diff --git a/libraries/MTConnect.NET-Common/Agents/MTConnectAgent.cs b/libraries/MTConnect.NET-Common/Agents/MTConnectAgent.cs
index 8a807c379..7e455f4c6 100644
--- a/libraries/MTConnect.NET-Common/Agents/MTConnectAgent.cs
+++ b/libraries/MTConnect.NET-Common/Agents/MTConnectAgent.cs
@@ -1112,54 +1112,76 @@ private Device NormalizeDevice(IDevice device)
                     obj.Compositions = NormalizeCompositions(device.Compositions, obj, obj);
                     obj.Components = NormalizeComponents(device.Components, obj, obj);
 
+                    // Required DataItem backfill: enumerate `obj.DataItems` once
+                    // into a List<IDataItem> and project the type set into a HashSet
+                    // so each required-type lookup is O(1) instead of O(n) and we
+                    // avoid the per-required-type ToList() allocation. Preserves the
+                    // existing behavior pinned by NormalizeDeviceRequiredDataItemsTests.
+                    var dataItemList = obj.DataItems != null
+                        ? new List<IDataItem>(obj.DataItems)
+                        : new List<IDataItem>();
+#if NET472_OR_GREATER || NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_0_OR_GREATER
+                    var dataItemTypes = new HashSet<string>(dataItemList.Count);
+#else
+                    var dataItemTypes = new HashSet<string>();
+#endif
+                    for (var i = 0; i < dataItemList.Count; i++)
+                    {
+                        var t = dataItemList[i]?.Type;
+                        if (t != null) dataItemTypes.Add(t);
+                    }
+
                     // Add Required Availability DataItem
-                    if (obj.DataItems.IsNullOrEmpty() || !obj.DataItems.Any(o => o.Type == AvailabilityDataItem.TypeId))
+                    if (!dataItemTypes.Contains(AvailabilityDataItem.TypeId))
                     {
                         var availability = new AvailabilityDataItem(obj.Id);
                         availability.Device = obj;
                         availability.Container = obj;
                         availability.Name = AvailabilityDataItem.NameId;
-                        var x = obj.DataItems.ToList();
-                        x.Add(availability);
-                        obj.DataItems = x;
+                        dataItemList.Add(availability);
+                        dataItemTypes.Add(AvailabilityDataItem.TypeId);
                     }
 
                     // Add Required AssetChanged DataItem
-                    if (obj.DataItems.IsNullOrEmpty() || !obj.DataItems.Any(o => o.Type == AssetChangedDataItem.TypeId))
+                    if (!dataItemTypes.Contains(AssetChangedDataItem.TypeId))
                     {
                         var assetChanged = new AssetChangedDataItem(obj.Id);
                         assetChanged.Device = obj;
                         assetChanged.Container = obj;
                         assetChanged.Name = AssetChangedDataItem.NameId;
-                        var x = obj.DataItems.ToList();
-                        x.Add(assetChanged);
-                        obj.DataItems = x;
+                        dataItemList.Add(assetChanged);
+                        dataItemTypes.Add(AssetChangedDataItem.TypeId);
                     }
 
                     // Add Required AssetRemoved DataItem
-                    if (obj.DataItems.IsNullOrEmpty() || !obj.DataItems.Any(o => o.Type == AssetRemovedDataItem.TypeId))
+                    if (!dataItemTypes.Contains(AssetRemovedDataItem.TypeId))
                     {
                         var assetRemoved = new AssetRemovedDataItem(obj.Id);
                         assetRemoved.Device = obj;
                         assetRemoved.Container = obj;
                         assetRemoved.Name = AssetRemovedDataItem.NameId;
-                        var x = obj.DataItems.ToList();
-                        x.Add(assetRemoved);
-                        obj.DataItems = x;
+                        dataItemList.Add(assetRemoved);
+                        dataItemTypes.Add(AssetRemovedDataItem.TypeId);
                     }
 
                     // Add Required AssetCount DataItem
-                    if (obj.DataItems.IsNullOrEmpty() || !obj.DataItems.Any(o => o.Type == AssetCountDataItem.TypeId))
+                    if (!dataItemTypes.Contains(AssetCountDataItem.TypeId))
                     {
                         var assetcount = new AssetCountDataItem(obj.Id);
                         assetcount.Device = obj;
                         assetcount.Container = obj;
                         assetcount.Name = AssetCountDataItem.NameId;
-                        var x = obj.DataItems.ToList();
-                        x.Add(assetcount);
-                        obj.DataItems = x;
+                        // ASSET_COUNT is a DATA_SET representation per MTConnect Part 2
+                        // (UML _19_0_3_68e0225_1640602520420_217627_44). The generated
+                        // AssetCountDataItem still defaults Representation to VALUE; override
+                        // it here so the auto-injected DataItem matches the spec.
+                        assetcount.Representation = DataItemRepresentation.DATA_SET;
+                        dataItemList.Add(assetcount);
+                        dataItemTypes.Add(AssetCountDataItem.TypeId);
                     }
 
