diff --git a/src/EventListener/AccessDeniedListener.php b/src/EventListener/AccessDeniedListener.php
index 096416cc..ab99027b 100644
--- a/src/EventListener/AccessDeniedListener.php
+++ b/src/EventListener/AccessDeniedListener.php
@@ -63,13 +63,19 @@ class AccessDeniedListener implements EventSubscriberInterface
             return;
         }
 
-        /** @var Session $session */
-        $session = $request->getSession();
-
         $this->authLogger->warning('Access denied', [
             'uri' => $request->getRequestUri(),
         ]);
 
+        // API clients (/api, /mcp, webhooks) get the plain 401/403. A flash would start a
+        // session nobody reads and send a cookie, and a redirect would answer with HTML.
+        if (true === $this->security->getFirewallConfig($request)?->isStateless()) {
+            return;
+        }
+
+        /** @var Session $session */
+        $session = $request->getSession();
+
         if (null === $this->security->getUser()) {
             $session->getFlashBag()->add('info', 'Bitte melde dich an.');
 
diff --git a/tests/EventListener/AccessDeniedListenerTest.php b/tests/EventListener/AccessDeniedListenerTest.php
index 9c3c30cc..1f0bb1ff 100644
--- a/tests/EventListener/AccessDeniedListenerTest.php
+++ b/tests/EventListener/AccessDeniedListenerTest.php
@@ -11,6 +11,7 @@ use App\Security\DefaultRouteResolver;
 use PHPUnit\Framework\TestCase;
 use Psr\Log\LoggerInterface;
 use Symfony\Bundle\SecurityBundle\Security;
+use Symfony\Bundle\SecurityBundle\Security\FirewallConfig;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Session\Session;
@@ -83,6 +84,17 @@ class AccessDeniedListenerTest extends TestCase
         self::assertSame([], $this->session($request)->getFlashBag()->all());
     }
 
+    public function testStatelessFirewallsGetThePlainResponseWithoutStartingASession(): void
+    {
+        foreach ([null, new User('client@example.org')] as $user) {
+            $request = $this->request('/mcp');
+            $event = $this->dispatch($request, $user, stateless: true);
+
+            self::assertFalse($event->hasResponse());
+            self::assertFalse($this->session($request)->isStarted());
+        }
+    }
+
     /**
      * @param array<string, string> $server
      */
@@ -102,10 +114,11 @@ class AccessDeniedListenerTest extends TestCase
         return $session;
     }
 
-    private function dispatch(Request $request, ?UserInterface $user): ExceptionEvent
+    private function dispatch(Request $request, ?UserInterface $user, bool $stateless = false): ExceptionEvent
     {
         $security = $this->createStub(Security::class);
         $security->method('getUser')->willReturn($user);
+        $security->method('getFirewallConfig')->willReturn(new FirewallConfig($stateless ? 'mcp' : 'main', 'security.user_checker', stateless: $stateless));
 
         $resolver = $this->createStub(DefaultRouteResolver::class);
         $resolver->method('resolveUrl')->willReturn('/admin/dashboard');