diff --git a/gui/main.qml b/gui/main.qml
index e71477a..131fe75 100644
--- a/gui/main.qml
+++ b/gui/main.qml
@@ -168,16 +168,27 @@ ApplicationWindow {
         if (renamingIndex < 0) return
         var item = sessionTreeModel.get(renamingIndex)
         var newName = renamingText.trim()
-        if (newName !== "" && newName !== item.displayName) {
+        var oldName = item.displayName
+        if (newName !== "" && newName !== oldName) {
+            var success = false
             if (item.type === "session") {
-                if (ollie.renameSession(item.id, newName)) {
+                success = ollie.renameSession(item.id, newName)
+                if (success) {
                     sessionTreeModel.set(renamingIndex, { id: newName, displayName: newName })
                 }
             } else if (item.type === "agent") {
-                if (ollie.renameAgent(item.parentId, item.agentId, newName)) {
+                success = ollie.renameAgent(item.parentId, item.agentId, newName)
+                if (success) {
                     sessionTreeModel.set(renamingIndex, { agentId: newName, displayName: newName })
                 }
             }
+            if (!success) {
+                // Reset to old name and show error
+                sessionTreeModel.set(renamingIndex, { displayName: oldName })
+                renameErrorDialog.itemType = item.type
+                renameErrorDialog.attemptedName = newName
+                renameErrorDialog.open()
+            }
         }
         renamingIndex = -1
         renamingText = ""
@@ -242,6 +253,24 @@ ApplicationWindow {
         }
     }
 
+    // --- Rename Error Dialog ---
+    Dialog {
+        id: renameErrorDialog
+        title: "Rename Failed"
+        anchors.centerIn: parent
+        width: 320
+        modal: true
+        standardButtons: Dialog.Ok
+
+        property string itemType: "session"
+        property string attemptedName: ""
+
+        contentItem: Label {
+            text: "Could not rename " + renameErrorDialog.itemType + " to \"" + renameErrorDialog.attemptedName + "\".\n\nThe name may already be in use."
+            wrapMode: Text.Wrap
+        }
+    }
+
     // --- Settings Dialog ---
     Dialog {
         id: settingsDialog