+                    obj.DataItems = dataItemList;
+
 
                     // Generic Components
                     var genericComponents = obj.GetComponents()?.Where(o => o.GetType() == typeof(Component));
diff --git a/libraries/MTConnect.NET-Common/Headers/IMTConnectAssestsHeader.cs b/libraries/MTConnect.NET-Common/Headers/IMTConnectAssestsHeader.cs
index 0fe33725d..9fb4d7500 100644
--- a/libraries/MTConnect.NET-Common/Headers/IMTConnectAssestsHeader.cs
+++ b/libraries/MTConnect.NET-Common/Headers/IMTConnectAssestsHeader.cs
@@ -22,6 +22,12 @@ public interface IMTConnectAssetsHeader
         /// </summary>
         string Version { get; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        string SchemaVersion { get; }
+
         /// <summary>
         /// An identification defining where the Agent that published the Response Document is installed or hosted.
         /// </summary>
diff --git a/libraries/MTConnect.NET-Common/Headers/IMTConnectDevicesHeader.cs b/libraries/MTConnect.NET-Common/Headers/IMTConnectDevicesHeader.cs
index b9103c589..46335ff99 100644
--- a/libraries/MTConnect.NET-Common/Headers/IMTConnectDevicesHeader.cs
+++ b/libraries/MTConnect.NET-Common/Headers/IMTConnectDevicesHeader.cs
@@ -22,6 +22,12 @@ public interface IMTConnectDevicesHeader
         /// </summary>
         string Version { get; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        string SchemaVersion { get; }
+
         /// <summary>
         /// An identification defining where the Agent that published the Response Document is installed or hosted.
         /// </summary>
diff --git a/libraries/MTConnect.NET-Common/Headers/IMTConnectStreamsHeader.cs b/libraries/MTConnect.NET-Common/Headers/IMTConnectStreamsHeader.cs
index b2bac0dd4..eb305f60c 100644
--- a/libraries/MTConnect.NET-Common/Headers/IMTConnectStreamsHeader.cs
+++ b/libraries/MTConnect.NET-Common/Headers/IMTConnectStreamsHeader.cs
@@ -22,6 +22,12 @@ public interface IMTConnectStreamsHeader
         /// </summary>
         string Version { get; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        string SchemaVersion { get; }
+
         /// <summary>
         /// An identification defining where the Agent that published the Response Document is installed or hosted.
         /// </summary>
diff --git a/libraries/MTConnect.NET-Common/Headers/MTConnectAssestsHeader.cs b/libraries/MTConnect.NET-Common/Headers/MTConnectAssestsHeader.cs
index bd098b08e..0df2034d0 100644
--- a/libraries/MTConnect.NET-Common/Headers/MTConnectAssestsHeader.cs
+++ b/libraries/MTConnect.NET-Common/Headers/MTConnectAssestsHeader.cs
@@ -22,6 +22,12 @@ public class MTConnectAssetsHeader : IMTConnectAssetsHeader
         /// </summary>
         public string Version { get; set; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        public string SchemaVersion { get; set; }
+
         /// <summary>
         /// An identification defining where the Agent that published the Response Document is installed or hosted.
         /// </summary>
diff --git a/libraries/MTConnect.NET-Common/Headers/MTConnectDevicesHeader.cs b/libraries/MTConnect.NET-Common/Headers/MTConnectDevicesHeader.cs
index 61426373c..f5c5af650 100644
--- a/libraries/MTConnect.NET-Common/Headers/MTConnectDevicesHeader.cs
+++ b/libraries/MTConnect.NET-Common/Headers/MTConnectDevicesHeader.cs
@@ -22,6 +22,12 @@ public class MTConnectDevicesHeader : IMTConnectDevicesHeader
         /// </summary>
         public string Version { get; set; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        public string SchemaVersion { get; set; }
+
         /// <summary>
         /// An identification defining where the Agent that published the Response Document is installed or hosted.
         /// </summary>
diff --git a/libraries/MTConnect.NET-Common/Headers/MTConnectStreamsHeader.cs b/libraries/MTConnect.NET-Common/Headers/MTConnectStreamsHeader.cs
index 8c6dfd2a5..5109ce080 100644
--- a/libraries/MTConnect.NET-Common/Headers/MTConnectStreamsHeader.cs
+++ b/libraries/MTConnect.NET-Common/Headers/MTConnectStreamsHeader.cs
@@ -22,6 +22,12 @@ public class MTConnectStreamsHeader : IMTConnectStreamsHeader
         /// </summary>
         public string Version { get; set; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        public string SchemaVersion { get; set; }
+
         /// <summary>
         /// An identification defining where the Agent that published the Response Document is installed or hosted.
         /// </summary>
diff --git a/libraries/MTConnect.NET-JSON-cppagent/Assets/JsonAssetsHeader.cs b/libraries/MTConnect.NET-JSON-cppagent/Assets/JsonAssetsHeader.cs
index ed5e2ae79..8e5c188ac 100644
--- a/libraries/MTConnect.NET-JSON-cppagent/Assets/JsonAssetsHeader.cs
+++ b/libraries/MTConnect.NET-JSON-cppagent/Assets/JsonAssetsHeader.cs
@@ -15,6 +15,13 @@ public class JsonAssetsHeader
         [JsonPropertyName("version")]
         public string Version { get; set; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        [JsonPropertyName("schemaVersion")]
+        public string SchemaVersion { get; set; }
+
         [JsonPropertyName("sender")]
         public string Sender { get; set; }
 
@@ -30,6 +37,13 @@ public class JsonAssetsHeader
         [JsonPropertyName("testIndicator")]
         public bool TestIndicator { get; set; }
 
+        /// <summary>
+        /// Indicates if the MTConnect Agent is validating against the normative model.
+        /// Mirrors the cppagent v2 wire shape that emits `validation` on every Header.
+        /// </summary>
+        [JsonPropertyName("validation")]
+        public bool Validation { get; set; }
+
         [JsonPropertyName("creationTime")]
         public DateTime CreationTime { get; set; }
 
@@ -42,11 +56,13 @@ public JsonAssetsHeader(IMTConnectAssetsHeader header)
             {
                 InstanceId = header.InstanceId;
                 Version = header.Version;
+                SchemaVersion = header.SchemaVersion;
                 Sender = header.Sender;
                 AssetBufferSize = header.AssetBufferSize;
                 AssetCount = header.AssetCount;
                 DeviceModelChangeTime = header.DeviceModelChangeTime;
                 TestIndicator = header.TestIndicator;
+                Validation = header.Validation;
                 CreationTime = header.CreationTime;
             }
         }
@@ -57,11 +73,13 @@ public virtual IMTConnectAssetsHeader ToAssetsHeader()
             var header = new MTConnectAssetsHeader();
             header.InstanceId = InstanceId;
             header.Version = Version;
+            header.SchemaVersion = SchemaVersion;
             header.Sender = Sender;
             header.AssetBufferSize = AssetBufferSize;
             header.AssetCount = AssetCount;
             header.DeviceModelChangeTime = DeviceModelChangeTime;
             header.TestIndicator = TestIndicator;
+            header.Validation = Validation;
             header.CreationTime = CreationTime;
             return header;
         }
diff --git a/libraries/MTConnect.NET-JSON-cppagent/Devices/JsonDevicesHeader.cs b/libraries/MTConnect.NET-JSON-cppagent/Devices/JsonDevicesHeader.cs
index 3d2a679fd..74bb8bddd 100644
--- a/libraries/MTConnect.NET-JSON-cppagent/Devices/JsonDevicesHeader.cs
+++ b/libraries/MTConnect.NET-JSON-cppagent/Devices/JsonDevicesHeader.cs
@@ -36,6 +36,13 @@ public class JsonDevicesHeader
         [JsonPropertyName("testIndicator")]
         public bool TestIndicator { get; set; }
 
+        /// <summary>
+        /// Indicates if the MTConnect Agent is validating against the normative model.
+        /// Mirrors the cppagent v2 wire shape that emits `validation` on every Header.
+        /// </summary>
+        [JsonPropertyName("validation")]
+        public bool Validation { get; set; }
+
         [JsonPropertyName("creationTime")]
         public DateTime CreationTime { get; set; }
 
@@ -48,12 +55,14 @@ public JsonDevicesHeader(IMTConnectDevicesHeader header)
             {
                 InstanceId = header.InstanceId;
                 Version = header.Version;
+                SchemaVersion = header.SchemaVersion;
                 Sender = header.Sender;
                 BufferSize = header.BufferSize;
                 AssetBufferSize = header.AssetBufferSize;
                 AssetCount = header.AssetCount;
                 DeviceModelChangeTime = header.DeviceModelChangeTime;
                 TestIndicator = header.TestIndicator;
+                Validation = header.Validation;
                 CreationTime = header.CreationTime;
             }
         }
@@ -64,12 +73,14 @@ public IMTConnectDevicesHeader ToDevicesHeader()
             var header = new MTConnectDevicesHeader();
             header.InstanceId = InstanceId;
             header.Version = Version;
+            header.SchemaVersion = SchemaVersion;
             header.Sender = Sender;
             header.BufferSize = BufferSize;
             header.AssetBufferSize = AssetBufferSize;
             header.AssetCount = AssetCount;
             header.DeviceModelChangeTime = DeviceModelChangeTime;
             header.TestIndicator = TestIndicator;
+            header.Validation = Validation;
             header.CreationTime = CreationTime;
             return header;
         }
diff --git a/libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs b/libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs
index c2ee22557..8aea7a4f5 100644
--- a/libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs
+++ b/libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs
@@ -15,6 +15,13 @@ public class JsonStreamsHeader
         [JsonPropertyName("version")]
         public string Version { get; set; }
 
