diff --git a/src/Controller/Administrative/System/Document/DeleteController.php b/src/Controller/Administrative/System/Document/DeleteController.php
index 319b012..9228123 100644
--- a/src/Controller/Administrative/System/Document/DeleteController.php
+++ b/src/Controller/Administrative/System/Document/DeleteController.php
@@ -4,8 +4,10 @@ namespace App\Controller\Administrative\System\Document;
 
 use App\Entity\Upload;
 use App\Htmx\HxRedirectResponse;
+use App\Repository\SeasonTaskDocumentRepository;
 use Doctrine\ORM\EntityManagerInterface;
 use Psr\Log\LoggerInterface;
+use Symfony\Bridge\Doctrine\Attribute\MapEntity;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
@@ -17,14 +19,32 @@ class DeleteController extends AbstractController
     public function __construct(
         private readonly EntityManagerInterface $entityManager,
         private readonly LoggerInterface $logger,
+        private readonly SeasonTaskDocumentRepository $seasonTaskDocumentRepository,
     ) {
     }
 
     #[Route('/administrative/system/document/delete/{uuid}', name: 'app_administrative_system_document_delete')]
     #[IsGranted('ROLE_ADMINISTRATIVE')]
     #[IsGranted('DELETE', subject: 'document')]
-    public function index(Upload $document, Request $request): Response
-    {
+    public function index(
+        #[MapEntity(mapping: ['uuid' => 'uuid'])]
+        Upload $document,
+        Request $request,
+    ): Response {
+        // A document handed out for a season task is referenced by a NOT NULL foreign key, so
+        // deleting it fails in the database. It has to be replaced in that mapping first.
+        $seasonTaskDocuments = $this->seasonTaskDocumentRepository->findByDocument($document);
+
+        if (0 < count($seasonTaskDocuments) && true === $request->isMethod('POST')) {
+            $this->addFlash('error', 'Das Dokument ist als Saisonteam-Dokument hinterlegt und kann nicht gelöscht werden');
+            $this->logger->info('Refuse deleting document assigned to a season task', [
+                'document_id' => $document->getId(),
+                'document_filename' => $document->getOriginalFilename(),
+            ]);
+
+            return new HxRedirectResponse($this->generateUrl('app_administrative_system_document_index'));
+        }
+
         if (true === $request->isMethod('POST')) {
             $this->entityManager->remove($document);
             $this->entityManager->flush();
@@ -40,6 +60,7 @@ class DeleteController extends AbstractController
 
         return $this->render('administrative/system/document/modal_delete.html.twig', [
             'document' => $document,
+            'seasonTaskDocuments' => $seasonTaskDocuments,
         ]);
     }
 }
diff --git a/src/Controller/Administrative/System/Document/IndexController.php b/src/Controller/Administrative/System/Document/IndexController.php
index 51e8d05..1b7e173 100644
--- a/src/Controller/Administrative/System/Document/IndexController.php
+++ b/src/Controller/Administrative/System/Document/IndexController.php
@@ -2,6 +2,7 @@
 
 namespace App\Controller\Administrative\System\Document;
 
+use App\Repository\SeasonTaskDocumentRepository;
 use App\Repository\UploadRepository;
 use Knp\Component\Pager\PaginatorInterface;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
@@ -15,6 +16,7 @@ class IndexController extends AbstractController
     public function __construct(
         private readonly UploadRepository $uploadRepository,
         private readonly PaginatorInterface $paginator,
+        private readonly SeasonTaskDocumentRepository $seasonTaskDocumentRepository,
     ) {
     }
 
@@ -36,6 +38,7 @@ class IndexController extends AbstractController
 
         return $this->render('administrative/system/document/index.html.twig', [
             'pagination' => $pagination,
+            'seasonTaskDocuments' => $this->seasonTaskDocumentRepository->findGroupedByDocumentId(),
         ]);
     }
 }
diff --git a/src/Repository/SeasonTaskDocumentRepository.php b/src/Repository/SeasonTaskDocumentRepository.php
index 972bfa4..a0e21dd 100644
--- a/src/Repository/SeasonTaskDocumentRepository.php
+++ b/src/Repository/SeasonTaskDocumentRepository.php
@@ -4,6 +4,7 @@ namespace App\Repository;
 
 use App\Config\SeasonTask;
 use App\Entity\SeasonTaskDocument;
+use App\Entity\Upload;
 use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
 use Doctrine\Persistence\ManagerRegistry;
 
