diff --git a/test/UnitTests/Microsoft.Testing.Platform.UnitTests/Helpers/ExtensionValidationHelperTests.cs b/test/UnitTests/Microsoft.Testing.Platform.UnitTests/Helpers/ExtensionValidationHelperTests.cs
new file mode 100644
index 0000000000..42162c93b1
--- /dev/null
+++ b/test/UnitTests/Microsoft.Testing.Platform.UnitTests/Helpers/ExtensionValidationHelperTests.cs
@@ -0,0 +1,198 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Microsoft.Testing.Platform.Helpers;
+
+namespace Microsoft.Testing.Platform.UnitTests;
+
+[TestClass]
+public sealed class ExtensionValidationHelperTests
+{
+    // Generic overload: ValidateUniqueExtension<T>(IEnumerable<T>, IExtension, Func<T, IExtension>)
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenExistingExtensionsIsNull_ThrowsArgumentNullException()
+    {
+        IEnumerable<IExtension> existingExtensions = null!;
+        IExtension newExtension = CreateExtension("uid1");
+
+        Assert.ThrowsExactly<ArgumentNullException>(
+            () => existingExtensions.ValidateUniqueExtension(newExtension, x => x));
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenNewExtensionIsNull_ThrowsArgumentNullException()
+    {
+        IExtension[] existingExtensions = [];
+        IExtension newExtension = null!;
+
+        Assert.ThrowsExactly<ArgumentNullException>(
+            () => existingExtensions.ValidateUniqueExtension(newExtension, x => x));
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenExtensionSelectorIsNull_ThrowsArgumentNullException()
+    {
+        IExtension[] existingExtensions = [];
+        IExtension newExtension = CreateExtension("uid1");
+
+        Assert.ThrowsExactly<ArgumentNullException>(
+            () => existingExtensions.ValidateUniqueExtension<IExtension>(newExtension, null!));
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenCollectionIsEmpty_DoesNotThrow()
+    {
+        IExtension[] existingExtensions = [];
+        IExtension newExtension = CreateExtension("uid1");
+
+        existingExtensions.ValidateUniqueExtension(newExtension, x => x);
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenNoDuplicateUid_DoesNotThrow()
+    {
+        IExtension[] existingExtensions = [CreateExtension("uid-A"), CreateExtension("uid-B")];
+        IExtension newExtension = CreateExtension("uid-C");
+
+        existingExtensions.ValidateUniqueExtension(newExtension, x => x);
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenDuplicateUidExists_ErrorMessageContainsUid()
+    {
+        const string duplicateUid = "my-duplicate-uid";
+        IExtension[] existingExtensions = [CreateExtension(duplicateUid)];
+        IExtension newExtension = CreateExtension(duplicateUid);
+
+        InvalidOperationException ex = Assert.ThrowsExactly<InvalidOperationException>(
+            () => existingExtensions.ValidateUniqueExtension(newExtension, x => x));
+
+        Assert.Contains(duplicateUid, ex.Message);
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenDuplicateUidExists_ErrorMessageContainsTypeNames()
+    {
+        const string duplicateUid = "my-duplicate-uid";
+        FakeExtensionA existing = new(duplicateUid);
+        FakeExtensionB newExtension = new(duplicateUid);
+
+        InvalidOperationException ex = Assert.ThrowsExactly<InvalidOperationException>(
+            () => ((IExtension[])[existing]).ValidateUniqueExtension(newExtension, x => x));
+
+        Assert.Contains(typeof(FakeExtensionA).ToString(), ex.Message);
+        Assert.Contains(typeof(FakeExtensionB).ToString(), ex.Message);
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenMultipleDuplicatesExist_ErrorMessageContainsAllTypes()
+    {
+        const string duplicateUid = "shared-uid";
+        FakeExtensionA existing1 = new(duplicateUid);
+        FakeExtensionB existing2 = new(duplicateUid);
+        FakeExtensionC newExtension = new(duplicateUid);
+
+        InvalidOperationException ex = Assert.ThrowsExactly<InvalidOperationException>(