+        /// <summary>
+        /// The major and minor number of the MTConnect Standard schema the Response Document conforms to (for example "2.7").
+        /// Mirrors the cppagent v2 wire shape that emits `schemaVersion` on every Header.
+        /// </summary>
+        [JsonPropertyName("schemaVersion")]
+        public string SchemaVersion { get; set; }
+
         [JsonPropertyName("sender")]
         public string Sender { get; set; }
 
@@ -36,6 +43,13 @@ public class JsonStreamsHeader
         [JsonPropertyName("testIndicator")]
         public bool TestIndicator { get; set; }
 
+        /// <summary>
+        /// Indicates if the MTConnect Agent is validating against the normative model.
+        /// Mirrors the cppagent v2 wire shape that emits `validation` on every Header.
+        /// </summary>
+        [JsonPropertyName("validation")]
+        public bool Validation { get; set; }
+
         [JsonPropertyName("creationTime")]
         public DateTime CreationTime { get; set; }
 
@@ -48,6 +62,7 @@ public JsonStreamsHeader(IMTConnectStreamsHeader header)
             {
                 InstanceId = header.InstanceId;
                 Version = header.Version;
+                SchemaVersion = header.SchemaVersion;
                 Sender = header.Sender;
                 BufferSize = header.BufferSize;
                 FirstSequence = header.FirstSequence;
@@ -55,6 +70,7 @@ public JsonStreamsHeader(IMTConnectStreamsHeader header)
                 NextSequence = header.NextSequence;
                 DeviceModelChangeTime = header.DeviceModelChangeTime;
                 TestIndicator = header.TestIndicator;
+                Validation = header.Validation;
                 CreationTime = header.CreationTime;
             }
         }
@@ -65,6 +81,7 @@ public virtual IMTConnectStreamsHeader ToStreamsHeader()
             var header = new MTConnectStreamsHeader();
             header.InstanceId = InstanceId;
             header.Version = Version;
+            header.SchemaVersion = SchemaVersion;
             header.Sender = Sender;
             header.BufferSize = BufferSize;
             header.FirstSequence = FirstSequence;
@@ -72,6 +89,7 @@ public virtual IMTConnectStreamsHeader ToStreamsHeader()
             header.NextSequence = NextSequence;
             header.DeviceModelChangeTime = DeviceModelChangeTime;
             header.TestIndicator = TestIndicator;
+            header.Validation = Validation;
             header.CreationTime = CreationTime;
             return header;
         }
diff --git a/tests/MTConnect.NET-Common-Tests/Agents/NormalizeDeviceRequiredDataItemsTests.cs b/tests/MTConnect.NET-Common-Tests/Agents/NormalizeDeviceRequiredDataItemsTests.cs
new file mode 100644
index 000000000..000796346
--- /dev/null
+++ b/tests/MTConnect.NET-Common-Tests/Agents/NormalizeDeviceRequiredDataItemsTests.cs
@@ -0,0 +1,112 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Linq;
+using MTConnect.Agents;
+using MTConnect.Devices;
+using MTConnect.Devices.DataItems;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.Common.Agents
+{
+    /// <summary>
+    /// Pins the contract that <see cref="MTConnectAgent.AddDevice"/> backfills
+    /// the four required Device-level DataItems exactly once each, regardless
+    /// of the starting state of <c>device.DataItems</c>:
+    ///   - Availability
+    ///   - AssetChanged
+    ///   - AssetRemoved
+    ///   - AssetCount
+    ///
+    /// Lets the inner-loop perf optimization (cast DataItems once and use a
+    /// HashSet for type-id checks) refactor without regressing behavior.
+    /// </summary>
+    [TestFixture]
+    public class NormalizeDeviceRequiredDataItemsTests
+    {
+        private static readonly string[] RequiredTypeIds =
+        {
+            AvailabilityDataItem.TypeId,
+            AssetChangedDataItem.TypeId,
+            AssetRemovedDataItem.TypeId,
+            AssetCountDataItem.TypeId,
+        };
+
+        private static IEnumerable<IDataItem>? StartingDataItemsForCase(string startingState)
+        {
+            return startingState switch
+            {
+                "null" => null,
+                "empty" => new List<IDataItem>(),
+                "with_availability" => new List<IDataItem>
+                {
+                    new AvailabilityDataItem("dev1"),
+                },
+                "with_all_required" => new List<IDataItem>
+                {
+                    new AvailabilityDataItem("dev1"),
+                    new AssetChangedDataItem("dev1"),
+                    new AssetRemovedDataItem("dev1"),
+                    new AssetCountDataItem("dev1"),
+                },
+                _ => null,
+            };
+        }
+
+        [TestCase("null")]
+        [TestCase("empty")]
+        [TestCase("with_availability")]
+        [TestCase("with_all_required")]
+        public void AddDevice_backfills_all_required_dataItems_exactly_once(string startingState)
+        {
+            using var agent = new MTConnectAgent(uuid: "test-agent", initializeAgentDevice: false);
+            var device = new Device
+            {
+                Id = "dev1",
+                Uuid = "dev1-uuid",
+                Name = "dev1",
+                Type = Device.TypeId,
+                DataItems = StartingDataItemsForCase(startingState),
+            };
+
+            var added = agent.AddDevice(device, initializeDataItems: false);
+
+            Assert.That(added, Is.Not.Null, "AddDevice must return the normalized device.");
+            Assert.That(added.DataItems, Is.Not.Null, "Normalized DataItems must not be null after backfill.");
+
+            var types = added.DataItems!.Select(d => d.Type).ToList();
+            foreach (var requiredType in RequiredTypeIds)
+            {
+                Assert.That(types.Count(t => t == requiredType), Is.EqualTo(1),
+                    $"Required DataItem type '{requiredType}' must appear exactly once after AddDevice.");
+            }
+        }
+
+        [Test]
+        public void AddDevice_preserves_user_provided_dataItems_alongside_required_ones()
+        {
+            using var agent = new MTConnectAgent(uuid: "test-agent", initializeAgentDevice: false);
+            var custom = new DataItem(DataItemCategory.EVENT, "PROGRAM", null, "dev1-program");
+            var device = new Device
+            {
+                Id = "dev1",
+                Uuid = "dev1-uuid",
+                Name = "dev1",
+                Type = Device.TypeId,
+                DataItems = new List<IDataItem> { custom },
+            };
+
+            var added = agent.AddDevice(device, initializeDataItems: false);
+
+            Assert.That(added!.DataItems, Is.Not.Null);
+            Assert.That(added.DataItems!.Any(d => d.Id == "dev1-program"), Is.True,
+                "User-provided DataItems must survive the required-DataItem backfill.");
+            foreach (var requiredType in RequiredTypeIds)
+            {
+                Assert.That(added.DataItems!.Any(d => d.Type == requiredType), Is.True,
+                    $"Required DataItem type '{requiredType}' must still be backfilled when custom DataItems are present.");
+            }
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-Common-Tests/Headers/HeaderXmlDocExampleVersionTests.cs b/tests/MTConnect.NET-Common-Tests/Headers/HeaderXmlDocExampleVersionTests.cs
new file mode 100644
index 000000000..546dbf4e4
--- /dev/null
+++ b/tests/MTConnect.NET-Common-Tests/Headers/HeaderXmlDocExampleVersionTests.cs
@@ -0,0 +1,65 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.IO;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.Common.Headers
+{
+    /// <summary>
+    /// Pins the SchemaVersion XML-doc example string on the six Header
+    /// interfaces and classes. The example must reference the current
+    /// MTConnect Standard release ("2.7") rather than the stale "2.5"
+    /// snapshot, so consumers reading the IntelliSense don't think
