diff --git a/graccesscli/src/ZB.MOM.WW.GRAccess.Cli/GRAccess/GRAccessCommandDispatcher.cs b/graccesscli/src/ZB.MOM.WW.GRAccess.Cli/GRAccess/GRAccessCommandDispatcher.cs
index d6056f4..34fe325 100644
--- a/graccesscli/src/ZB.MOM.WW.GRAccess.Cli/GRAccess/GRAccessCommandDispatcher.cs
+++ b/graccesscli/src/ZB.MOM.WW.GRAccess.Cli/GRAccess/GRAccessCommandDispatcher.cs
@@ -502,6 +502,7 @@ namespace ZB.MOM.WW.GRAccess.Cli.GRAccess
                     return AtomicObjectEdit(obj, editObj =>
                     {
                         var editAttr = FindAttributeForMutation(editObj, Arg(args, "attribute"));
+                        EnsureMutableViaSetValue(editAttr, Arg(args, "attribute"));
                         editAttr.SetValue(CreateMxValue(Arg(args, "value"), Arg(args, "data-type", "string")));
                         return CommandSummary(editAttr, "Set attribute value");
                     });
@@ -1622,11 +1623,40 @@ namespace ZB.MOM.WW.GRAccess.Cli.GRAccess
             }
 
             var attr = FindAttributeForMutation(obj, attributeName);
+            EnsureMutableViaSetValue(attr, attributeName);
             var body = File.ReadAllText(file);
             attr.SetValue(CreateMxValue(body, "string"));
             return CommandSummary(attr, $"Set script {attributeName}");
         }
 
+        // Checks whether `attr.SetValue` will actually persist on this attribute,
+        // or whether the attribute is package-only (in which case SetValue silently
+        // no-ops despite returning Successful=true). Discovered via investigation
+        // documented in the writeback-gap notes: ScriptExtension text fields
+        // (ExecuteText, DeclarationsText, StartupText, ShutdownText, OnScanText,
+        // OffScanText, Expression) are all `MxCategoryPackageOnly_Lockable` —
+        // editable only via the package import/export round-trip, never via the
+        // generic `IAttribute.SetValue` path. Same for any other attribute whose
+        // category starts with `MxCategoryPackageOnly`.
+        private static void EnsureMutableViaSetValue(IAttribute attr, string attributeName)
+        {
+            string category;
+            try { category = attr.AttributeCategory.ToString(); }
+            catch { return; } // best-effort; if we can't read the category, let the write proceed
+
+            if (category != null && category.StartsWith("MxCategoryPackageOnly", StringComparison.OrdinalIgnoreCase))
+            {
+                throw new InvalidOperationException(
+                    $"Attribute '{attributeName}' has category '{category}'. " +
+                    "Package-only attributes silently no-op via IAttribute.SetValue and can only be " +
+                    "edited through the package import/export round-trip. " +
+                    "For ScriptExtension bodies (ExecuteText / DeclarationsText / StartupText / etc.), " +
+                    "edit the source object in the System Platform IDE and re-import, " +
+                    "or use a future package-rewrite path. " +
+                    "See docs/script-parsing.md:8.");
+            }
+        }
+
         private static string ObjectScriptCreate(IGalaxy galaxy, IgObject obj, IDictionary<string, object> args)
         {
             var scriptName = Arg(args, "script");
@@ -1691,6 +1721,7 @@ namespace ZB.MOM.WW.GRAccess.Cli.GRAccess
             if (!string.IsNullOrWhiteSpace(expression))
             {
                 var attr = FindAttributeForMutation(obj, scriptName + ".Expression");
+                EnsureMutableViaSetValue(attr, scriptName + ".Expression");
                 attr.SetValue(CreateMxValue(expression, "string"));
                 yield return CommandSummary(attr, $"Set script {scriptName}.Expression");
             }