diff --git a/test/Microsoft.NET.TestFramework/Commands/TestCommand.cs b/test/Microsoft.NET.TestFramework/Commands/TestCommand.cs
index b5446a2c5b2e..ff5401650c53 100644
--- a/test/Microsoft.NET.TestFramework/Commands/TestCommand.cs
+++ b/test/Microsoft.NET.TestFramework/Commands/TestCommand.cs
@@ -218,10 +218,15 @@ public virtual CommandResult Execute(IEnumerable<string> args)
                     command.OnOutputLine(line => CommandOutputHandler.Invoke(line));
                 }
 
-                if (StandardOutputEncoding is not null)
-                {
-                    command.StandardOutputEncoding(StandardOutputEncoding);
-                }
+                // Decode captured stdout as UTF-8 by default so non-ASCII output (e.g. localized
+                // strings such as the French "Bienvenue à .Net!") is not corrupted by the host
+                // console's active code page. Child dotnet/MSBuild processes emit UTF-8; when
+                // StandardOutputEncoding is left null, Process decodes the redirected stream using
+                // Console.OutputEncoding, which on Windows is the OEM code page (e.g. CP437/850) and
+                // turns UTF-8 bytes like 0xC3 0xA0 ('à') into mojibake ('├á'). Because the active code
+                // page varies by agent, tests asserting on non-ASCII output failed intermittently. On
+                // non-Windows the default capture encoding is already UTF-8, so this is a no-op there.
+                command.StandardOutputEncoding(StandardOutputEncoding ?? Encoding.UTF8);
             }
 
             string fileToShow = Path.GetFileNameWithoutExtension(spec.FileName!).Equals("dotnet", StringComparison.OrdinalIgnoreCase) ?
diff --git a/test/TestAssets/TestProjects/KitchenSink/TestApp/Program.cs b/test/TestAssets/TestProjects/KitchenSink/TestApp/Program.cs
index 20987f7bd003..547a8960af08 100644
--- a/test/TestAssets/TestProjects/KitchenSink/TestApp/Program.cs
+++ b/test/TestAssets/TestProjects/KitchenSink/TestApp/Program.cs
@@ -6,6 +6,7 @@
 using System.Globalization;
 using System.Resources;
 using System.Reflection;
+using System.Text;
 
 namespace TestApp
 {
@@ -13,6 +14,14 @@ public class Program
     {
         public static void Main(string[] args)
         {
+            // Emit UTF-8 deterministically so the localized (non-ASCII) strings this app prints
+            // survive being captured by a test harness. When stdout is redirected, Console defaults
+            // its output encoding to the host's console code page (e.g. the OEM code page CP437/CP850
+            // on Windows CI agents), which varies by machine. That made the encoding of characters
+            // such as the French 'à' non-deterministic and the satellite-assembly test flaky. Forcing
+            // UTF-8 here (a no-op on platforms that already default to UTF-8, and BOM-free because
+            // Console strips the preamble) pairs with the harness capturing stdout as UTF-8.
+            Console.OutputEncoding = Encoding.UTF8;
             Console.WriteLine(TestLibrary.Helper.GetMessage());
             VerifySatelliteAssemblies();
         }