diff --git a/Inventory_frontend b/Inventory_frontend
index db630e3..40d0753 160000
--- a/Inventory_frontend
+++ b/Inventory_frontend
@@ -1 +1 @@
-Subproject commit db630e315b2a5bd3ccd89abaa1c3bce1f9a78ef5
+Subproject commit 40d0753637f53092a557c7ebf162b243fa6780e9
diff --git a/migrations/Version20260323100000.php b/migrations/Version20260323100000.php
new file mode 100644
index 0000000..a9bc07b
--- /dev/null
+++ b/migrations/Version20260323100000.php
@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Scaffold missing composant slots for existing composants that have
+ * a typeComposant with skeleton requirements but no corresponding slots.
+ */
+final class Version20260323100000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Scaffold missing composant slots from skeleton requirements for existing composants';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // Piece slots
+        $this->addSql(<<<'SQL'
+                INSERT INTO composant_piece_slots (id, "composantid", "typepieceid", quantity, position, "createdat", "updatedat")
+                SELECT
+                  'cl' || substr(md5(random()::text || clock_timestamp()::text || spr.id), 1, 24),
+                  c.id,
+                  spr."typepieceid",
+                  1,
+                  spr.position,
+                  NOW(),
+                  NOW()
+                FROM composants c
+                JOIN skeleton_piece_requirements spr ON spr."modeltypeid" = c."typecomposantid"
+                WHERE c."typecomposantid" IS NOT NULL
+                  AND NOT EXISTS (
+                    SELECT 1 FROM composant_piece_slots cps
+                    WHERE cps."composantid" = c.id AND cps."typepieceid" = spr."typepieceid"
+                  )
+            SQL);
+
+        // Product slots
+        $this->addSql(<<<'SQL'
+                INSERT INTO composant_product_slots (id, "composantid", "typeproductid", "familycode", position, "createdat", "updatedat")
+                SELECT
+                  'cl' || substr(md5(random()::text || clock_timestamp()::text || spr.id), 1, 24),
+                  c.id,
+                  spr."typeproductid",
+                  spr."familycode",
+                  spr.position,
+                  NOW(),
+                  NOW()
+                FROM composants c
+                JOIN skeleton_product_requirements spr ON spr."modeltypeid" = c."typecomposantid"
+                WHERE c."typecomposantid" IS NOT NULL
+                  AND NOT EXISTS (
+                    SELECT 1 FROM composant_product_slots cps
+                    WHERE cps."composantid" = c.id AND cps."typeproductid" = spr."typeproductid"
+                  )
+            SQL);
+
+        // Subcomponent slots
+        $this->addSql(<<<'SQL'
+                INSERT INTO composant_subcomponent_slots (id, "composantid", alias, "familycode", "typecomposantid", position, "createdat", "updatedat")
+                SELECT
+                  'cl' || substr(md5(random()::text || clock_timestamp()::text || spr.id), 1, 24),
+                  c.id,
+                  spr.alias,
+                  spr."familycode",
+                  spr."typecomposantid",
+                  spr.position,
+                  NOW(),
+                  NOW()
+                FROM composants c
+                JOIN skeleton_subcomponent_requirements spr ON spr."modeltypeid" = c."typecomposantid"
+                WHERE c."typecomposantid" IS NOT NULL
+                  AND NOT EXISTS (
+                    SELECT 1 FROM composant_subcomponent_slots css
+                    WHERE css."composantid" = c.id
+                      AND COALESCE(css."typecomposantid", '') = COALESCE(spr."typecomposantid", '')
+                      AND COALESCE(css.alias, '') = COALESCE(spr.alias, '')
+                  )
+            SQL);
+    }
+
+    public function down(Schema $schema): void
+    {
+        // No-op: slots created by this migration are valid data
+    }
+}
diff --git a/src/Entity/Composant.php b/src/Entity/Composant.php
index 4435b86..9fe6fd5 100644
--- a/src/Entity/Composant.php
+++ b/src/Entity/Composant.php
@@ -16,6 +16,7 @@ use ApiPlatform\Metadata\Post;
 use ApiPlatform\Metadata\Put;
 use App\Entity\Trait\CuidEntityTrait;
 use App\Repository\ComposantRepository;
+use App\State\ComposantProcessor;
 use DateTimeImmutable;
 use Doctrine\Common\Collections\ArrayCollection;
 use Doctrine\Common\Collections\Collection;
@@ -33,7 +34,7 @@ use Symfony\Component\Serializer\Attribute\Groups;
     operations: [
         new Get(security: "is_granted('ROLE_VIEWER')"),
         new GetCollection(security: "is_granted('ROLE_VIEWER')"),
-        new Post(security: "is_granted('ROLE_GESTIONNAIRE')"),
+        new Post(security: "is_granted('ROLE_GESTIONNAIRE')", processor: ComposantProcessor::class),
         new Put(security: "is_granted('ROLE_GESTIONNAIRE')"),
         new Patch(security: "is_granted('ROLE_GESTIONNAIRE')"),
         new Delete(security: "is_granted('ROLE_GESTIONNAIRE')"),
diff --git a/src/EventSubscriber/UniqueConstraintSubscriber.php b/src/EventSubscriber/UniqueConstraintSubscriber.php
index dd5fbb4..91f9f60 100644
--- a/src/EventSubscriber/UniqueConstraintSubscriber.php
+++ b/src/EventSubscriber/UniqueConstraintSubscriber.php
@@ -16,7 +16,7 @@ final class UniqueConstraintSubscriber implements EventSubscriberInterface
     public static function getSubscribedEvents(): array
     {
         return [
-            KernelEvents::EXCEPTION => 'onKernelException',
+            KernelEvents::EXCEPTION => ['onKernelException', 256],
         ];
     }
 
