diff --git a/config/bootstrap.php b/config/bootstrap.php
index e05ec9fe2..3732ec7ee 100644
--- a/config/bootstrap.php
+++ b/config/bootstrap.php
@@ -202,16 +202,39 @@
  * If you don't use these checks you can safely remove this code
  * and the mobiledetect package from composer.json.
  */
-ServerRequest::addDetector('mobile', function ($request) {
-    $detector = new MobileDetect();
-
-    return $detector->isMobile();
-});
-ServerRequest::addDetector('tablet', function ($request) {
-    $detector = new MobileDetect();
+$deviceDetector = static function (ServerRequest $request): MobileDetect {
+    // Read the headers from the request instead of the global $_SERVER.
+    $detector = new MobileDetect(config: ['autoInitOfHttpHeaders' => false]);
+    $knownHeaders = array_merge(
+        $detector->getUaHttpHeaders(),
+        array_keys($detector->getMobileHeaders()),
+        $detector->getCloudFrontHttpHeaders(),
+    );
+    $serverParams = $request->getServerParams();
+    $headers = [];
+    foreach ($knownHeaders as $name) {
+        if (isset($serverParams[$name])) {
+            // MobileDetect expects strings, but withHeader() can store arrays.
+            $headers[$name] = implode(', ', (array)$serverParams[$name]);
+        }
+    }
+    $detector->setHttpHeaders($headers);
+    // MobileDetect throws when no User-Agent is set at all.
+    if (!$detector->hasUserAgent()) {
+        $detector->setUserAgent('');
+    }
 
-    return $detector->isTablet();
-});
+    return $detector;
+};
+ServerRequest::addDetector(
+    'mobile',
+    static fn(ServerRequest $request): bool => $deviceDetector($request)->isMobile(),
+);
+ServerRequest::addDetector(
+    'tablet',
+    static fn(ServerRequest $request): bool => $deviceDetector($request)->isTablet(),
+);
+unset($deviceDetector);
 
 /*
  * You can enable default locale format parsing by adding calls
diff --git a/tests/TestCase/RequestDetectorTest.php b/tests/TestCase/RequestDetectorTest.php
new file mode 100644
index 000000000..9a5882c69
--- /dev/null
+++ b/tests/TestCase/RequestDetectorTest.php
@@ -0,0 +1,104 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * CakePHP(tm) : Rapid Development Framework (https://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ * @link          https://cakephp.org CakePHP(tm) Project
+ * @license       https://opensource.org/licenses/mit-license.php MIT License
+ */
+namespace App\Test\TestCase;
+
+use Cake\Http\ServerRequest;
+use Cake\TestSuite\TestCase;
+use ErrorException;
+
+/**
+ * Tests the mobile and tablet detectors registered in config/bootstrap.php.
+ */
+class RequestDetectorTest extends TestCase
+{
+    protected const IPHONE_USER_AGENT = 'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) '
+        . 'AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1';
+
+    protected const IPAD_USER_AGENT = 'Mozilla/5.0 (iPad; CPU OS 17_0 like Mac OS X) '
+        . 'AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1';
+
+    protected const DESKTOP_USER_AGENT = 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) '
+        . 'AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36';
+
+    /**
+     * Test the detectors read the User-Agent from the request environment.
+     *
+     * @return void
+     */
+    public function testDetectorsUseRequestEnvironment(): void
+    {
+        $phone = new ServerRequest(['environment' => ['HTTP_USER_AGENT' => self::IPHONE_USER_AGENT]]);
+        $this->assertTrue($phone->is('mobile'));
+        $this->assertFalse($phone->is('tablet'));
+
+        $tablet = new ServerRequest(['environment' => ['HTTP_USER_AGENT' => self::IPAD_USER_AGENT]]);
+        $this->assertTrue($tablet->is('mobile'));
+        $this->assertTrue($tablet->is('tablet'));
+
+        $desktop = new ServerRequest(['environment' => ['HTTP_USER_AGENT' => self::DESKTOP_USER_AGENT]]);
+        $this->assertFalse($desktop->is('mobile'));
+        $this->assertFalse($desktop->is('tablet'));
+    }
+
+    /**
+     * Test the detectors follow a User-Agent header changed after construction.
+     *
+     * @return void
+     */
+    public function testDetectorsUseChangedHeader(): void
+    {
+        $request = new ServerRequest(['environment' => ['HTTP_USER_AGENT' => self::DESKTOP_USER_AGENT]]);
+        $request = $request->withHeader('User-Agent', self::IPHONE_USER_AGENT);
+
+        $this->assertTrue($request->is('mobile'));
+    }
+
+    /**
+     * Test the detectors handle array header values and non-string server params.
+     *
+     * @return void
+     */
+    public function testDetectorsWithArrayValues(): void
+    {
+        $request = new ServerRequest(['environment' => ['argv' => ['bin/cake'], 'argc' => 1]]);
+        $request = $request->withHeader('User-Agent', [self::IPHONE_USER_AGENT])
+            ->withHeader('Accept', ['text/html']);
+
+        // Fail on "Array to string conversion" warnings instead of only reporting them.
+        set_error_handler(function (int $severity, string $message): never {
+            throw new ErrorException($message, 0, $severity);
+        });
+        try {
+            $this->assertTrue($request->is('mobile'));
+            $this->assertFalse($request->is('tablet'));
+        } finally {
+            restore_error_handler();
+        }
+    }
+
+    /**
+     * Test the detectors do not fail without a User-Agent.
+     *
+     * @return void
+     */
+    public function testDetectorsWithoutUserAgent(): void
+    {
+        $request = new ServerRequest();
+
+        $this->assertFalse($request->is('mobile'));
+        $this->assertFalse($request->is('tablet'));
+    }
+}