diff --git a/src/NosCore.GameObject/Services/ExchangeService/ExchangeService.cs b/src/NosCore.GameObject/Services/ExchangeService/ExchangeService.cs
index 9563cff4b..7aceeb252 100644
--- a/src/NosCore.GameObject/Services/ExchangeService/ExchangeService.cs
+++ b/src/NosCore.GameObject/Services/ExchangeService/ExchangeService.cs
@@ -11,6 +11,7 @@
 using NosCore.GameObject.Ecs.Extensions;
 using NosCore.GameObject.Ecs.Interfaces;
 using NosCore.GameObject.Networking.ClientSession;
+using NosCore.GameObject.Services.BroadcastService;
 using NosCore.GameObject.Services.InventoryService;
 using NosCore.GameObject.Services.ItemGenerationService;
 using NosCore.Packets.Enumerations;
@@ -28,7 +29,8 @@ namespace NosCore.GameObject.Services.ExchangeService
 {
     public class ExchangeService(IItemGenerationService itemBuilderService,
             IOptions<WorldConfiguration> worldConfiguration, ILogger<ExchangeService> logger, IExchangeRequestRegistry exchangeRegistry,
-            ILogLanguageLocalizer<LogLanguageKey> logLanguage, IGameLanguageLocalizer gameLanguageLocalizer)
+            ILogLanguageLocalizer<LogLanguageKey> logLanguage, IGameLanguageLocalizer gameLanguageLocalizer,
+            ISessionRegistry sessionRegistry)
         : IExchangeService
     {
         public void SetGold(long visualId, long gold, long bankGold)
@@ -204,12 +206,25 @@ public bool CheckExchange(long visualId, long targetId)
                 logger.LogError(logLanguage[LogLanguageKey.TRY_REMOVE_FAILED], data.Value);
             }
 
+            SetInExchange(data.Key, false);
+            SetInExchange(data.Value, false);
+
             return new ExcClosePacket
             {
                 Type = resultType
             };
         }
 
+        // The lifecycle owns the flag. Set at the seven call sites instead, it was simply
+        // never set at all, which left every InExchangeOrShop guard inert.
+        private void SetInExchange(long visualId, bool value)
+        {
+            if (sessionRegistry.TryGetCharacter(c => c.VisualId == visualId, out var character))
+            {
+                character.InExchange = value;
+            }
+        }
+
         public bool OpenExchange(long visualId, long targetVisualId)
         {
             if (CheckExchange(visualId) || CheckExchange(targetVisualId))
@@ -222,6 +237,8 @@ public bool OpenExchange(long visualId, long targetVisualId)
             exchangeRegistry.SetExchangeRequest(targetVisualId, visualId);
             exchangeRegistry.SetExchangeData(visualId, new ExchangeData());
             exchangeRegistry.SetExchangeData(targetVisualId, new ExchangeData());
+            SetInExchange(visualId, true);
+            SetInExchange(targetVisualId, true);
             return true;
         }
 
@@ -257,6 +274,21 @@ bool IsFullTransfer
                 }
             }
 
+            // The offer captured these references when the item was put in the window. Confirm
+            // each is still there, at that amount, before anything is created: the destination
+            // item is built fresh, so a source that has since gone would duplicate it.
+            foreach (var transfer in pendingTransfers)
+            {
+                if (!transfer.OriginInventory.TryGetValue(transfer.OriginalItem.ItemInstanceId, out var live)
+                    || live.ItemInstance == null
+                    || live.ItemInstance.ItemVNum != transfer.OriginalItem.ItemInstance.ItemVNum
+                    || live.ItemInstance.Amount < transfer.Amount)
+                {
+                    logger.LogError(logLanguage[LogLanguageKey.INVALID_EXCHANGE]);
+                    return new List<KeyValuePair<long, IvnPacket>>();
+                }
+            }
+
             var addedItems = new List<(IInventoryService Inventory, InventoryItemInstance Item)>();
 
             foreach (var transfer in pendingTransfers)
diff --git a/test/NosCore.GameObject.Tests/Services/ExchangeService/ExchangeServiceTests.cs b/test/NosCore.GameObject.Tests/Services/ExchangeService/ExchangeServiceTests.cs
index 745994c9b..b8ecb7d69 100644
--- a/test/NosCore.GameObject.Tests/Services/ExchangeService/ExchangeServiceTests.cs
+++ b/test/NosCore.GameObject.Tests/Services/ExchangeService/ExchangeServiceTests.cs
@@ -56,7 +56,8 @@ public void Setup()
             };
 