@@ -43,4 +44,31 @@ class SeasonTaskDocumentRepository extends ServiceEntityRepository
     {
         return $this->findOneBy(['taskKey' => $task->value]);
     }
+
+    /**
+     * Every mapping that points at the document, including rows whose task key no longer
+     * resolves: those still hold the foreign key that blocks deleting the upload.
+     *
+     * @return array<int, SeasonTaskDocument>
+     */
+    public function findByDocument(Upload $document): array
+    {
+        return $this->findBy(['document' => $document]);
+    }
+
+    /**
+     * The mappings grouped by the id of the document they point at, for the document list.
+     *
+     * @return array<int, array<int, SeasonTaskDocument>>
+     */
+    public function findGroupedByDocumentId(): array
+    {
+        $grouped = [];
+
+        foreach ($this->findAll() as $seasonTaskDocument) {
+            $grouped[$seasonTaskDocument->getDocument()->getId()][] = $seasonTaskDocument;
+        }
+
+        return $grouped;
+    }
 }
diff --git a/templates/administrative/system/document/index.html.twig b/templates/administrative/system/document/index.html.twig
index d3e0556..c9b744c 100644
--- a/templates/administrative/system/document/index.html.twig
+++ b/templates/administrative/system/document/index.html.twig
@@ -31,6 +31,7 @@
                 </thead>
                 <tbody>
                 {% for document in pagination %}
+                    {% set assignments = seasonTaskDocuments[document.id] ?? [] %}
                     <tr>
                         <td>
                             {{ document.displayName ? document.displayName : document.originalFilename }}
@@ -38,7 +39,10 @@
                         <td>
                             {# A slot belongs to exactly one document, so this is what tells staff
                                which file a feature actually picks up. #}
-                            {{ document.token ? document.token.label() : '-' }}
+                            {% set usages = (document.token ? [document.token.label()] : [])|merge(
+                                assignments|filter(a => a.task)|map(a => 'Saisonteam: ' ~ a.task.label())
+                            ) %}
+                            {{ usages is empty ? '-' : usages|join(', ') }}
                         </td>
                         <td>
                             {{ document.size|file_size }}
@@ -51,13 +55,21 @@
                         </td>
                         <td>
                             <div class="flex items-center space-x-2 justify-end">
-                                <button type="button"
-                                        class="text-red-500"
-                                        hx-get="{{ path('app_administrative_system_document_delete', { 'uuid': document.uuid }) }}"
-                                        hx-target="body"
-                                        hx-swap="beforeend">
-                                    {{ icon('delete') }}
-                                </button>
+                                {% if assignments is empty %}
+                                    <button type="button"
+                                            class="text-red-500"
+                                            hx-get="{{ path('app_administrative_system_document_delete', { 'uuid': document.uuid }) }}"
+                                            hx-target="body"
+                                            hx-swap="beforeend">
+                                        {{ icon('delete') }}
+                                    </button>
+                                {% else %}
+                                    {# Held by a NOT NULL foreign key; replace it under Saisonteam-Dokumente first. #}
+                                    <span class="text-gray-300 cursor-not-allowed"
+                                          title="Als Saisonteam-Dokument hinterlegt, kann nicht gelöscht werden">
+                                        {{ icon('delete') }}
+                                    </span>
+                                {% endif %}
                                 <button type="button"
                                         hx-get="{{ path('app_administrative_system_document_replace', { 'uuid': document.uuid }) }}"
                                         hx-target="body"
diff --git a/templates/administrative/system/document/modal_delete.html.twig b/templates/administrative/system/document/modal_delete.html.twig
index 1842e77..7aa09c5 100644
--- a/templates/administrative/system/document/modal_delete.html.twig
+++ b/templates/administrative/system/document/modal_delete.html.twig
@@ -1,7 +1,25 @@
 {% extends 'htmx_confirmation_modal.html.twig' %}
 