+            () => ((IExtension[])[existing1, existing2]).ValidateUniqueExtension(newExtension, x => x));
+
+        Assert.Contains(typeof(FakeExtensionA).ToString(), ex.Message);
+        Assert.Contains(typeof(FakeExtensionB).ToString(), ex.Message);
+        Assert.Contains(typeof(FakeExtensionC).ToString(), ex.Message);
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WithWrapperType_SelectsExtensionViaSelector()
+    {
+        const string duplicateUid = "wrapper-uid";
+        ExtensionWrapper existing = new(CreateExtension(duplicateUid));
+        IExtension newExtension = CreateExtension(duplicateUid);
+
+        Assert.ThrowsExactly<InvalidOperationException>(
+            () => ((ExtensionWrapper[])[existing]).ValidateUniqueExtension(newExtension, w => w.Extension));
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WithWrapperType_WhenNoDuplicate_DoesNotThrow()
+    {
+        ExtensionWrapper existing = new(CreateExtension("uid-X"));
+        IExtension newExtension = CreateExtension("uid-Y");
+
+        ((ExtensionWrapper[])[existing]).ValidateUniqueExtension(newExtension, w => w.Extension);
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_WhenUidsDifferOnlyByCase_DoesNotThrow()
+    {
+        IExtension[] existingExtensions = [CreateExtension("uid-A")];
+        IExtension newExtension = CreateExtension("uid-a");
+
+        existingExtensions.ValidateUniqueExtension(newExtension, x => x);
+    }
+
+    // Simple overload: ValidateUniqueExtension(IEnumerable<IExtension>, IExtension)
+    [TestMethod]
+    public void ValidateUniqueExtension_SimpleOverload_WhenExistingExtensionsIsNull_ThrowsArgumentNullException()
+    {
+        IEnumerable<IExtension> existingExtensions = null!;
+        IExtension newExtension = CreateExtension("uid1");
+
+        Assert.ThrowsExactly<ArgumentNullException>(
+            () => existingExtensions.ValidateUniqueExtension(newExtension));
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_SimpleOverload_WhenNewExtensionIsNull_ThrowsArgumentNullException()
+    {
+        IExtension[] existingExtensions = [];
+
+        Assert.ThrowsExactly<ArgumentNullException>(
+            () => existingExtensions.ValidateUniqueExtension(null!));
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_SimpleOverload_WhenNoDuplicate_DoesNotThrow()
+    {
+        IExtension[] existingExtensions = [CreateExtension("uid-A"), CreateExtension("uid-B")];
+        IExtension newExtension = CreateExtension("uid-C");
+
+        existingExtensions.ValidateUniqueExtension(newExtension);
+    }
+
+    [TestMethod]
+    public void ValidateUniqueExtension_SimpleOverload_WhenDuplicateUidExists_ThrowsInvalidOperationException()
+    {
+        const string duplicateUid = "my-duplicate-uid";
+        IExtension[] existingExtensions = [CreateExtension(duplicateUid)];
+        IExtension newExtension = CreateExtension(duplicateUid);
+
+        InvalidOperationException ex = Assert.ThrowsExactly<InvalidOperationException>(
+            () => existingExtensions.ValidateUniqueExtension(newExtension));
+
+        Assert.Contains(duplicateUid, ex.Message);
+    }
+
+    private static IExtension CreateExtension(string uid) => new FakeExtension(uid);
+
+    private abstract class FakeExtensionBase(string uid) : IExtension
+    {
+        public string Uid => uid;
+
+        public string Version => "1.0";
+
+        public string DisplayName => uid;
+
+        public string Description => uid;
+
+        public Task<bool> IsEnabledAsync() => Task.FromResult(true);
+    }
+
+    private sealed class FakeExtension(string uid) : FakeExtensionBase(uid);
+
+    private sealed class FakeExtensionA(string uid) : FakeExtensionBase(uid);
+
+    private sealed class FakeExtensionB(string uid) : FakeExtensionBase(uid);
+
+    private sealed class FakeExtensionC(string uid) : FakeExtensionBase(uid);
+
+    private sealed record ExtensionWrapper(IExtension Extension);
+}