[release/10.0.1xx] Fix first-run experience output interfering with completion commands

.github/copilot-instructions.md

@@ -6,6 +6,13 @@ Coding Style and Changes:
 Testing:
 - Large changes should always include test changes.
 - The Skip parameter of the Fact attribute to point to the specific issue link.
+- To run tests in this repo:
+  - Use the repo-local dotnet instance: `./.dotnet/dotnet`
+  - For MSTest-style projects: `dotnet test path/to/project.csproj --filter "FullyQualifiedName~TestName"`
+  - For XUnit test assemblies: `dotnet exec artifacts/bin/redist/Debug/TestAssembly.dll -method "*TestMethodName*"`
+  - Examples:
+    - `dotnet test test/dotnet.Tests/dotnet.Tests.csproj --filter "Name~ItShowsTheAppropriateMessageToTheUser"`
+    - `dotnet exec artifacts/bin/redist/Debug/dotnet.Tests.dll -method "*ItShowsTheAppropriateMessageToTheUser*"`
 
 Output Considerations:
 - When considering how output should look, solicit advice from baronfel.

src/Cli/dotnet/Program.cs

@@ -389,7 +389,7 @@ 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

@@ -103,12 +103,12 @@ public void UsingDotnetForTheFirstTimeWithNonVerbsDoesNotPrintEula()
                 .StartWith(firstTimeNonVerbUseMessage);
         }
 
-        [WindowsOnlyFact]
+        [Fact]
         public void ItShowsTheAppropriateMessageToTheUser()
         {
 
             var expectedVersion = GetDotnetVersion();
-            _fixture.FirstDotnetVerbUseCommandResult.StdOut
+            _fixture.FirstDotnetVerbUseCommandResult.StdErr
                 .Should()
                 .ContainVisuallySameFragment(string.Format(
                     Configurer.LocalizableStrings.FirstTimeMessageWelcome,
@@ -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,