diff --git a/code-reviews/Driver.TwinCAT/findings.md b/code-reviews/Driver.TwinCAT/findings.md
index 91a418e..53267c2 100644
--- a/code-reviews/Driver.TwinCAT/findings.md
+++ b/code-reviews/Driver.TwinCAT/findings.md
@@ -103,7 +103,7 @@ Discovery `ToDriverDataType` maps `Structure` to `String`, compounding the incon
 does not support UDT tags, and `BrowseSymbolsAsync` already correctly yields
 `DataType = null` for them.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — `BuildTag` now parses the `DataType` field first and rejects `TwinCATDataType.Structure` with an `InvalidOperationException` that names the tag and explains the limitation; configuration-time failure replaces the previous silent garbage read or late `NotSupportedException`.
 
 ### Driver.TwinCAT-004
 
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriverFactoryExtensions.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriverFactoryExtensions.cs
index fe2f710..2dde259 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriverFactoryExtensions.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriverFactoryExtensions.cs
@@ -63,17 +63,34 @@ public static class TwinCATDriverFactoryExtensions
         };
     }
 
-    private static TwinCATTagDefinition BuildTag(TwinCATTagDto t, string driverInstanceId) =>
-        new(
+    private static TwinCATTagDefinition BuildTag(TwinCATTagDto t, string driverInstanceId)
+    {
+        var dataType = ParseEnum<TwinCATDataType>(t.DataType, t.Name, driverInstanceId, "DataType");
+
+        // Driver.TwinCAT-003: Structure-typed pre-declared tags are not supported. The driver's
+        // atomic surface cannot read/write UDT blobs — MapToClrType falls through to typeof(int)
+        // and ConvertForWrite throws NotSupportedException, producing garbage reads or late
+        // runtime failures. BrowseSymbolsAsync already correctly yields DataType = null for
+        // Structure symbols so they never appear in the discovered address space. Reject here
+        // with a clear error so operators get a configuration-time failure, not a silent wrong value.
+        if (dataType == TwinCATDataType.Structure)
+            throw new InvalidOperationException(
+                $"TwinCAT tag '{t.Name ?? "<unnamed>"}' in '{driverInstanceId}' specifies " +
+                "DataType 'Structure'. The driver does not support UDT/FB-instance pre-declared tags. " +
+                "Use EnableControllerBrowse to discover UDT members individually, or declare " +
+                "individual atomic-typed tags for the fields you need.");
+
+        return new TwinCATTagDefinition(
             Name: t.Name ?? throw new InvalidOperationException(
                 $"TwinCAT config for '{driverInstanceId}' has a tag missing Name"),
             DeviceHostAddress: t.DeviceHostAddress ?? throw new InvalidOperationException(
                 $"TwinCAT tag '{t.Name}' in '{driverInstanceId}' missing DeviceHostAddress"),
             SymbolPath: t.SymbolPath ?? throw new InvalidOperationException(
                 $"TwinCAT tag '{t.Name}' in '{driverInstanceId}' missing SymbolPath"),
-            DataType: ParseEnum<TwinCATDataType>(t.DataType, t.Name, driverInstanceId, "DataType"),
+            DataType: dataType,
             Writable: t.Writable ?? true,
             WriteIdempotent: t.WriteIdempotent ?? false);
+    }
 
     private static T ParseEnum<T>(string? raw, string? tagName, string driverInstanceId, string field)
         where T : struct, Enum