feat: Add ISdkResolver to decouple SDK fetching from platform files

src/BuildScriptGenerator/BuildScriptGeneratorServiceCollectionExtensions.cs

@@ -41,6 +41,8 @@ public static IServiceCollection AddBuildScriptGeneratorServices(this IServiceCo
             services.AddSingleton<DefaultPlatformsInformationProvider>();
             services.AddSingleton<PlatformsInstallationScriptProvider>();
             services.AddSingleton<IExternalSdkProvider, ExternalSdkProvider>();
+            services.AddSingleton<IMcrSdkProvider, McrSdkProvider>();
+            services.AddSingleton<ISdkResolver, SdkResolver>();
             services.AddHttpClient("general", httpClient =>
             {
                 // NOTE: Setting user agent is required to avoid receiving 403 Forbidden response.

src/BuildScriptGenerator/Contracts/IMcrSdkProvider.cs

@@ -0,0 +1,39 @@
+// --------------------------------------------------------------------------------------------
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+// --------------------------------------------------------------------------------------------
+
+using System.Threading.Tasks;
+
+namespace Microsoft.Oryx.BuildScriptGenerator
+{
+    /// <summary>
+    /// Interface for MCR (Microsoft Container Registry) SDK provider that pulls
+    /// SDK tarballs from Docker images hosted in MCR.
+    /// </summary>
+    public interface IMcrSdkProvider
+    {
+        /// <summary>
+        /// The default base URL for MCR SDK images.
+        /// Images follow the convention: {BaseUrl}/{platformName}:{version}-{debianFlavor}.
+        /// </summary>
+        public const string DefaultMcrSdkImageBaseUrl = "mcr.microsoft.com/oryx/sdks";
+
+        /// <summary>
+        /// The directory inside the Docker image where the SDK tarball is stored.
+        /// </summary>
+        public const string SdkDirectoryInImage = "/sdks";
+
+        /// <summary>
+        /// Pulls an SDK tarball from a Docker image in MCR and stores it in the local cache.
+        /// The tarball is extracted from the image and placed at the same cache directory
+        /// used by <see cref="IExternalSdkProvider"/> so that existing installation scripts
+        /// can locate and extract it.
+        /// </summary>
+        /// <param name="platformName">The name of the platform (e.g., "nodejs", "python", "dotnet").</param>
+        /// <param name="version">The version of the SDK to pull.</param>
+        /// <param name="debianFlavor">The Debian flavor (e.g., "bookworm", "bullseye").</param>
+        /// <returns>True if the SDK was successfully pulled and cached; false otherwise.</returns>
+        Task<bool> PullSdkAsync(string platformName, string version, string debianFlavor);
+    }
+}

src/BuildScriptGenerator/Contracts/ISdkResolver.cs

@@ -0,0 +1,29 @@
+// --------------------------------------------------------------------------------------------
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+// --------------------------------------------------------------------------------------------
+
+namespace Microsoft.Oryx.BuildScriptGenerator
+{
+    /// <summary>
+    /// Resolves SDK binaries by trying multiple sources in priority order:
+    /// MCR Docker images → External SDK provider (blob storage) → CDN fallback.
+    /// </summary>
+    public interface ISdkResolver
+    {
+        /// <summary>
+        /// Attempts to fetch the SDK tarball for the given platform and version from available
+        /// SDK sources (MCR, External SDK provider). If a source succeeds, the tarball will be
+        /// available in the shared SDK cache directory and the caller can generate an installation
+        /// script that skips the binary download.
+        /// </summary>
+        /// <param name="platformName">The name of the platform (e.g., "nodejs", "python", "dotnet").</param>
+        /// <param name="version">The version of the SDK to fetch.</param>
+        /// <param name="debianFlavor">The Debian flavor (e.g., "bookworm", "bullseye").</param>
+        /// <returns>
+        /// True if the SDK was successfully fetched and cached from any source;
+        /// false if no source could provide the SDK (caller should fall back to CDN download).
+        /// </returns>
+        bool TryFetchSdk(string platformName, string version, string debianFlavor);
+    }
+}

src/BuildScriptGenerator/DefaultPlatformsInformationProvider.cs

@@ -52,6 +52,11 @@ public IEnumerable<PlatformInfo> GetPlatformsInfo(RepositoryContext context)
                 this.outputWriter.WriteLine("External SDK provider is enabled.");
             }
 
+            if (this.commonOptions.EnableMcrSdkProvider)
+            {
+                this.outputWriter.WriteLine("MCR SDK provider is enabled.");
+            }
+
             foreach (var platform in this.platforms)
             {
                 // Check if a platform is enabled or not

src/BuildScriptGenerator/DotNetCore/DotnetCorePlatform.cs

@@ -34,7 +34,7 @@ internal class DotNetCorePlatform : IProgrammingPlatform
         private readonly BuildScriptGeneratorOptions commonOptions;
         private readonly DotNetCorePlatformInstaller platformInstaller;
         private readonly GlobalJsonSdkResolver globalJsonSdkResolver;
-        private readonly IExternalSdkProvider externalSdkProvider;
+        private readonly ISdkResolver sdkResolver;
         private readonly TelemetryClient telemetryClient;
 
