Reject --add-source when package source mapping is enabled

src/Cli/dotnet/CliStrings.resx

@@ -455,6 +455,9 @@ setx PATH "%PATH%;{0}"
   <data name="FailedToFindToolAssembly" xml:space="preserve">
     <value>{0}: failed to find toolAssembly for {1}</value>
   </data>
+  <data name="CannotUseAddSourceWithSourceMapping" xml:space="preserve">
+    <value>The --add-source option cannot be combined with package source mapping. To use an additional source, update your NuGet configuration file's source mapping settings. Learn more: https://aka.ms/nuget-package-source-mapping</value>
+  </data>
   <data name="FailedToLoadNuGetSource" xml:space="preserve">
     <value>Failed to load NuGet source {0}</value>
   </data>

src/Cli/dotnet/NugetPackageDownloader/NuGetPackageDownloader.cs

@@ -371,6 +371,25 @@ private IEnumerable<PackageSource> LoadOverrideSources(PackageSourceLocation pac
 
     }
 
+    private void ValidateSourceMappingCompatibility(PackageSourceLocation sourceLocation, PackageSourceMapping mapping)
+    {
+        // When mapping feature is active and user specified extra feeds via CLI, this creates a conflict
+        // because the extra feeds won't be in the mapping configuration
+        if (mapping?.IsEnabled != true)
+        {
+            return;
+        }
+
+        var extraFeeds = sourceLocation?.AdditionalSourceFeed;
+        if (extraFeeds == null || extraFeeds.Length == 0)
+        {
+            return;
+        }
+
+        // Found both mapping and CLI-added feeds - this combination won't work
+        throw new NuGetPackageInstallerException(CliStrings.CannotUseAddSourceWithSourceMapping);
+    }
+
     private List<PackageSource> LoadDefaultSources(PackageId packageId, PackageSourceLocation packageSourceLocation = null, PackageSourceMapping packageSourceMapping = null)
     {
         List<PackageSource> defaultSources = [];
@@ -395,6 +414,12 @@ private List<PackageSource> LoadDefaultSources(PackageId packageId, PackageSourc
 
         packageSourceMapping ??= PackageSourceMapping.GetPackageSourceMapping(settings);
 
+        // Ensure compatibility between source mapping configuration and CLI options
+        if (_shouldUsePackageSourceMapping)
+        {
+            ValidateSourceMappingCompatibility(packageSourceLocation, packageSourceMapping);
+        }
+
         // filter package patterns if enabled
         if (_shouldUsePackageSourceMapping && packageSourceMapping?.IsEnabled == true)
         {