+    /// the agent only supports an older revision.
+    ///
+    /// Files covered (line 27 in each):
+    /// - libraries/MTConnect.NET-Common/Headers/IMTConnectAssestsHeader.cs
+    /// - libraries/MTConnect.NET-Common/Headers/IMTConnectDevicesHeader.cs
+    /// - libraries/MTConnect.NET-Common/Headers/IMTConnectStreamsHeader.cs
+    /// - libraries/MTConnect.NET-Common/Headers/MTConnectAssestsHeader.cs
+    /// - libraries/MTConnect.NET-Common/Headers/MTConnectDevicesHeader.cs
+    /// - libraries/MTConnect.NET-Common/Headers/MTConnectStreamsHeader.cs
+    /// </summary>
+    [TestFixture]
+    public class HeaderXmlDocExampleVersionTests
+    {
+        private static readonly string[] HeaderFileRelativePaths =
+        {
+            "libraries/MTConnect.NET-Common/Headers/IMTConnectAssestsHeader.cs",
+            "libraries/MTConnect.NET-Common/Headers/IMTConnectDevicesHeader.cs",
+            "libraries/MTConnect.NET-Common/Headers/IMTConnectStreamsHeader.cs",
+            "libraries/MTConnect.NET-Common/Headers/MTConnectAssestsHeader.cs",
+            "libraries/MTConnect.NET-Common/Headers/MTConnectDevicesHeader.cs",
+            "libraries/MTConnect.NET-Common/Headers/MTConnectStreamsHeader.cs",
+        };
+
+        private static string FindRepoRoot()
+        {
+            var dir = new DirectoryInfo(TestContext.CurrentContext.TestDirectory);
+            while (dir != null)
+            {
+                if (Directory.Exists(Path.Combine(dir.FullName, "libraries")) &&
+                    Directory.Exists(Path.Combine(dir.FullName, "tests")))
+                {
+                    return dir.FullName;
+                }
+                dir = dir.Parent;
+            }
+            Assert.Fail("Could not locate repo root from test working directory.");
+            return string.Empty;
+        }
+
+        [TestCaseSource(nameof(HeaderFileRelativePaths))]
+        public void Header_xmldoc_example_does_not_reference_stale_version_2_5(string relativePath)
+        {
+            var fullPath = Path.Combine(FindRepoRoot(), relativePath);
+            Assert.That(File.Exists(fullPath), Is.True, $"Header source file not found at {fullPath}.");
+
+            var text = File.ReadAllText(fullPath);
+
+            Assert.That(text.Contains("(for example \"2.5\")"), Is.False,
+                $"{relativePath} must not pin its SchemaVersion XML-doc example to the stale \"2.5\" string.");
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/Assets/JsonAssetsHeaderSchemaVersionTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/Assets/JsonAssetsHeaderSchemaVersionTests.cs
new file mode 100644
index 000000000..24ba4ca0f
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/Assets/JsonAssetsHeaderSchemaVersionTests.cs
@@ -0,0 +1,111 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using MTConnect.Assets.Json;
+using MTConnect.Headers;
+using MTConnect.Tests.JsonCppagent.TestHelpers;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent.Assets
+{
+    /// <summary>
+    /// Pins the JSON-cppagent Assets Header behavior: every emitted
+    /// `MTConnectAssets.Header` envelope must include `schemaVersion` mapped
+    /// from the source `IMTConnectAssetsHeader.SchemaVersion`, plus the
+    /// existing `testIndicator` regression pin.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.schemaVersion` and
+    ///   `Header.testIndicator` on every Assets envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130,
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131.
+    /// </summary>
+    [TestFixture]
+    [Category("CppAgentHeaderFieldsPresent")]
+    [Category("ComplianceMatrix")]
+    public class JsonAssetsHeaderSchemaVersionTests
+    {
+        [TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(JsonHeaderWireShapeMatrix.SchemaVersionCases))]
+        public void Constructor_with_source_header_copies_schemaVersion(string schemaVersion)
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                InstanceId = 1,
+                Version = $"{schemaVersion}.0.0",
+                SchemaVersion = schemaVersion,
+                Sender = "agent",
+            };
+
+            var json = new JsonAssetsHeader(source);
+
+            Assert.That(json.SchemaVersion, Is.EqualTo(schemaVersion),
+                "JsonAssetsHeader must copy SchemaVersion from the source IMTConnectAssetsHeader.");
+        }
+
+        [TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(JsonHeaderWireShapeMatrix.SchemaVersionCases))]
+        public void Serialized_assets_header_emits_schemaVersion_property(string schemaVersion)
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                SchemaVersion = schemaVersion,
+            };
+
+            var jsonHeader = new JsonAssetsHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("schemaVersion", out var v), Is.True,
+                "Serialized JsonAssetsHeader must expose 'schemaVersion' on the wire.");
+            Assert.That(v.GetString(), Is.EqualTo(schemaVersion));
+        }
+
+        [Test]
+        public void Serialized_assets_header_emits_testIndicator_property()
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                TestIndicator = false,
+            };
+
+            var jsonHeader = new JsonAssetsHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("testIndicator", out var v), Is.True,
+                "Serialized JsonAssetsHeader must expose 'testIndicator' on the wire.");
+            Assert.That(v.GetBoolean(), Is.False);
+        }
+
+        [Test]
+        public void Reverse_mapping_round_trips_schemaVersion()
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                SchemaVersion = "2.5",
+            };
+
+            var roundTripped = new JsonAssetsHeader(source).ToAssetsHeader();
+
+            Assert.That(roundTripped.SchemaVersion, Is.EqualTo("2.5"),
+                "ToAssetsHeader must preserve SchemaVersion through the round trip.");
+        }
+
+        [Test]
+        public void Constructor_with_null_source_does_not_throw()
+        {
+            var jsonHeader = new JsonAssetsHeader(null);
+
+            Assert.That(jsonHeader.SchemaVersion, Is.Null);
+        }
+
+        [Test]
+        public void Default_constructor_leaves_schemaVersion_unset()
+        {
+            var jsonHeader = new JsonAssetsHeader();
+
+            Assert.That(jsonHeader.SchemaVersion, Is.Null);
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/Assets/JsonAssetsHeaderValidationTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/Assets/JsonAssetsHeaderValidationTests.cs
new file mode 100644
index 000000000..4e3c644d9
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/Assets/JsonAssetsHeaderValidationTests.cs
@@ -0,0 +1,77 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using MTConnect.Assets.Json;
+using MTConnect.Headers;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent.Assets
+{
+    /// <summary>
+    /// Pins the JSON-cppagent Assets Header behavior: every emitted
+    /// `MTConnectAssets.Header` envelope must include `validation`
+    /// mapped from the source `IMTConnectAssetsHeader.Validation`,
+    /// matching the cppagent v2 wire shape.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.validation`
+    ///   on every Assets envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130,
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131.
+    /// </summary>
+    [TestFixture]
+    [Category("CppAgentHeaderFieldsPresent")]
+    public class JsonAssetsHeaderValidationTests
+    {
+        [Test]
+        public void Constructor_with_source_header_copies_validation()
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                InstanceId = 1,
+                Version = "2.5.0.0",
+                SchemaVersion = "2.5",
+                Sender = "agent",
+                Validation = true,
+            };
+
+            var json = new JsonAssetsHeader(source);