             ItemProvider = new GameObject.Services.ItemGenerationService.ItemGenerationService(items, NullLoggerFactory.Instance, TestHelpers.Instance.LogLanguageLocalizer);
-            ExchangeProvider = new GameObject.Services.ExchangeService.ExchangeService(ItemProvider, WorldConfiguration, NullLogger<NosCore.GameObject.Services.ExchangeService.ExchangeService>.Instance, new ExchangeRequestRegistry(), TestHelpers.Instance.LogLanguageLocalizer, TestHelpers.Instance.GameLanguageLocalizer);
+            ExchangeProvider = new GameObject.Services.ExchangeService.ExchangeService(ItemProvider, WorldConfiguration, NullLogger<NosCore.GameObject.Services.ExchangeService.ExchangeService>.Instance, new ExchangeRequestRegistry(), TestHelpers.Instance.LogLanguageLocalizer, TestHelpers.Instance.GameLanguageLocalizer,
+                TestHelpers.Instance.SessionRegistry);
         }
 
         [TestMethod]
@@ -183,7 +184,8 @@ private async Task SessionAndTargetAreSetUp()
             _sessionB = await TestHelpers.Instance.GenerateSessionAsync();
             _realExchange = new GameObject.Services.ExchangeService.ExchangeService(
                 ItemProvider!, WorldConfiguration!, NullLogger<NosCore.GameObject.Services.ExchangeService.ExchangeService>.Instance, new ExchangeRequestRegistry(),
-                TestHelpers.Instance.LogLanguageLocalizer, TestHelpers.Instance.GameLanguageLocalizer);
+                TestHelpers.Instance.LogLanguageLocalizer, TestHelpers.Instance.GameLanguageLocalizer,
+                TestHelpers.Instance.SessionRegistry);
             _realExchange.OpenExchange(_sessionA.Character.CharacterId, _sessionB.Character.VisualId);
         }
 
@@ -332,5 +334,30 @@ private void ProcessExchangeShouldReturnItemsForBoth()
             var itemList = ExchangeProvider.ProcessExchange(1, 2, inventory1, inventory2);
             Assert.IsTrue((itemList.Count(s => s.Key == 1) == 2) && (itemList.Count(s => s.Key == 2) == 2));
         }
+
+        [TestMethod]
+        public void AnOfferedItemThatLeftTheInventoryDoesNotReachTheOtherSide()
+        {
+            IInventoryService giver =
+                new GameObject.Services.InventoryService.InventoryService(new List<ItemDto> { new Item { VNum = 1012, Type = NoscorePocketType.Main } },
+                    WorldConfiguration!, NullLogger<NosCore.GameObject.Services.InventoryService.InventoryService>.Instance);
+            IInventoryService receiver =
+                new GameObject.Services.InventoryService.InventoryService(new List<ItemDto> { new Item { VNum = 1012, Type = NoscorePocketType.Main } },
+                    WorldConfiguration!, NullLogger<NosCore.GameObject.Services.InventoryService.InventoryService>.Instance);
+
+            var offered = giver.AddItemToPocket(InventoryItemInstance.Create(ItemProvider!.Create(1012, 1), 0))!.First();
+
+            ExchangeProvider!.OpenExchange(1, 2);
+            ExchangeProvider.AddItems(1, offered, 1);
+
+            // The destination item is built fresh rather than moved, so dropping the offered
+            // item after putting it in the window used to hand over a second copy of it.
+            giver.Remove(offered.ItemInstanceId);
+
+            var itemList = ExchangeProvider.ProcessExchange(1, 2, giver, receiver);
+
+            Assert.AreEqual(0, itemList.Count, "a vanished offer must not transfer");
+            Assert.AreEqual(0, receiver.Count, "the receiver must not gain a duplicate");
+        }
     }
 }