module.php';
+            if (!is_file($configPath)) {
+                return $this->settingsCache[$moduleName] = [];
+            }
+
+            $config = require $configPath;
+
+            return $this->settingsCache[$moduleName] = is_array($config) ? $config : [];
+        }
+
+        private function modulePath(string $moduleName): string
+        {
+            return $this->projectRoot . '/modules/' . trim($moduleName, '/');
+        }
+    }
+}
+
+namespace {
+
+    use ModulesCore\LegacyCompat;
+
+    if (!function_exists('app')) {
+        function app(): \ModulesCore\LegacyCompatAppContext
+        {
+            return LegacyCompat::app();
+        }
+    }
+
+    if (!function_exists('modules')) {
+        function modules(): \ModulesCore\LegacyCompatModuleRuntime
+        {
+            return LegacyCompat::modules();
+        }
+    }
+
+    if (!function_exists('module_fn')) {
+        function module_fn(string $moduleName, string $functionName, mixed ...$args): mixed
+        {
+            return modules()->call($moduleName, $functionName, ...$args);
+        }
+    }
+
+    if (!function_exists('module_debug_enabled')) {
+        function module_debug_enabled(string $moduleName): bool
+        {
+            $settings = modules()->settings($moduleName);
+            $debug = $settings['debug'] ?? null;
+
+            return is_array($debug) && (bool) ($debug['enabled'] ?? false);
+        }
+    }
+
+    if (!function_exists('nexus_display_timezone_name')) {
+        function nexus_display_timezone_name(): string
+        {
+            $timezone = trim((string) date_default_timezone_get());
+
+            return $timezone !== '' ? $timezone : 'Europe/Berlin';
+        }
+    }
+}