diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/MqttTagDefinitionFactory.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/MqttTagDefinitionFactory.cs
index 709ddc0d..7de88b4d 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/MqttTagDefinitionFactory.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/MqttTagDefinitionFactory.cs
@@ -431,6 +431,22 @@ public static class MqttTagDefinitionFactory
                 + "never bind and will read BadNodeIdUnknown in Sparkplug B mode.");
         }
 
+        // isArray on a PLAIN-shaped tag (lmxopcua#770). The composer materialises the node from isArray
+        // (ValueRank=1) in every mode, but only the Sparkplug ingest path ever feeds it an array: a Plain
+        // tag decodes ONE scalar per message, so the node would be an array fed scalar values. Warned only
+        // when the Plain reader takes the blob and the Sparkplug one does not (a Sparkplug blob carrying a
+        // leftover topic key is a legitimate array tag and must stay quiet — a gate stricter than the runtime).
+        if (accepted && ReadArrayShape(root).IsArray
+            && FromTagConfig(root.GetRawText(), InspectionRawPath, out _)
+            && !FromSparkplugTagConfig(root.GetRawText(), InspectionRawPath, out _))
+        {
+            advisories.Add(
+                "'isArray' is set on a Plain-mode tag: Plain MQTT decodes one scalar per message and never "
+                + "produces an array, so the tag would be an array node fed scalar values. 'isArray' is "
+                + "honoured only in Sparkplug B mode (array metrics) — remove it, or switch the driver to "
+                + "Sparkplug B.");
+        }
+
         // The ONE key whose verdict depends on the DRIVER'S mode, reported as an
         // advisory because no tag-level gate can see that mode. A present-but-invalid payloadFormat is
         // fatal to the Plain parse and invisible to the Sparkplug one, so a union accept here can only
diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/RawTagCsvMapper.cs b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/RawTagCsvMapper.cs
index b521cf58..c9d39f80 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/RawTagCsvMapper.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/RawTagCsvMapper.cs
@@ -225,11 +225,9 @@ public static class RawTagCsvMapper
                 arrayLength = (uint)n;
             }
 
-            if (TagArrayConfig.Validate(isArray, arrayLength) is { } arrErr)
-            {
-                return Invalid(rowNumber, cells, arrErr);
-            }
-
+            // No positive-length requirement (lmxopcua#770): IsArray with a blank/0 ArrayLength is a
+            // variable-length array (ArrayDimensions=[0]), which browse-commit and the composer both produce and
+            // honour, so the export of such a tag must re-import. TagArrayConfig.Validate stays the editor's rule.
             json = TagArrayConfig.Set(json, isArray, arrayLength);
 
             // Alarm flags — an alarm is authored iff Alarm.AlarmType is present.
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttTagDefinitionFactoryTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttTagDefinitionFactoryTests.cs
index 74a41eec..7c4ac4c0 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttTagDefinitionFactoryTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttTagDefinitionFactoryTests.cs
@@ -72,6 +72,18 @@ public sealed class MqttTagDefinitionFactoryTests
         => MqttTagDefinitionFactory.FromTagConfig(
             """{"topic":"a/b","payloadFormat":"Json","dataType":"Double"}""", RawPath, out _).ShouldBeFalse();
 
+    [Fact]
+    public void Inspect_IsArrayOnPlainTag_Warns_lmxopcua_770()
+    {
+        var warnings = MqttTagDefinitionFactory.Inspect("""{"topic":"a/b","payloadFormat":"Json","isArray":true}""");
+        warnings.ShouldHaveSingleItem().ShouldContain("isArray");
+    }
+
+    [Fact]
+    public void Inspect_IsArrayOnSparkplugTag_StaysQuiet()
+        => MqttTagDefinitionFactory.Inspect(
+            """{"groupId":"G","edgeNodeId":"E","metricName":"M","topic":"leftover","isArray":true}""").ShouldBeEmpty();
+
     [Fact]
     public void Inspect_WildcardTopic_ReturnsWarning()
     {
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/RawTagCsvRoundTripTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/RawTagCsvRoundTripTests.cs
index 31dc679f..634d07e0 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/RawTagCsvRoundTripTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/RawTagCsvRoundTripTests.cs
@@ -68,6 +68,18 @@ public sealed class RawTagCsvRoundTripTests
         AssertRoundTrip(DriverTypeNames.Galaxy, new[] { galaxy });
     }
 
+    [Fact]
+    public void Variable_length_array_round_trips_lmxopcua_770()
+    {
+        // IsArray with no ArrayLength is what browse-commit and the composer produce (ArrayDimensions=[0]);
+        // the CSV import used to reject its own export.
+        var variable = new RawCsvExportTag(
+            null, "profile", "Float", TagAccessLevel.Read, false, null,
+            """{"nodeId":"nsu=urn:plc;s=Profile","isArray":true}""");
+
+        AssertRoundTrip(DriverTypeNames.OpcUaClient, new[] { variable });
+    }
+
     [Fact]
     public void Export_header_matches_the_driver_column_dictionary()
     {