+{% block title %}{{ seasonTaskDocuments is empty ? 'Bist du sicher?' : 'Löschen nicht möglich' }}{% endblock %}
+
 {% block content %}
-    <div>
-        Möchtest du das Dokument <em>{{ document }}</em> wirklich löschen?
-    </div>
-{% endblock %}
\ No newline at end of file
+    {% if seasonTaskDocuments is empty %}
+        <div>
+            Möchtest du das Dokument <em>{{ document }}</em> wirklich löschen?
+        </div>
+    {% else %}
+        <div>
+            Das Dokument <em>{{ document }}</em> ist als Saisonteam-Dokument hinterlegt
+            {%- set labels = seasonTaskDocuments|map(m => m.task ? m.task.label() : null)|filter(l => l is not null) -%}
+            {%- if labels is not empty %} ({{ labels|join(', ') }}){% endif %}
+            und kann deshalb nicht gelöscht werden. Ersetze es zuerst unter
+            <a class="underline" href="{{ path('app_administrative_system_season_task_document_index') }}">Saisonteam-Dokumente</a>.
+        </div>
+    {% endif %}
+{% endblock %}
+
+{% block confirm %}
+    {% if seasonTaskDocuments is empty %}{{ parent() }}{% endif %}
+{% endblock %}
+
+{% block button_cancel %}{{ seasonTaskDocuments is empty ? 'Abbrechen' : 'Schließen' }}{% endblock %}
diff --git a/templates/htmx_confirmation_modal.html.twig b/templates/htmx_confirmation_modal.html.twig
index 64baf23..f3744c6 100644
--- a/templates/htmx_confirmation_modal.html.twig
+++ b/templates/htmx_confirmation_modal.html.twig
@@ -14,9 +14,11 @@
         </div>
         {% block content %}{% endblock %}
         <div class="mt-6 flex items-center justify-between">
+            {% block confirm %}
             <button type="button" class="btn bg-red-500" {% block button_attribute %}hx-post="{{ app.request.uri }}"{% endblock %}>
                 {% block button_confirm %}Ja{% endblock %}
             </button>
+            {% endblock %}
             <button type="button" class="btn" {{ stimulus_action('modal', 'close') }}>
                 {% block button_cancel %}Abbrechen{% endblock %}
             </button>
diff --git a/tests/Controller/Administrative/System/Document/DeleteControllerTest.php b/tests/Controller/Administrative/System/Document/DeleteControllerTest.php
new file mode 100644
index 0000000..cd8197d
--- /dev/null
+++ b/tests/Controller/Administrative/System/Document/DeleteControllerTest.php
@@ -0,0 +1,173 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Controller\Administrative\System\Document;
+
+use App\Config\SeasonTask;
+use App\Entity\SeasonTaskDocument;
+use App\Entity\Upload;
+use App\Entity\User;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Bundle\FrameworkBundle\KernelBrowser;
+use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * A library document handed out for a season task is held by a NOT NULL foreign key. Deleting it
+ * used to end in a 500 that htmx swallowed, so the modal just stayed open; it now has to be
+ * refused before it reaches the database.
+ */
+class DeleteControllerTest extends WebTestCase
+{
+    private const LIST_URL = '/administrative/system/document';
+
+    private KernelBrowser $client;
+
+    private EntityManagerInterface $entityManager;
+
+    protected function setUp(): void
+    {
+        $this->client = static::createClient(server: ['HTTPS' => true]);
+        $this->entityManager = self::getContainer()->get(EntityManagerInterface::class);
+
+        $this->entityManager->createQuery(sprintf('DELETE FROM %s', SeasonTaskDocument::class))->execute();
+        $this->entityManager->createQuery(sprintf('DELETE FROM %s', Upload::class))->execute();
+        $this->entityManager->createQuery(sprintf('DELETE FROM %s', User::class))->execute();
+
+        $this->client->loginUser($this->administrative());
+    }
+
+    /**
+     * The mapping holds a foreign key on upload; left behind, it breaks every later test that
+     * clears the upload table.
+     */
+    protected function tearDown(): void
+    {
+        $this->entityManager->createQuery(sprintf('DELETE FROM %s', SeasonTaskDocument::class))->execute();
+
+        parent::tearDown();
+    }
+
+    public function testTheModalOffersNoConfirmationForAnAssignedDocument(): void
+    {
+        $document = $this->assigned($this->document('Vertrag.pdf'), SeasonTask::CONTRACT);
+
+        $crawler = $this->client->request('GET', $this->deleteUrl($document));
+
+        $this->assertSame(Response::HTTP_OK, $this->client->getResponse()->getStatusCode());
+        $this->assertStringContainsString('kann deshalb nicht gelöscht werden', $crawler->text());