+
+            Assert.That(json.Validation, Is.True,
+                "JsonAssetsHeader must copy Validation from the source IMTConnectAssetsHeader.");
+        }
+
+        [Test]
+        public void Serialized_assets_header_emits_validation_property()
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                Validation = true,
+            };
+
+            var jsonHeader = new JsonAssetsHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("validation", out var v), Is.True,
+                "Serialized JsonAssetsHeader must expose 'validation' on the wire.");
+            Assert.That(v.GetBoolean(), Is.True);
+        }
+
+        [Test]
+        public void Reverse_mapping_round_trips_validation()
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                Validation = true,
+            };
+
+            var roundTripped = new JsonAssetsHeader(source).ToAssetsHeader();
+
+            Assert.That(roundTripped.Validation, Is.True,
+                "ToAssetsHeader must preserve Validation through the round trip.");
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/CppAgentHeaderFieldsRegressionGuardTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/CppAgentHeaderFieldsRegressionGuardTests.cs
new file mode 100644
index 000000000..8a3b55255
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/CppAgentHeaderFieldsRegressionGuardTests.cs
@@ -0,0 +1,71 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Reflection;
+using System.Text.Json.Serialization;
+using MTConnect.Assets.Json;
+using MTConnect.Devices.Json;
+using MTConnect.Streams.Json;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent
+{
+    /// <summary>
+    /// Reflection guard. Pins the contract: every JSON-cppagent header DTO must
+    /// expose `schemaVersion` and `testIndicator` as serializable properties so a
+    /// future regression cannot silently drop either field from the wire.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.schemaVersion` and
+    ///   `Header.testIndicator` on every Streams / Devices / Assets envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130 (schemaVersion),
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131 (testIndicator).
+    /// </summary>
+    [TestFixture]
+    public class CppAgentHeaderFieldsRegressionGuardTests
+    {
+        private static readonly System.Type[] HeaderDtos =
+        {
+            typeof(JsonStreamsHeader),
+            typeof(JsonDevicesHeader),
+            typeof(JsonAssetsHeader),
+        };
+
+        [TestCaseSource(nameof(HeaderDtos))]
+        public void Header_dto_exposes_schemaVersion_property(System.Type headerType)
+        {
+            var property = headerType.GetProperty("SchemaVersion",
+                BindingFlags.Public | BindingFlags.Instance);
+
+            Assert.That(property, Is.Not.Null,
+                $"{headerType.Name} must expose a public SchemaVersion property.");
+            Assert.That(property!.PropertyType, Is.EqualTo(typeof(string)),
+                $"{headerType.Name}.SchemaVersion must be a string.");
+
+            var jsonAttr = property.GetCustomAttribute<JsonPropertyNameAttribute>();
+            Assert.That(jsonAttr, Is.Not.Null,
+                $"{headerType.Name}.SchemaVersion must carry [JsonPropertyName] for cppagent wire shape.");
+            Assert.That(jsonAttr!.Name, Is.EqualTo("schemaVersion"),
+                $"{headerType.Name}.SchemaVersion must serialize as 'schemaVersion'.");
+        }
+
+        [TestCaseSource(nameof(HeaderDtos))]
+        public void Header_dto_exposes_testIndicator_property(System.Type headerType)
+        {
+            var property = headerType.GetProperty("TestIndicator",
+                BindingFlags.Public | BindingFlags.Instance);
+
+            Assert.That(property, Is.Not.Null,
+                $"{headerType.Name} must expose a public TestIndicator property.");
+            Assert.That(property!.PropertyType, Is.EqualTo(typeof(bool)),
+                $"{headerType.Name}.TestIndicator must be a bool.");
+
+            var jsonAttr = property.GetCustomAttribute<JsonPropertyNameAttribute>();
+            Assert.That(jsonAttr, Is.Not.Null,
+                $"{headerType.Name}.TestIndicator must carry [JsonPropertyName] for cppagent wire shape.");
+            Assert.That(jsonAttr!.Name, Is.EqualTo("testIndicator"),
+                $"{headerType.Name}.TestIndicator must serialize as 'testIndicator'.");
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/Devices/JsonDevicesHeaderSchemaVersionTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/Devices/JsonDevicesHeaderSchemaVersionTests.cs
new file mode 100644
index 000000000..988603a3e
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/Devices/JsonDevicesHeaderSchemaVersionTests.cs
@@ -0,0 +1,111 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using MTConnect.Devices.Json;
+using MTConnect.Headers;
+using MTConnect.Tests.JsonCppagent.TestHelpers;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent.Devices
+{
+    /// <summary>
+    /// Pins the JSON-cppagent Devices Header behavior: every emitted
+    /// `MTConnectDevices.Header` envelope must include `schemaVersion` mapped
+    /// from the source `IMTConnectDevicesHeader.SchemaVersion`, plus the
+    /// existing `testIndicator` regression pin.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.schemaVersion` and
+    ///   `Header.testIndicator` on every Devices envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130,
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131.
+    /// </summary>
+    [TestFixture]
+    [Category("CppAgentHeaderFieldsPresent")]
+    [Category("ComplianceMatrix")]
+    public class JsonDevicesHeaderSchemaVersionTests
+    {
+        [TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(JsonHeaderWireShapeMatrix.SchemaVersionCases))]
+        public void Constructor_with_source_header_copies_schemaVersion(string schemaVersion)
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                InstanceId = 1,
+                Version = $"{schemaVersion}.0.0",
+                SchemaVersion = schemaVersion,
+                Sender = "agent",
+            };
+
+            var json = new JsonDevicesHeader(source);
+
+            Assert.That(json.SchemaVersion, Is.EqualTo(schemaVersion),
+                "JsonDevicesHeader must copy SchemaVersion from the source IMTConnectDevicesHeader.");
+        }
+
+        [TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(JsonHeaderWireShapeMatrix.SchemaVersionCases))]
+        public void Serialized_devices_header_emits_schemaVersion_property(string schemaVersion)
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                SchemaVersion = schemaVersion,
+            };
+
+            var jsonHeader = new JsonDevicesHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("schemaVersion", out var v), Is.True,
+                "Serialized JsonDevicesHeader must expose 'schemaVersion' on the wire.");
+            Assert.That(v.GetString(), Is.EqualTo(schemaVersion));
+        }
+
+        [Test]
+        public void Serialized_devices_header_emits_testIndicator_property()
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                TestIndicator = false,
+            };
+
+            var jsonHeader = new JsonDevicesHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("testIndicator", out var v), Is.True,
+                "Serialized JsonDevicesHeader must expose 'testIndicator' on the wire.");
+            Assert.That(v.GetBoolean(), Is.False);
+        }
+
+        [Test]
+        public void Reverse_mapping_round_trips_schemaVersion()
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                SchemaVersion = "2.5",
+            };
+
+            var roundTripped = new JsonDevicesHeader(source).ToDevicesHeader();
+
+            Assert.That(roundTripped.SchemaVersion, Is.EqualTo("2.5"),
