diff --git a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/GlobalJsonBuilderTests.cs b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/GlobalJsonBuilderTests.cs
index 7e6dcb59..89e34d6f 100644
--- a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/GlobalJsonBuilderTests.cs
+++ b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/GlobalJsonBuilderTests.cs
@@ -280,4 +280,36 @@ public async Task CreateAsync_WithoutOptionalSettings_OnlyIncludesVersion(
         // Assert
         _ = await Assert.That(content).Contains("\"version\"");
     }
+
+    [Test]
+    [Arguments("")]
+    [Arguments(" ")]
+    public async Task SetRuntimeSdk_WithEmptyOrWhitespace_ThrowsArgumentException(string runtimeVersion)
+    {
+        // Arrange
+        await using var directory = new TemporaryDirectoryBuilder();
+        await using var builder = new GlobalJsonBuilder(directory, Constants.RuntimeSdkDefault);
+
+        // Act
+        void Act() => builder.SetRuntimeSdk(runtimeVersion);
+
+        // Assert
+        _ = Assert.Throws<ArgumentException>(Act);
+    }
+
+    [Test]
+    public async Task CreateAsync_WithCanceledToken_ThrowsOperationCanceledException()
+    {
+        // Arrange
+        await using var directory = new TemporaryDirectoryBuilder();
+        await using var builder = new GlobalJsonBuilder(directory, Constants.RuntimeSdkDefault);
+        using var cts = new CancellationTokenSource();
+        await cts.CancelAsync();
+
+        // Act
+        async Task Act() => await builder.CreateAsync(cts.Token);
+
+        // Assert
+        _ = await Assert.ThrowsAsync<OperationCanceledException>(Act);
+    }
 }
diff --git a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/ProjectBuilderExtensionsTests.cs b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/ProjectBuilderExtensionsTests.cs
index 0057ee70..9c01a3ed 100644
--- a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/ProjectBuilderExtensionsTests.cs
+++ b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/ProjectBuilderExtensionsTests.cs
@@ -217,4 +217,66 @@ public async Task AddVBFile_WritesContentToFile(CancellationToken cancellationTo
         // Assert
         _ = await Assert.That(fileContent).IsEqualTo(content);
     }
+
+    [Test]
+    public async Task AddVBFile_WithEmptyFileName_ThrowsArgumentException()
+    {
+        // Arrange
+        await using var directory = new TemporaryDirectoryBuilder();
+        await using var builder = new ProjectBuilder(directory, Constants.VBNetProjectFileName);
+        var fileName = string.Empty;
+        const string content = "Public Class Program End Class";
+
+        // Act
+        void Act() => builder.AddVBFile(fileName, content);
+
+        // Assert
+        _ = Assert.Throws<ArgumentException>(Act);
+    }
+
+    [Test]
+    public async Task AddVBFile_WithWhitespaceFileName_ThrowsArgumentException()
+    {
+        // Arrange
+        await using var directory = new TemporaryDirectoryBuilder();
+        await using var builder = new ProjectBuilder(directory, Constants.VBNetProjectFileName);
+        const string content = "Public Class Program End Class";
+
+        // Act
+        void Act() => builder.AddVBFile(" ", content);
+
+        // Assert
+        _ = Assert.Throws<ArgumentException>(Act);
+    }
+
+    [Test]
+    public async Task AddVBFile_WithEmptyContent_ThrowsArgumentException()
+    {
+        // Arrange
+        await using var directory = new TemporaryDirectoryBuilder();
+        await using var builder = new ProjectBuilder(directory, Constants.VBNetProjectFileName);
+        const string fileName = "Program";
+        var content = string.Empty;
+
+        // Act
+        void Act() => builder.AddVBFile(fileName, content);
+
+        // Assert
+        _ = Assert.Throws<ArgumentException>(Act);
+    }
+
+    [Test]
+    public async Task AddVBFile_WithWhitespaceContent_ThrowsArgumentException()
+    {
+        // Arrange
+        await using var directory = new TemporaryDirectoryBuilder();
+        await using var builder = new ProjectBuilder(directory, Constants.VBNetProjectFileName);
+        const string fileName = "Program";
+
+        // Act
+        void Act() => builder.AddVBFile(fileName, " ");
+
+        // Assert
+        _ = Assert.Throws<ArgumentException>(Act);
+    }
 }
diff --git a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/ProjectFactoryExtensionsTests.cs b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/ProjectFactoryExtensionsTests.cs
new file mode 100644
index 00000000..bf11c312
--- /dev/null
+++ b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/ProjectFactoryExtensionsTests.cs
@@ -0,0 +1,44 @@
+namespace NetEvolve.ProjectBuilders.Tests.Unit.Builders;
+
+using System;
+using System.Threading.Tasks;
+using NetEvolve.ProjectBuilders.Abstractions;
+using NetEvolve.ProjectBuilders.Builders;
+
+public class ProjectFactoryExtensionsTests
+{
+    [Test]
+    [Arguments("")]
+    [Arguments(" ")]
+    public async Task AddGlobalJson_WithEmptyOrWhitespaceRuntimeVersion_ThrowsArgumentException(string runtimeVersion)
+    {
+        // Arrange
+        await using var factory = ProjectFactory.Create();
+
+        // Act
+        void Act() => factory.AddGlobalJson(runtimeVersion);
+
+        // Assert
+        _ = Assert.Throws<ArgumentException>(Act);
+    }
+
+    [Test]
+    public async Task AddGlobalJson_WithConfigure_InvokesCallbackWithGlobalJsonBuilder()
+    {
+        // Arrange
+        await using var factory = ProjectFactory.Create();
+        IGlobalJsonBuilder? received = null;
+        var calls = 0;
+
+        // Act
+        _ = factory.AddGlobalJson(configure: gb =>
+        {
+            calls++;
+            received = gb;
+        });
+
+        // Assert
+        _ = await Assert.That(calls).IsEqualTo(1);
+        _ = await Assert.That(received).IsNotNull().And.IsTypeOf<GlobalJsonBuilder>();
+    }
+}
diff --git a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/SubdirectoryBuilderTests.cs b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/SubdirectoryBuilderTests.cs
index 778a93f5..45919e0d 100644
--- a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/SubdirectoryBuilderTests.cs
+++ b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/SubdirectoryBuilderTests.cs
@@ -90,7 +90,8 @@ public async Task CreateFile_WithExistingFileName_ThrowsArgumentException()
         void Act() => _ = subdirectory.CreateFile("file.txt");
 
         // Assert
-        _ = Assert.Throws<ArgumentException>(Act);
+        var ex = Assert.Throws<ArgumentException>(Act);
+        _ = await Assert.That(ex.Message).Contains("file.txt").And.Contains("already exists");
     }
 
     [Test]
diff --git a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/TemporaryDirectoryBuilderTests.cs b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/TemporaryDirectoryBuilderTests.cs
index 06c2276e..917358dc 100644
--- a/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/TemporaryDirectoryBuilderTests.cs
+++ b/tests/NetEvolve.ProjectBuilders.Tests.Unit/Builders/TemporaryDirectoryBuilderTests.cs
@@ -36,7 +36,8 @@ public async Task CreateFile_WithExistingFileName_ThrowsArgumentException()
         void Act() => directory.CreateFile(fileName);
 
         // Assert
-        _ = Assert.Throws<ArgumentException>(Act);
+        var ex = Assert.Throws<ArgumentException>(Act);
+        _ = await Assert.That(ex.Message).Contains(fileName).And.Contains("already exists");
     }
 
     [Test]