diff --git a/ManagerService.Tests/Controllers/AiControllerTests.cs b/ManagerService.Tests/Controllers/AiControllerTests.cs
index 665a4ab..bc28bf4 100644
--- a/ManagerService.Tests/Controllers/AiControllerTests.cs
+++ b/ManagerService.Tests/Controllers/AiControllerTests.cs
@@ -140,6 +140,42 @@ namespace ManagerService.Tests.Controllers
             Assert.Equal(42, db.Instances.First().AiTokensThisMonth);
         }
 
+        /// <summary>
+        /// Deux tours d'une même conversation partagent leur ConversationId — c'est le seul
+        /// lien entre eux côté serveur, l'historique étant reconstruit par le client.
+        /// ⚠️ Aucun client ne l'envoyait jusqu'au 2026-08-13 : le repli `Guid.NewGuid()`
+        /// s'appliquait à chaque appel, donc chaque question formait sa propre conversation.
+        /// </summary>
+        [Fact]
+        public async Task Chat_SameConversationId_LinksBothTurns()
+        {
+            using var db = DbContextFactory.Create();
+            SeedAssistantInstance(db);
+
+            var controller = BuildController(db);
+            var first = MakeRequest("i1");
+            first.ConversationId = "conv-1";
+            var second = MakeRequest("i1");
+            second.ConversationId = "conv-1";
+
+            await controller.Chat(first);
+            await controller.Chat(second);
+
+            Assert.Equal(2, db.VisitorQuestions.Count());
+            Assert.All(db.VisitorQuestions, q => Assert.Equal("conv-1", q.ConversationId));
+        }
+
+        [Fact]
+        public async Task Chat_NoConversationId_FallsBackToAGeneratedOne()
+        {
+            using var db = DbContextFactory.Create();
+            SeedAssistantInstance(db);
+
+            await BuildController(db).Chat(MakeRequest("i1"));
+
+            Assert.False(string.IsNullOrWhiteSpace(db.VisitorQuestions.First().ConversationId));
+        }
+
         private static void SeedAssistantInstance(MyInfoMateDbContext db)
         {
             db.Instances.Add(new Instance