+                "ToDevicesHeader must preserve SchemaVersion through the round trip.");
+        }
+
+        [Test]
+        public void Constructor_with_null_source_does_not_throw()
+        {
+            var jsonHeader = new JsonDevicesHeader(null);
+
+            Assert.That(jsonHeader.SchemaVersion, Is.Null);
+        }
+
+        [Test]
+        public void Default_constructor_leaves_schemaVersion_unset()
+        {
+            var jsonHeader = new JsonDevicesHeader();
+
+            Assert.That(jsonHeader.SchemaVersion, Is.Null);
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/Devices/JsonDevicesHeaderValidationTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/Devices/JsonDevicesHeaderValidationTests.cs
new file mode 100644
index 000000000..bf0c1d8a5
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/Devices/JsonDevicesHeaderValidationTests.cs
@@ -0,0 +1,77 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using MTConnect.Devices.Json;
+using MTConnect.Headers;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent.Devices
+{
+    /// <summary>
+    /// Pins the JSON-cppagent Devices Header behavior: every emitted
+    /// `MTConnectDevices.Header` envelope must include `validation`
+    /// mapped from the source `IMTConnectDevicesHeader.Validation`,
+    /// matching the cppagent v2 wire shape.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.validation`
+    ///   on every Devices envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130,
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131.
+    /// </summary>
+    [TestFixture]
+    [Category("CppAgentHeaderFieldsPresent")]
+    public class JsonDevicesHeaderValidationTests
+    {
+        [Test]
+        public void Constructor_with_source_header_copies_validation()
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                InstanceId = 1,
+                Version = "2.5.0.0",
+                SchemaVersion = "2.5",
+                Sender = "agent",
+                Validation = true,
+            };
+
+            var json = new JsonDevicesHeader(source);
+
+            Assert.That(json.Validation, Is.True,
+                "JsonDevicesHeader must copy Validation from the source IMTConnectDevicesHeader.");
+        }
+
+        [Test]
+        public void Serialized_devices_header_emits_validation_property()
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                Validation = true,
+            };
+
+            var jsonHeader = new JsonDevicesHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("validation", out var v), Is.True,
+                "Serialized JsonDevicesHeader must expose 'validation' on the wire.");
+            Assert.That(v.GetBoolean(), Is.True);
+        }
+
+        [Test]
+        public void Reverse_mapping_round_trips_validation()
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                Validation = true,
+            };
+
+            var roundTripped = new JsonDevicesHeader(source).ToDevicesHeader();
+
+            Assert.That(roundTripped.Validation, Is.True,
+                "ToDevicesHeader must preserve Validation through the round trip.");
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderSchemaVersionXmlDocTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderSchemaVersionXmlDocTests.cs
new file mode 100644
index 000000000..055fa9cf5
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderSchemaVersionXmlDocTests.cs
@@ -0,0 +1,66 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.IO;
+using System.Text.RegularExpressions;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent
+{
+    /// <summary>
+    /// Pins the XML-doc summary requirement on the SchemaVersion property in
+    /// the three JSON-cppagent header DTO files. Each property must carry a
+    /// `<summary>` block immediately above the `[JsonPropertyName("schemaVersion")]`
+    /// attribute so consumers reading IntelliSense see the cppagent v2
+    /// wire-shape semantics inline.
+    ///
+    /// Files covered:
+    /// - libraries/MTConnect.NET-JSON-cppagent/Assets/JsonAssetsHeader.cs
+    /// - libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs
+    /// - libraries/MTConnect.NET-JSON-cppagent/Devices/JsonDevicesHeader.cs
+    /// </summary>
+    [TestFixture]
+    public class JsonHeaderSchemaVersionXmlDocTests
+    {
+        private static readonly string[] HeaderFileRelativePaths =
+        {
+            "libraries/MTConnect.NET-JSON-cppagent/Assets/JsonAssetsHeader.cs",
+            "libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs",
+            "libraries/MTConnect.NET-JSON-cppagent/Devices/JsonDevicesHeader.cs",
+        };
+
+        private static string FindRepoRoot()
+        {
+            var dir = new DirectoryInfo(TestContext.CurrentContext.TestDirectory);
+            while (dir != null)
+            {
+                if (Directory.Exists(Path.Combine(dir.FullName, "libraries")) &&
+                    Directory.Exists(Path.Combine(dir.FullName, "tests")))
+                {
+                    return dir.FullName;
+                }
+                dir = dir.Parent;
+            }
+            Assert.Fail("Could not locate repo root from test working directory.");
+            return string.Empty;
+        }
+
+        [TestCaseSource(nameof(HeaderFileRelativePaths))]
+        public void Header_dto_schemaVersion_has_xmldoc_summary(string relativePath)
+        {
+            var fullPath = Path.Combine(FindRepoRoot(), relativePath);
+            Assert.That(File.Exists(fullPath), Is.True, $"Header source file not found at {fullPath}.");
+
+            var text = File.ReadAllText(fullPath);
+
+            // Match a `<summary>...</summary>` block followed (with optional
+            // whitespace and other attributes) by `[JsonPropertyName("schemaVersion")]`.
+            var pattern = new Regex(
+                @"<summary>[\s\S]+?</summary>\s*(?:///[^\n]*\n\s*)*\[JsonPropertyName\(""schemaVersion""\)\]",
+                RegexOptions.Multiline);
+
+            Assert.That(pattern.IsMatch(text), Is.True,
+                $"{relativePath} must precede [JsonPropertyName(\"schemaVersion\")] with a <summary> XML-doc block.");
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderWireShapeE2ETests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderWireShapeE2ETests.cs
new file mode 100644
index 000000000..97a74f70f
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderWireShapeE2ETests.cs
@@ -0,0 +1,115 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using MTConnect.Assets.Json;
+using MTConnect.Devices.Json;
+using MTConnect.Headers;
+using MTConnect.Streams.Json;
+using MTConnect.Tests.JsonCppagent.TestHelpers;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent
+{
+    /// <summary>
+    /// End-to-end wire-shape pin: every JSON-cppagent envelope's `Header` element
+    /// must carry both `schemaVersion` and `testIndicator` after serialization with
+    /// the default `JsonSerializer` settings, regardless of which field on the
+    /// source DTO carries a non-default value. The (envelopeKind, schemaVersion)
+    /// matrix is sourced from <see cref="JsonHeaderWireShapeMatrix"/> so the
+    /// per-envelope and cross-envelope E2E sets stay in lockstep.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.schemaVersion` and
+    ///   `Header.testIndicator` on every Streams / Devices / Assets envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130 (schemaVersion),
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131 (testIndicator).
+    /// </summary>
+    [TestFixture]
+    [Category("ComplianceMatrix")]
+    public class JsonHeaderWireShapeE2ETests
+    {
+        [TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(JsonHeaderWireShapeMatrix.Cases))]
+        public void Envelope_carries_schemaVersion_and_testIndicator(string envelopeKind, string schemaVersion)
+        {
+            string serialized = envelopeKind switch
+            {
