Fix first-run experience to output to stderr instead of stdout

Co-authored-by: baronfel <[email protected]>

Author: Copilot

Program.cs

@@ -0,0 +1,2 @@
+// See https://aka.ms/new-console-template for more information
+Console.WriteLine("Hello, World!");

sdk.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

src/Cli/dotnet/Program.cs

@@ -378,7 +378,7 @@ private static void ConfigureDotNetForFirstTimeUse(
         var environmentPath = EnvironmentPathFactory.CreateEnvironmentPath(isDotnetBeingInvokedFromNativeInstaller, environmentProvider);
         _ = new DotNetCommandFactory(alwaysRunOutOfProc: true);
         var aspnetCertificateGenerator = new AspNetCoreCertificateGenerator();
-        var reporter = Reporter.Output;
+        var reporter = Reporter.Error;
         var dotnetConfigurer = new DotnetFirstTimeUseConfigurer(
             firstTimeUseNoticeSentinel,
             aspNetCertificateSentinel,

test/dotnet.Tests/GivenThatTheUserIsRunningDotNetForTheFirstTime.cs

@@ -118,6 +118,30 @@ public void ItShowsTheAppropriateMessageToTheUser()
                 .And.NotContain("Restore completed in");
         }
 
+        [WindowsOnlyFact]
+        public void FirstRunExperienceMessagesShouldGoToStdErr()
+        {
+            // This test ensures that first-run experience messages go to stderr, 
+            // not stdout, to avoid interfering with completion commands and other
+            // tools that parse stdout. See: https://github.com/dotnet/sdk/issues/50444
+            var expectedVersion = GetDotnetVersion();
+            
+            // StdErr should contain first-run messages
+            _fixture.FirstDotnetVerbUseCommandResult.StdErr
+                .Should()
+                .ContainVisuallySameFragment(string.Format(
+                    Configurer.LocalizableStrings.FirstTimeMessageWelcome,
+                    DotnetFirstTimeUseConfigurer.ParseDotNetVersion(expectedVersion),
+                    expectedVersion))
+                .And.ContainVisuallySameFragment(Configurer.LocalizableStrings.FirstTimeMessageMoreInformation);
+                
+            // StdOut should NOT contain first-run messages (they should only be in stderr)
+            _fixture.FirstDotnetVerbUseCommandResult.StdOut
+                .Should()
+                .NotContain("Welcome to .NET")
+                .And.NotContain("Write your first app");
+        }
+
         [Fact]
         public void ItCreatesAFirstUseSentinelFileUnderTheDotDotNetFolder()
         {
@@ -164,7 +188,7 @@ public void ItShowsTheTelemetryNoticeWhenInvokingACommandAfterInternalReportInst
 
             var expectedVersion = GetDotnetVersion();
 
-            result.StdOut
+            result.StdErr
                 .Should()
                 .ContainVisuallySameFragment(string.Format(
                     Configurer.LocalizableStrings.FirstTimeMessageWelcome,