+        $this->assertStringContainsString('Vertrag', $crawler->text());
+        $this->assertCount(0, $crawler->filter('button[hx-post]'));
+    }
+
+    public function testDeletingAnAssignedDocumentIsRefused(): void
+    {
+        $document = $this->assigned($this->document('Vertrag.pdf'), SeasonTask::CONTRACT);
+        $id = $document->getId();
+
+        $this->client->request('POST', $this->deleteUrl($document));
+
+        $this->assertSame(Response::HTTP_OK, $this->client->getResponse()->getStatusCode());
+        $this->assertSame(self::LIST_URL, $this->client->getResponse()->headers->get('HX-Redirect'));
+
+        $this->entityManager->clear();
+        $this->assertNotNull($this->entityManager->find(Upload::class, $id));
+    }
+
+    /**
+     * A row whose task key no longer resolves still holds the foreign key.
+     */
+    public function testAnAssignmentToARemovedTaskStillBlocksDeletion(): void
+    {
+        $document = $this->document('Alt.pdf');
+        $this->entityManager->persist((new SeasonTaskDocument())->setTaskKey('removed_task')->setDocument($document));
+        $this->entityManager->flush();
+        $id = $document->getId();
+
+        $this->client->request('POST', $this->deleteUrl($document));
+
+        // Refused by the guard, not by the database: that would be the 500.
+        $this->assertSame(self::LIST_URL, $this->client->getResponse()->headers->get('HX-Redirect'));
+
+        $this->entityManager->clear();
+        $this->assertNotNull($this->entityManager->find(Upload::class, $id));
+    }
+
+    public function testAnUnassignedDocumentIsDeleted(): void
+    {
+        $document = $this->document('Handbuch.pdf');
+        $id = $document->getId();
+
+        $crawler = $this->client->request('GET', $this->deleteUrl($document));
+        $this->assertCount(1, $crawler->filter('button[hx-post]'));
+
+        $this->client->request('POST', $this->deleteUrl($document));
+
+        $this->assertSame(self::LIST_URL, $this->client->getResponse()->headers->get('HX-Redirect'));
+
+        $this->entityManager->clear();
+        $this->assertNull($this->entityManager->find(Upload::class, $id));
+    }
+
+    public function testTheListMarksAnAssignedDocumentAndOffersNoDelete(): void
+    {
+        $assigned = $this->assigned($this->document('Vertrag.pdf'), SeasonTask::CONTRACT);
+        $free = $this->document('Handbuch.pdf');
+
+        $crawler = $this->client->request('GET', self::LIST_URL);
+
+        $this->assertSame(Response::HTTP_OK, $this->client->getResponse()->getStatusCode());
+        $this->assertStringContainsString('Saisonteam: Vertrag', $crawler->text());
+        $this->assertCount(0, $crawler->filter(sprintf('button[hx-get="%s"]', $this->deleteUrl($assigned))));
+        $this->assertCount(1, $crawler->filter(sprintf('button[hx-get="%s"]', $this->deleteUrl($free))));
+    }
+
+    private function deleteUrl(Upload $document): string
+    {
+        return sprintf('/administrative/system/document/delete/%s', $document->getUuid());
+    }
+
+    private function assigned(Upload $document, SeasonTask $task): Upload
+    {
+        $this->entityManager->persist((new SeasonTaskDocument())->setTaskKey($task->value)->setDocument($document));
+        $this->entityManager->flush();
+
+        return $document;
+    }
+
+    private function document(string $filename): Upload
+    {
+        // No file on disk: neither the guard nor the list reads it, and deleting a missing file
+        // is a no-op for UploadHandler::removeUploadedFile().
+        $upload = (new Upload())
+            ->setType(Upload::TYPE_DOCUMENT)
+            ->setFilename(sprintf('test-%s-%s', bin2hex(random_bytes(4)), $filename))
+            ->setOriginalFilename($filename)
+            ->setMimeType('application/pdf')
+            ->setSize(1024)
+        ;
+
+        $this->entityManager->persist($upload);
+        $this->entityManager->flush();
+
+        return $upload;
+    }
+
+    private function administrative(): User
+    {
+        $user = (new User())
+            ->setFirstName('Anna')
+            ->setLastName('Administrative')
+            ->setEmail('anna.administrative@example.com')
+            ->setRoles(['ROLE_ADMINISTRATIVE'])
+            ->setBusProAddressId(1)
+            ->setBusProPersonId(1)
+        ;
+
+        $this->entityManager->persist($user);
+        $this->entityManager->flush();
+
+        return $user;
+    }
+}