+                "Streams" => JsonSerializer.Serialize(new JsonStreamsHeader(new MTConnectStreamsHeader
+                {
+                    InstanceId = 42,
+                    Version = $"{schemaVersion}.0.0",
+                    SchemaVersion = schemaVersion,
+                    Sender = "agent",
+                    TestIndicator = false,
+                })),
+                "Devices" => JsonSerializer.Serialize(new JsonDevicesHeader(new MTConnectDevicesHeader
+                {
+                    InstanceId = 42,
+                    Version = $"{schemaVersion}.0.0",
+                    SchemaVersion = schemaVersion,
+                    Sender = "agent",
+                    TestIndicator = false,
+                })),
+                "Assets" => JsonSerializer.Serialize(new JsonAssetsHeader(new MTConnectAssetsHeader
+                {
+                    InstanceId = 42,
+                    Version = $"{schemaVersion}.0.0",
+                    SchemaVersion = schemaVersion,
+                    Sender = "agent",
+                    TestIndicator = false,
+                })),
+                _ => throw new System.ArgumentOutOfRangeException(nameof(envelopeKind), envelopeKind, null),
+            };
+
+            using var doc = JsonDocument.Parse(serialized);
+            Assert.That(doc.RootElement.GetProperty("schemaVersion").GetString(), Is.EqualTo(schemaVersion));
+            Assert.That(doc.RootElement.GetProperty("testIndicator").GetBoolean(), Is.False);
+        }
+
+        [Test]
+        public void Streams_envelope_round_trip_preserves_both_fields()
+        {
+            var source = new MTConnectStreamsHeader
+            {
+                SchemaVersion = "2.5",
+                TestIndicator = true,
+            };
+
+            var roundTripped = new JsonStreamsHeader(source).ToStreamsHeader();
+
+            Assert.That(roundTripped.SchemaVersion, Is.EqualTo("2.5"));
+            Assert.That(roundTripped.TestIndicator, Is.True);
+        }
+
+        [Test]
+        public void Devices_envelope_round_trip_preserves_both_fields()
+        {
+            var source = new MTConnectDevicesHeader
+            {
+                SchemaVersion = "2.5",
+                TestIndicator = true,
+            };
+
+            var roundTripped = new JsonDevicesHeader(source).ToDevicesHeader();
+
+            Assert.That(roundTripped.SchemaVersion, Is.EqualTo("2.5"));
+            Assert.That(roundTripped.TestIndicator, Is.True);
+        }
+
+        [Test]
+        public void Assets_envelope_round_trip_preserves_both_fields()
+        {
+            var source = new MTConnectAssetsHeader
+            {
+                SchemaVersion = "2.5",
+                TestIndicator = true,
+            };
+
+            var roundTripped = new JsonAssetsHeader(source).ToAssetsHeader();
+
+            Assert.That(roundTripped.SchemaVersion, Is.EqualTo("2.5"));
+            Assert.That(roundTripped.TestIndicator, Is.True);
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderWireShapeMatrixTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderWireShapeMatrixTests.cs
new file mode 100644
index 000000000..319e38221
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/JsonHeaderWireShapeMatrixTests.cs
@@ -0,0 +1,67 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Linq;
+using MTConnect.Tests.JsonCppagent.TestHelpers;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent
+{
+    /// <summary>
+    /// Pins the shared (envelopeKind, version) compliance matrix used by both
+    /// per-envelope and cross-envelope wire-shape fixtures. Centralizing the
+    /// matrix prevents per-envelope tests from drifting away from the cross-
+    /// envelope E2E set as new schema versions are added.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.schemaVersion` and
+    ///   `Header.testIndicator` on every envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130 (schemaVersion),
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131 (testIndicator).
+    /// </summary>
+    [TestFixture]
+    [Category("ComplianceMatrix")]
+    public class JsonHeaderWireShapeMatrixTests
+    {
+        [Test]
+        public void Matrix_exposes_three_envelope_kinds()
+        {
+            var kinds = JsonHeaderWireShapeMatrix.Cases
+                .Select(c => c.Arguments[0]!.ToString())
+                .Distinct()
+                .OrderBy(s => s)
+                .ToArray();
+
+            Assert.That(kinds, Is.EqualTo(new[] { "Assets", "Devices", "Streams" }));
+        }
+
+        [Test]
+        public void Matrix_covers_v20_v23_v25_for_each_envelope()
+        {
+            var expectedVersions = new[] { "2.0", "2.3", "2.5" };
+
+            foreach (var kind in new[] { "Assets", "Devices", "Streams" })
+            {
+                var versions = JsonHeaderWireShapeMatrix.Cases
+                    .Where(c => (string)c.Arguments[0]! == kind)
+                    .Select(c => (string)c.Arguments[1]!)
+                    .OrderBy(v => v)
+                    .ToArray();
+
+                Assert.That(versions, Is.EqualTo(expectedVersions),
+                    $"{kind} envelope must include v2.0, v2.3, and v2.5 in the compliance matrix.");
+            }
+        }
+
+        [Test]
+        public void Matrix_class_is_internal_static()
+        {
+            var t = typeof(JsonHeaderWireShapeMatrix);
+            Assert.That(t.IsAbstract && t.IsSealed, Is.True,
+                "JsonHeaderWireShapeMatrix must be a static class.");
+            Assert.That(t.IsNotPublic, Is.True,
+                "JsonHeaderWireShapeMatrix must be internal to the test project.");
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/Streams/JsonStreamsHeaderSchemaVersionTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/Streams/JsonStreamsHeaderSchemaVersionTests.cs
new file mode 100644
index 000000000..8c356356c
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/Streams/JsonStreamsHeaderSchemaVersionTests.cs
@@ -0,0 +1,114 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using MTConnect.Headers;
+using MTConnect.Streams.Json;
+using MTConnect.Tests.JsonCppagent.TestHelpers;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent.Streams
+{
+    /// <summary>
+    /// Pins the JSON-cppagent Streams Header behavior: every emitted
+    /// `MTConnectStreams.Header` envelope must include `schemaVersion`,
+    /// matching the cppagent v2 wire shape, and `testIndicator` must
+    /// remain on the wire (regression pin against #131 reverting).
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.schemaVersion` and
+    ///   `Header.testIndicator` on every Streams envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130 (schemaVersion),
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131 (testIndicator).
+    /// </summary>
+    [TestFixture]
+    [Category("CppAgentHeaderFieldsPresent")]
+    [Category("ComplianceMatrix")]
+    public class JsonStreamsHeaderSchemaVersionTests
+    {
+        [TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(JsonHeaderWireShapeMatrix.SchemaVersionCases))]
+        public void Constructor_with_source_header_copies_schemaVersion(string schemaVersion)
+        {
+            var source = new MTConnectStreamsHeader
+            {
+                InstanceId = 1,
+                Version = $"{schemaVersion}.0.0",
+                SchemaVersion = schemaVersion,
+                Sender = "agent",
+            };
+
+            var json = new JsonStreamsHeader(source);
+
+            Assert.That(json.SchemaVersion, Is.EqualTo(schemaVersion),
+                "JsonStreamsHeader must copy SchemaVersion from the source IMTConnectStreamsHeader.");
+        }
+
+        [TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(JsonHeaderWireShapeMatrix.SchemaVersionCases))]
+        public void Serialized_streams_header_emits_schemaVersion_property(string schemaVersion)
+        {
