diff --git a/src/Shared/Infrastructure/ApiPlatform/Serializer/ContractRelationDenormalizer.php b/src/Shared/Infrastructure/ApiPlatform/Serializer/ContractRelationDenormalizer.php
new file mode 100644
index 0000000..d173c67
--- /dev/null
+++ b/src/Shared/Infrastructure/ApiPlatform/Serializer/ContractRelationDenormalizer.php
@@ -0,0 +1,96 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Shared\Infrastructure\ApiPlatform\Serializer;
+
+use ApiPlatform\Metadata\IriConverterInterface;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Component\Serializer\Normalizer\DenormalizerAwareInterface;
+use Symfony\Component\Serializer\Normalizer\DenormalizerAwareTrait;
+use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
+use Throwable;
+
+use function array_key_exists;
+use function is_string;
+
+/**
+ * Modular monolith: cross-module relations are typed with a Shared\Domain\Contract
+ * interface (e.g. UserInterface, TaskTagInterface) instead of the concrete entity,
+ * to keep modules decoupled. Doctrine maps those back to the concrete entity through
+ * resolve_target_entities.
+ *
+ * API Platform denormalizes *single* interface relations fine (the concrete class is
+ * derived from the IRI), but blows up on *collections*: the collection value type stays
+ * the interface, which is not a registered API resource, so no normalizer supports it
+ * and the request fails with NotNormalizableValueException.
+ *
+ * This denormalizer bridges that gap for every contract interface, reusing Doctrine's
+ * resolve_target_entities mapping (no per-entity config):
+ *  - a string value is an IRI -> resolved through the IriConverter
+ *  - an array value is an embedded object -> denormalized into the concrete entity
+ */
+final class ContractRelationDenormalizer implements DenormalizerInterface, DenormalizerAwareInterface
+{
+    use DenormalizerAwareTrait;
+
+    private const CONTRACT_NAMESPACE = 'App\Shared\Domain\Contract\\';
+
+    /** @var array<string, ?class-string> */
+    private array $resolved = [];
+
+    public function __construct(
+        private readonly IriConverterInterface $iriConverter,
+        private readonly EntityManagerInterface $entityManager,
+    ) {}
+
+    public function supportsDenormalization(mixed $data, string $type, ?string $format = null, array $context = []): bool
+    {
+        return null !== $this->concreteClassFor($type);
+    }
+
+    public function denormalize(mixed $data, string $type, ?string $format = null, array $context = []): ?object
+    {
+        $concrete = $this->concreteClassFor($type);
+        if (null === $concrete) {
+            return null;
+        }
+
+        if (is_string($data)) {
+            return $this->iriConverter->getResourceFromIri($data, $context);
+        }
+
+        // Embedded object payload: denormalize into the resolved concrete entity.
+        return $this->denormalizer->denormalize($data, $concrete, $format, $context);
+    }
+
+    public function getSupportedTypes(?string $format): array
+    {
+        // Support depends on the runtime-resolved Doctrine mapping, so it cannot be
+        // statically cached by the serializer.
+        return ['object' => false];
+    }
+
+    /**
+     * @return ?class-string the concrete entity a contract interface resolves to, or null
+     */
+    private function concreteClassFor(string $type): ?string
+    {
+        if (array_key_exists($type, $this->resolved)) {
+            return $this->resolved[$type];
+        }
+
+        if (!str_starts_with($type, self::CONTRACT_NAMESPACE) || !interface_exists($type)) {
+            return $this->resolved[$type] = null;
+        }
+
+        try {
+            $name = $this->entityManager->getClassMetadata($type)->getName();
+        } catch (Throwable) {
+            // Not a Doctrine-mapped (resolve_target_entities) interface.
+            return $this->resolved[$type] = null;
+        }
+
+        return $this->resolved[$type] = ($name !== $type ? $name : null);
+    }
+}
diff --git a/tests/Functional/Module/Shared/InterfaceCollectionDenormalizationTest.php b/tests/Functional/Module/Shared/InterfaceCollectionDenormalizationTest.php
new file mode 100644
index 0000000..6437c98
--- /dev/null
+++ b/tests/Functional/Module/Shared/InterfaceCollectionDenormalizationTest.php
@@ -0,0 +1,119 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Functional\Module\Shared;
+
+use App\Module\Core\Domain\Entity\User;
+use App\Module\ProjectManagement\Domain\Entity\Project;
+use App\Module\ProjectManagement\Domain\Entity\TaskTag;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Bundle\FrameworkBundle\KernelBrowser;
+use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
+
+/**
+ * Regression: cross-module to-many relations are typed with a Shared contract
+ * interface (TaskTagInterface[], UserInterface[]). API Platform cannot
+ * denormalize a collection whose value type is an interface (no resource
+ * normalizer supports it), so every POST/PATCH carrying such a collection
+ * blew up with NotNormalizableValueException.
