Add test for dotnet test with UseAppHost=false in MTP

test/TestAssets/TestProjects/TestProjectMTPWithUseAppHostFalse/Program.cs

@@ -0,0 +1,57 @@
+using Microsoft.Testing.Platform.Builder;
+using Microsoft.Testing.Platform.Capabilities.TestFramework;
+using Microsoft.Testing.Platform.Extensions.Messages;
+using Microsoft.Testing.Platform.Extensions.TestFramework;
+
+var testApplicationBuilder = await TestApplication.CreateBuilderAsync(args);
+
+testApplicationBuilder.RegisterTestFramework(_ => new TestFrameworkCapabilities(), (_, __) => new DummyTestAdapter());
+
+using var testApplication = await testApplicationBuilder.BuildAsync();
+return await testApplication.RunAsync();
+
+public class DummyTestAdapter : ITestFramework, IDataProducer
+{
+	public string Uid => nameof(DummyTestAdapter);
+
+	public string Version => "2.0.0";
+
+	public string DisplayName => nameof(DummyTestAdapter);
+
+	public string Description => nameof(DummyTestAdapter);
+
+	public Task<bool> IsEnabledAsync() => Task.FromResult(true);
+
+	public Type[] DataTypesProduced => [];
+
+	public Task<CreateTestSessionResult> CreateTestSessionAsync(CreateTestSessionContext context)
+		=> Task.FromResult(new CreateTestSessionResult() { IsSuccess = true });
+
+	public Task<CloseTestSessionResult> CloseTestSessionAsync(CloseTestSessionContext context)
+		=> Task.FromResult(new CloseTestSessionResult() { IsSuccess = true });
+
+	public Task ExecuteRequestAsync(ExecuteRequestContext context)
+	{
+		// Simple dummy test that always passes
+		context.MessageBus.PublishAsync(this, new TestNodeUpdateMessage(
+			context.Request.Session.SessionUid,
+			new TestNode()
+			{
+				Uid = "dummy_test_1",
+				DisplayName = "Dummy Test 1",
+				Properties = new PropertyBag()
+			}));
+
+		context.MessageBus.PublishAsync(this, new TestResultMessage(
+			context.Request.Session.SessionUid,
+			new PassedTestResult()
+			{
+				Uid = "dummy_test_1",
+				DisplayName = "Dummy Test 1",
+				Duration = TimeSpan.FromMilliseconds(1)
+			}));
+
+		context.Complete();
+		return Task.CompletedTask;
+	}
+}

test/TestAssets/TestProjects/TestProjectMTPWithUseAppHostFalse/TestProjectMTPWithUseAppHostFalse.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), testAsset.props))\testAsset.props" />
+
+	<PropertyGroup>
+		<TargetFramework>$(CurrentTargetFramework)</TargetFramework>
+		<OutputType>Exe</OutputType>
+		<UseAppHost>false</UseAppHost>
+
+		<ImplicitUsings>enable</ImplicitUsings>
+		<Nullable>enable</Nullable>
+
+		<ManagePackageVersionsCentrally>false</ManagePackageVersionsCentrally>
+	</PropertyGroup>
+
+	<ItemGroup>
+		<PackageReference Include="Microsoft.Testing.Platform" Version="$(MicrosoftTestingPlatformVersion)" />		
+	</ItemGroup>
+</Project>

test/TestAssets/TestProjects/TestProjectMTPWithUseAppHostFalse/dotnet.config

@@ -0,0 +1,5 @@
+[general]
+telemetry-output=false
+
+[testing.platform]
+use-microsoft-testing-platform=true

test/dotnet.Tests/CommandTests/Test/GivenDotnetTestBuildsAndRunsTests.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Xml.Linq;
 using Microsoft.DotNet.Cli.Commands;
 using Microsoft.DotNet.Cli.Commands.Test;
 using CommandResult = Microsoft.DotNet.Cli.Utils.CommandResult;
@@ -374,5 +375,26 @@ public void RunningWithGlobalPropertyShouldProperlyPropagate(string configuratio
 
             result.ExitCode.Should().Be(ExitCodes.Success);
         }
+
+        [Fact]
+        public void RunMTPProjectWithUseAppHostFalseAndArchMismatch_ShouldFailWithProperError()
+        {
+            TestAsset testInstance = _testAssetsManager.CopyTestAsset("TestProjectMTPWithUseAppHostFalse", Guid.NewGuid().ToString())
+                .WithSource();
+
+            // Call test with wrong architecture
+            CommandResult result = new DotnetTestCommand(Log, disableNewOutput: false)
+                .WithWorkingDirectory(testInstance.Path)
+                .Execute("--arch", "wrongArchitecture");
+
+            // Verify proper error is shown for architecture mismatch
+            if (!TestContext.IsLocalized())
+            {
+                // Should provide clear error about architecture mismatch when UseAppHost=false
+                result.StdOut.Should().Contain("architecture");
+            }
+
+            result.ExitCode.Should().Be(ExitCodes.GenericFailure);
+        }
     }
 }