+            var source = new MTConnectStreamsHeader
+            {
+                SchemaVersion = schemaVersion,
+            };
+
+            var jsonHeader = new JsonStreamsHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("schemaVersion", out var v), Is.True,
+                "Serialized JsonStreamsHeader must expose 'schemaVersion' on the wire.");
+            Assert.That(v.GetString(), Is.EqualTo(schemaVersion));
+        }
+
+        [Test]
+        public void Serialized_streams_header_emits_testIndicator_property()
+        {
+            // Regression pin against #131: testIndicator must remain on the wire
+            // even when the source flag is the default `false`.
+            var source = new MTConnectStreamsHeader
+            {
+                TestIndicator = false,
+            };
+
+            var jsonHeader = new JsonStreamsHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("testIndicator", out var v), Is.True,
+                "Serialized JsonStreamsHeader must expose 'testIndicator' on the wire.");
+            Assert.That(v.GetBoolean(), Is.False);
+        }
+
+        [Test]
+        public void Reverse_mapping_round_trips_schemaVersion()
+        {
+            var source = new MTConnectStreamsHeader
+            {
+                SchemaVersion = "2.5",
+            };
+
+            var roundTripped = new JsonStreamsHeader(source).ToStreamsHeader();
+
+            Assert.That(roundTripped.SchemaVersion, Is.EqualTo("2.5"),
+                "ToStreamsHeader must preserve SchemaVersion through the round trip.");
+        }
+
+        [Test]
+        public void Constructor_with_null_source_does_not_throw()
+        {
+            // Null-source ctor branch is covered for 100% line coverage.
+            var jsonHeader = new JsonStreamsHeader(null);
+
+            Assert.That(jsonHeader.SchemaVersion, Is.Null);
+        }
+
+        [Test]
+        public void Default_constructor_leaves_schemaVersion_unset()
+        {
+            var jsonHeader = new JsonStreamsHeader();
+
+            Assert.That(jsonHeader.SchemaVersion, Is.Null);
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/Streams/JsonStreamsHeaderValidationTests.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/Streams/JsonStreamsHeaderValidationTests.cs
new file mode 100644
index 000000000..21b1fb88f
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/Streams/JsonStreamsHeaderValidationTests.cs
@@ -0,0 +1,77 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using MTConnect.Headers;
+using MTConnect.Streams.Json;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent.Streams
+{
+    /// <summary>
+    /// Pins the JSON-cppagent Streams Header behavior: every emitted
+    /// `MTConnectStreams.Header` envelope must include `validation`
+    /// mapped from the source `IMTConnectStreamsHeader.Validation`,
+    /// matching the cppagent v2 wire shape.
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.validation`
+    ///   on every Streams envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130,
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131.
+    /// </summary>
+    [TestFixture]
+    [Category("CppAgentHeaderFieldsPresent")]
+    public class JsonStreamsHeaderValidationTests
+    {
+        [Test]
+        public void Constructor_with_source_header_copies_validation()
+        {
+            var source = new MTConnectStreamsHeader
+            {
+                InstanceId = 1,
+                Version = "2.5.0.0",
+                SchemaVersion = "2.5",
+                Sender = "agent",
+                Validation = true,
+            };
+
+            var json = new JsonStreamsHeader(source);
+
+            Assert.That(json.Validation, Is.True,
+                "JsonStreamsHeader must copy Validation from the source IMTConnectStreamsHeader.");
+        }
+
+        [Test]
+        public void Serialized_streams_header_emits_validation_property()
+        {
+            var source = new MTConnectStreamsHeader
+            {
+                Validation = true,
+            };
+
+            var jsonHeader = new JsonStreamsHeader(source);
+            var serialized = JsonSerializer.Serialize(jsonHeader);
+            using var doc = JsonDocument.Parse(serialized);
+
+            Assert.That(doc.RootElement.TryGetProperty("validation", out var v), Is.True,
+                "Serialized JsonStreamsHeader must expose 'validation' on the wire.");
+            Assert.That(v.GetBoolean(), Is.True);
+        }
+
+        [Test]
+        public void Reverse_mapping_round_trips_validation()
+        {
+            var source = new MTConnectStreamsHeader
+            {
+                Validation = true,
+            };
+
+            var roundTripped = new JsonStreamsHeader(source).ToStreamsHeader();
+
+            Assert.That(roundTripped.Validation, Is.True,
+                "ToStreamsHeader must preserve Validation through the round trip.");
+        }
+    }
+}
diff --git a/tests/MTConnect.NET-JSON-cppagent-Tests/TestHelpers/JsonHeaderWireShapeMatrix.cs b/tests/MTConnect.NET-JSON-cppagent-Tests/TestHelpers/JsonHeaderWireShapeMatrix.cs
new file mode 100644
index 000000000..cc03566c6
--- /dev/null
+++ b/tests/MTConnect.NET-JSON-cppagent-Tests/TestHelpers/JsonHeaderWireShapeMatrix.cs
@@ -0,0 +1,77 @@
+// Copyright (c) 2026 TrakHound Inc., All Rights Reserved.
+// TrakHound Inc. licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using NUnit.Framework;
+
+namespace MTConnect.Tests.JsonCppagent.TestHelpers
+{
+    /// <summary>
+    /// Shared compliance matrix yielding (envelopeKind, schemaVersion) tuples
+    /// consumed by per-envelope and cross-envelope JSON-cppagent header
+    /// wire-shape fixtures. Centralizing the matrix prevents per-envelope
+    /// tests from drifting away from the cross-envelope E2E set as new
+    /// schema versions are added.
+    ///
+    /// envelopeKind values: "Assets", "Streams", "Devices".
+    /// schemaVersion values: "2.0", "2.3", "2.5".
+    ///
+    /// Source authority:
+    /// - Reference shape: cppagent v2.7.0.7 emits `Header.schemaVersion` and
+    ///   `Header.testIndicator` on every Streams / Devices / Assets envelope.
+    /// - Public defect tracker:
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/130 (schemaVersion),
+    ///   https://github.com/TrakHound/MTConnect.NET/issues/131 (testIndicator).
+    /// </summary>
+    internal static class JsonHeaderWireShapeMatrix
+    {
+        public static readonly string[] EnvelopeKinds =
+        {
+            "Streams",
+            "Devices",
+            "Assets",
+        };
+
+        public static readonly string[] SchemaVersions =
+        {
+            "2.0",
+            "2.3",
+            "2.5",
+        };
+
+        /// <summary>
+        /// NUnit TestCaseSource-shaped enumeration of (envelopeKind, schemaVersion).
+        /// Use as `[TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(Cases))]`.
+        /// </summary>
+        public static IEnumerable<TestCaseData> Cases
+        {
+            get
+            {
+                foreach (var kind in EnvelopeKinds)
+                {
+                    foreach (var version in SchemaVersions)
+                    {
+                        yield return new TestCaseData(kind, version)
+                            .SetName($"{kind}_envelope_v{version}");
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Per-envelope schema-version cases for fixtures scoped to a single
+        /// envelope kind. Use as
+        /// `[TestCaseSource(typeof(JsonHeaderWireShapeMatrix), nameof(SchemaVersions))]`.
+        /// </summary>
+        public static IEnumerable<TestCaseData> SchemaVersionCases
+        {
+            get
+            {
+                foreach (var version in SchemaVersions)
+                {
+                    yield return new TestCaseData(version).SetName($"v{version}");
+                }
+            }
+        }
+    }
+}