+ *
+ * @internal
+ */
+final class InterfaceCollectionDenormalizationTest extends WebTestCase
+{
+    protected function tearDown(): void
+    {
+        $conn = self::getContainer()->get(EntityManagerInterface::class)->getConnection();
+        $conn->executeStatement("DELETE FROM time_entry_task_type WHERE time_entry_id IN (SELECT id FROM time_entry WHERE title = 'iface-denorm-te')");
+        $conn->executeStatement("DELETE FROM time_entry WHERE title = 'iface-denorm-te'");
+        $conn->executeStatement("DELETE FROM task_collaborator WHERE task_id IN (SELECT id FROM task WHERE title = 'iface-denorm-task')");
+        $conn->executeStatement("DELETE FROM task WHERE title = 'iface-denorm-task'");
+        parent::tearDown();
+    }
+
+    public function testPostTimeEntryWithInterfaceTypedTags(): void
+    {
+        $client = self::createClient();
+        $em     = self::getContainer()->get(EntityManagerInterface::class);
+        $this->loginAdmin($client);
+
+        $userId = $this->adminId($em);
+        $tagId  = $this->aTaskTagId($em);
+
+        $client->request('POST', '/api/time_entries', server: [
+            'CONTENT_TYPE' => 'application/json',
+            'HTTP_ACCEPT'  => 'application/json',
+        ], content: json_encode([
+            'title'     => 'iface-denorm-te',
+            'startedAt' => '2026-06-22T10:00:00+02:00',
+            'stoppedAt' => '2026-06-22T11:00:00+02:00',
+            'user'      => '/api/users/'.$userId,
+            'tags'      => ['/api/task_tags/'.$tagId],
+        ]));
+
+        self::assertResponseStatusCodeSame(201, $client->getResponse()->getContent() ?: '');
+        $data = json_decode($client->getResponse()->getContent(), true);
+        self::assertCount(1, $data['tags'] ?? []);
+    }
+
+    public function testPostTaskWithInterfaceTypedCollaborators(): void
+    {
+        $client = self::createClient();
+        $em     = self::getContainer()->get(EntityManagerInterface::class);
+        $this->loginAdmin($client);
+
+        $userId    = $this->adminId($em);
+        $projectId = $this->aProjectId($em);
+
+        $client->request('POST', '/api/tasks', server: [
+            'CONTENT_TYPE' => 'application/json',
+            'HTTP_ACCEPT'  => 'application/json',
+        ], content: json_encode([
+            'title'         => 'iface-denorm-task',
+            'project'       => '/api/projects/'.$projectId,
+            'collaborators' => ['/api/users/'.$userId],
+        ]));
+
+        self::assertResponseStatusCodeSame(201, $client->getResponse()->getContent() ?: '');
+        $data = json_decode($client->getResponse()->getContent(), true);
+        self::assertCount(1, $data['collaborators'] ?? []);
+    }
+
+    private function loginAdmin(KernelBrowser $client): void
+    {
+        $em   = self::getContainer()->get(EntityManagerInterface::class);
+        $user = $em->getRepository(User::class)->findOneBy(['username' => 'admin']);
+        self::assertInstanceOf(User::class, $user);
+        $client->loginUser($user);
+    }
+
+    private function adminId(EntityManagerInterface $em): int
+    {
+        $user = $em->getRepository(User::class)->findOneBy(['username' => 'admin']);
+        self::assertInstanceOf(User::class, $user);
+
+        return $user->getId();
+    }
+
+    private function aTaskTagId(EntityManagerInterface $em): int
+    {
+        $tag = $em->getRepository(TaskTag::class)->findOneBy([]);
+        if (null === $tag) {
+            $tag = new TaskTag();
+            $tag->setLabel('iface-denorm-tag');
+            $em->persist($tag);
+            $em->flush();
+        }
+
+        return $tag->getId();
+    }
+
+    private function aProjectId(EntityManagerInterface $em): int
+    {
+        $project = $em->getRepository(Project::class)->findOneBy([]);
+        self::assertInstanceOf(Project::class, $project);
+
+        return $project->getId();
+    }
+}