         /// <summary>
@@ -47,6 +47,7 @@ internal class DotNetCorePlatform : IProgrammingPlatform
         /// <param name="dotNetCoreScriptGeneratorOptions">The options if .NET platform.</param>
         /// <param name="platformInstaller">The <see cref="DotNetCorePlatformInstaller"/>.</param>
         /// <param name="globalJsonSdkResolver">The <see cref="GlobalJsonSdkResolver"/>.</param>
+        /// <param name="sdkResolver">The <see cref="ISdkResolver"/>.</param>
         public DotNetCorePlatform(
             IDotNetCoreVersionProvider versionProvider,
             ILogger<DotNetCorePlatform> logger,
@@ -55,7 +56,7 @@ public DotNetCorePlatform(
             IOptions<DotNetCoreScriptGeneratorOptions> dotNetCoreScriptGeneratorOptions,
             DotNetCorePlatformInstaller platformInstaller,
             GlobalJsonSdkResolver globalJsonSdkResolver,
-            IExternalSdkProvider externalSdkProvider,
+            ISdkResolver sdkResolver,
             TelemetryClient telemetryClient)
         {
             this.versionProvider = versionProvider;
@@ -65,7 +66,7 @@ public DotNetCorePlatform(
             this.commonOptions = commonOptions.Value;
             this.platformInstaller = platformInstaller;
             this.globalJsonSdkResolver = globalJsonSdkResolver;
-            this.externalSdkProvider = externalSdkProvider;
+            this.sdkResolver = sdkResolver;
             this.telemetryClient = telemetryClient;
         }
 
@@ -244,36 +245,15 @@ public string GetInstallerScriptSnippet(
                 }
                 else
                 {
-                    if (this.commonOptions.EnableExternalSdkProvider)
-                    {
-                        this.logger.LogDebug("DotNetCore SDK version {version} is not installed. External SDK provider is enabled so trying to fetch SDK using it.", dotNetCorePlatformDetectorResult.SdkVersion);
-
-                        try
-                        {
-                            var blobName = BlobNameHelper.GetBlobNameForVersion(this.Name, dotNetCorePlatformDetectorResult.SdkVersion, this.commonOptions.DebianFlavor);
-                            var isExternalFetchSuccess = this.externalSdkProvider.RequestBlobAsync(this.Name, blobName).Result;
-                            if (isExternalFetchSuccess)
-                            {
-                                this.logger.LogDebug("DotNetCore SDK version {version} is fetched successfully using external SDK provider. So generating an installation script snippet which skips platform binary download.", dotNetCorePlatformDetectorResult.SdkVersion);
-                                installationScriptSnippet = this.platformInstaller.GetInstallerScriptSnippet(dotNetCorePlatformDetectorResult.SdkVersion, skipSdkBinaryDownload: true);
-                            }
-                            else
-                            {
-                                this.logger.LogDebug("DotNetCore SDK version {version} is not fetched successfully using external SDK provider. So generating an installation script snippet for it.", dotNetCorePlatformDetectorResult.SdkVersion);
-                                installationScriptSnippet = this.platformInstaller.GetInstallerScriptSnippet(dotNetCorePlatformDetectorResult.SdkVersion);
-                            }
-                        }
-                        catch (Exception ex)
-                        {
-                            this.logger.LogError(ex, "Error while fetching DotNetCore SDK version version {version} using external SDK provider.", dotNetCorePlatformDetectorResult.SdkVersion);
-                            installationScriptSnippet = this.platformInstaller.GetInstallerScriptSnippet(dotNetCorePlatformDetectorResult.SdkVersion);
-                        }
-                    }
-                    else
-                    {
-                        this.logger.LogDebug("DotNetCore SDK version {globalJsonSdkVersion} is not installed. So generating an installation script snippet for it.", dotNetCorePlatformDetectorResult.SdkVersion);
-                        installationScriptSnippet = this.platformInstaller.GetInstallerScriptSnippet(dotNetCorePlatformDetectorResult.SdkVersion);
-                    }
+                    this.logger.LogDebug(
+                        "DotNetCore SDK version {version} is not installed. Trying to fetch SDK.",
+                        dotNetCorePlatformDetectorResult.SdkVersion);
+
+                    var sdkFetched = this.sdkResolver.TryFetchSdk(
+                        this.Name, dotNetCorePlatformDetectorResult.SdkVersion, this.commonOptions.DebianFlavor);
+
+                    installationScriptSnippet = this.platformInstaller.GetInstallerScriptSnippet(
+                        dotNetCorePlatformDetectorResult.SdkVersion, skipSdkBinaryDownload: sdkFetched);
                 }
             }
             else