diff --git a/tests/Module/Commercial/Api/ClientSubResourceApiTest.php b/tests/Module/Commercial/Api/ClientSubResourceApiTest.php
index c5df358..059661f 100644
--- a/tests/Module/Commercial/Api/ClientSubResourceApiTest.php
+++ b/tests/Module/Commercial/Api/ClientSubResourceApiTest.php
@@ -94,6 +94,51 @@ final class ClientSubResourceApiTest extends AbstractCommercialApiTestCase
         self::assertSame('L\'adresse email n\'est pas valide.', $byPath['email']);
     }
 
+    /**
+     * Regression ERP-110 (bug subresource Link toProperty) : POST d'un contact sur
+     * un client qui en a DEJA >= 2 ne doit pas exploser en 500
+     * (NonUniqueResultException sur la resolution du parent), mais creer (201).
+     */
+    public function testPostContactOnClientWithTwoExistingContactsReturns201(): void
+    {
+        $client = $this->createAdminClient();
+        $seed   = $this->seedClient('Contact Multi');
+        $this->seedContact($seed, 'Alpha');
+        $this->seedContact($seed, 'Beta');
+
+        $client->request('POST', '/api/clients/'.$seed->getId().'/contacts', [
+            'headers' => ['Content-Type' => self::LD, 'Accept' => self::LD],
+            'json'    => ['firstName' => 'Gamma'],
+        ]);
+
+        self::assertResponseStatusCodeSame(201);
+    }
+
+    /**
+     * Meme contexte (>= 2 contacts existants) : un email invalide doit produire
+     * une 422 par champ (la validation est bien atteinte), pas une 500.
+     */
+    public function testPostInvalidContactOnPopulatedClientReturns422OnField(): void
+    {
+        $client = $this->createAdminClient();
+        $seed   = $this->seedClient('Contact Multi Bad');
+        $this->seedContact($seed, 'Alpha');
+        $this->seedContact($seed, 'Beta');
+
+        $response = $client->request('POST', '/api/clients/'.$seed->getId().'/contacts', [
+            'headers' => ['Content-Type' => self::LD, 'Accept' => self::LD],
+            'json'    => ['firstName' => 'Gamma', 'email' => 'pas-un-email'],
+        ]);
+
+        self::assertResponseStatusCodeSame(422);
+        $byPath = [];
+        foreach ($response->toArray(false)['violations'] ?? [] as $v) {
+            $byPath[$v['propertyPath']] = $v['message'];
+        }
+        self::assertArrayHasKey('email', $byPath);
+        self::assertSame('L\'adresse email n\'est pas valide.', $byPath['email']);
+    }
+
     public function testPatchContactNormalizes(): void
     {
         $client  = $this->createAdminClient();