@@ -28,10 +28,17 @@ final class UniqueConstraintSubscriber implements EventSubscriberInterface
             return;
         }
 
+        $constraint = $this->detectConstraintName($exception);
+        $error      = match ($constraint) {
+            'unique_category_name' => 'Un élément avec ce nom existe déjà dans cette catégorie.',
+            default                => 'Un élément avec cette valeur existe déjà.',
+        };
+
         $event->setResponse(new JsonResponse(
             [
-                'success' => false,
-                'error'   => 'nom duplique',
+                'success'    => false,
+                'error'      => $error,
+                'constraint' => $constraint,
             ],
             JsonResponse::HTTP_CONFLICT
         ));
@@ -47,4 +54,15 @@ final class UniqueConstraintSubscriber implements EventSubscriberInterface
 
         return null;
     }
+
+    private function detectConstraintName(UniqueConstraintViolationException $exception): ?string
+    {
+        $message = $exception->getMessage();
+
+        if (preg_match('/constraint\s+"([^"]+)"/', $message, $matches)) {
+            return $matches[1];
+        }
+
+        return null;
+    }
 }
diff --git a/src/State/ComposantProcessor.php b/src/State/ComposantProcessor.php
new file mode 100644
index 0000000..eb87125
--- /dev/null
+++ b/src/State/ComposantProcessor.php
@@ -0,0 +1,110 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\State;
+
+use ApiPlatform\Metadata\Operation;
+use ApiPlatform\Metadata\Post;
+use ApiPlatform\State\ProcessorInterface;
+use App\Entity\Composant;
+use App\Entity\ComposantPieceSlot;
+use App\Entity\ComposantProductSlot;
+use App\Entity\ComposantSubcomponentSlot;
+use App\Entity\ModelType;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Component\DependencyInjection\Attribute\Autowire;
+
+/**
+ * Initializes skeleton slots when a Composant is created with a typeComposant.
+ *
+ * Reads the SkeletonPieceRequirements, SkeletonProductRequirements and
+ * SkeletonSubcomponentRequirements from the ModelType and creates matching
+ * ComposantPieceSlot / ComposantProductSlot / ComposantSubcomponentSlot rows.
+ */
+final class ComposantProcessor implements ProcessorInterface
+{
+    public function __construct(
+        #[Autowire(service: 'api_platform.doctrine.orm.state.persist_processor')]
+        private readonly ProcessorInterface $decorated,
+        private readonly EntityManagerInterface $entityManager,
+    ) {}
+
+    /**
+     * @param array<string, mixed> $uriVariables
+     * @param array<string, mixed> $context
+     */
+    public function process(mixed $data, Operation $operation, array $uriVariables = [], array $context = []): mixed
+    {
+        if (!$data instanceof Composant) {
+            return $this->decorated->process($data, $operation, $uriVariables, $context);
+        }
+
+        $isCreation = $operation instanceof Post;
+
+        // Persist the entity first
+        $result = $this->decorated->process($data, $operation, $uriVariables, $context);
+
+        // On creation, scaffold slots from the ModelType skeleton
+        if ($isCreation) {
+            $this->initializeSlotsFromSkeleton($data);
+        }
+
+        return $result;
+    }
+
+    private function initializeSlotsFromSkeleton(Composant $composant): void
+    {
+        $modelType = $composant->getTypeComposant();
+
+        if (!$modelType) {
+            return;
+        }
+
+        // Ensure the ModelType's skeleton collections are loaded
+        $modelType = $this->entityManager->getRepository(ModelType::class)->find($modelType->getId());
+
+        if (!$modelType) {
+            return;
+        }
+
+        $hasNewSlots = false;
+
+        // Piece slots
+        foreach ($modelType->getSkeletonPieceRequirements() as $req) {
+            $slot = new ComposantPieceSlot();
+            $slot->setComposant($composant);
+            $slot->setTypePiece($req->getTypePiece());
+            $slot->setPosition($req->getPosition());
+            $this->entityManager->persist($slot);
+            $hasNewSlots = true;
+        }
+
+        // Product slots
+        foreach ($modelType->getSkeletonProductRequirements() as $req) {
+            $slot = new ComposantProductSlot();
+            $slot->setComposant($composant);
+            $slot->setTypeProduct($req->getTypeProduct());
+            $slot->setFamilyCode($req->getFamilyCode());
+            $slot->setPosition($req->getPosition());
+            $this->entityManager->persist($slot);
+            $hasNewSlots = true;
+        }
+
+        // Subcomponent slots
+        foreach ($modelType->getSkeletonSubcomponentRequirements() as $req) {
+            $slot = new ComposantSubcomponentSlot();
+            $slot->setComposant($composant);
+            $slot->setAlias($req->getAlias());
+            $slot->setFamilyCode($req->getFamilyCode());
+            $slot->setTypeComposant($req->getTypeComposant());
+            $slot->setPosition($req->getPosition());
+            $this->entityManager->persist($slot);
+            $hasNewSlots = true;
+        }
+
+        if ($hasNewSlots) {
+            $this->entityManager->flush();
+        }
+    }
+}