Fix guest AppHost update with older CLI

src/Aspire.Cli/Commands/UpdateCommand.cs

@@ -15,6 +15,7 @@
 using Aspire.Cli.Utils;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.Logging;
+using Semver;
 using Spectre.Console;
 
 namespace Aspire.Cli.Commands;
@@ -242,6 +243,12 @@ protected override async Task<CommandResult> ExecuteAsync(ParseResult parseResul
                 ConfirmBinding = confirmBinding,
                 NuGetConfigDirBinding = nugetConfigDirBinding
             };
+            var cliUpdateResult = await TryUpdateCliBeforeGuestProjectUpdateAsync(project, projectFile, channel, confirmBinding, parseResult, cancellationToken);
+            if (cliUpdateResult is not null)
+            {
+                return cliUpdateResult;
+            }
+
             await project.UpdatePackagesAsync(updateContext, cancellationToken);
 
             // After successful project update, check if CLI update is available and prompt
@@ -312,6 +319,78 @@ protected override async Task<CommandResult> ExecuteAsync(ParseResult parseResul
         return CommandResult.FromExitCode(0);
     }
 
+    private async Task<CommandResult?> TryUpdateCliBeforeGuestProjectUpdateAsync(
+        IAppHostProject project,
+        FileInfo projectFile,
+        PackageChannel channel,
+        PromptBinding<bool> confirmBinding,
+        ParseResult parseResult,
+        CancellationToken cancellationToken)
+    {
+        if (_cliDownloader is null ||
+            string.IsNullOrEmpty(channel.CliDownloadBaseUrl) ||
+            project.LanguageId.Equals(KnownLanguageId.CSharp, StringComparison.OrdinalIgnoreCase) ||
+            projectFile.Directory is not { } projectDirectory)
+        {
+            return null;
+        }
+
+        var targetSdkVersion = await GetLatestGuestSdkVersionAsync(channel, projectDirectory, cancellationToken);
+        if (targetSdkVersion is null ||
+            !SemVersion.TryParse(VersionHelper.GetDefaultSdkVersion(), SemVersionStyles.Strict, out var currentCliVersion) ||
+            SemVersion.PrecedenceComparer.Compare(targetSdkVersion, currentCliVersion) <= 0)
+        {
+            return null;
+        }
+
+        var shouldUpdateCli = await InteractionService.PromptConfirmAsync(
+            UpdateCommandStrings.UpdateCliBeforeGuestProjectUpdatePrompt,
+            binding: confirmBinding,
+            cancellationToken: cancellationToken);
+
+        if (!shouldUpdateCli)
+        {
+            return null;
+        }
+
+        var dotNetToolUpdateCommand = GetDotNetToolUpdateCommand();
+        if (dotNetToolUpdateCommand is not null)
+        {
+            InteractionService.DisplayMessage(KnownEmojis.Information, UpdateCommandStrings.DotNetToolSelfUpdateMessage);
+            InteractionService.DisplayPlainText($"  {dotNetToolUpdateCommand}");
+            InteractionService.DisplayMessage(KnownEmojis.Information, UpdateCommandStrings.ProjectUpdateSkippedAfterCliUpdateMessage);
+            return CommandResult.Success();
+        }
+
+        var selfUpdateResult = await ExecuteSelfUpdateAsync(parseResult, cancellationToken, channel.Name);
+        if (selfUpdateResult.ExitCode == ExitCodeConstants.Success)
+        {
+            InteractionService.DisplayMessage(KnownEmojis.Information, UpdateCommandStrings.ProjectUpdateSkippedAfterCliUpdateMessage);
+        }
+
+        return selfUpdateResult;
+    }
+
+    private async Task<SemVersion?> GetLatestGuestSdkVersionAsync(PackageChannel channel, DirectoryInfo projectDirectory, CancellationToken cancellationToken)
+    {
+        try
+        {
+            var sdkPackage = await channel.GetLatestGuestAppHostSdkPackageAsync(projectDirectory, cancellationToken);
+            return sdkPackage is not null && SemVersion.TryParse(sdkPackage.Version, SemVersionStyles.Strict, out var version)
+                ? version
+                : null;
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Failed to check target Aspire SDK version before project update");
+            return null;
+        }
+    }
+
     private async Task<CommandResult> ExecuteSelfUpdateAsync(ParseResult parseResult, CancellationToken cancellationToken, string? selectedChannel = null)
     {
         var channel = selectedChannel ?? parseResult.GetValue(_channelOption) ?? parseResult.GetValue(_qualityOption);

src/Aspire.Cli/Packaging/PackageChannel.cs

@@ -14,6 +14,8 @@ namespace Aspire.Cli.Packaging;
 
 internal class PackageChannel(string name, PackageChannelQuality quality, PackageMapping[]? mappings, INuGetPackageCache nuGetPackageCache, bool configureGlobalPackagesFolder = false, string? cliDownloadBaseUrl = null, string? pinnedVersion = null, ILogger? logger = null)
 {
+    private const string GuestAppHostSdkPackageId = "Aspire.Hosting";
+
     public string Name { get; } = name;
     public PackageChannelQuality Quality { get; } = quality;
     public PackageMapping[]? Mappings { get; } = mappings;
@@ -219,6 +221,31 @@ public async Task<IEnumerable<NuGetPackage>> GetPackagesAsync(string packageId,
         return filteredPackages;
     }
 
+    public async Task<NuGetPackage?> GetLatestGuestAppHostSdkPackageAsync(DirectoryInfo workingDirectory, CancellationToken cancellationToken)
+    {
+        // Guest AppHost sdk.version resolves to the base Aspire.Hosting package because
+        // the managed server restores that package to evaluate and generate the AppHost.
+        var packages = await GetPackagesAsync(GuestAppHostSdkPackageId, workingDirectory, cancellationToken);
+
+        NuGetPackage? latestPackage = null;
+        SemVersion? latestVersion = null;
+        foreach (var package in packages)
+        {
+            if (!SemVersion.TryParse(package.Version, SemVersionStyles.Strict, out var version))
+            {
+                continue;
+            }
+
+            if (latestVersion is null || SemVersion.PrecedenceComparer.Compare(version, latestVersion) > 0)
+            {
+                latestPackage = package;
+                latestVersion = version;
+            }
+        }
+
+        return latestPackage;
+    }
+
     public async Task<IEnumerable<NuGetPackage>> GetPackageVersionsAsync(string packageId, DirectoryInfo workingDirectory, CancellationToken cancellationToken)
     {
         var tasks = new List<Task<IEnumerable<NuGetPackage>>>();

src/Aspire.Cli/Projects/DotNetBasedAppHostServerProject.cs

@@ -254,7 +254,8 @@ private XDocument CreateProjectFile(IEnumerable<IntegrationReference> integratio
     /// </summary>
     public async Task<(string ProjectPath, string? ChannelName)> CreateProjectFilesAsync(
         IEnumerable<IntegrationReference> integrations,
-        CancellationToken cancellationToken = default)
+        CancellationToken cancellationToken = default,
+        string? requestedChannel = null)
     {
         // Clean obj folder to ensure fresh NuGet restore
         var objPath = Path.Combine(_projectModelPath, "obj");
@@ -321,7 +322,8 @@ private XDocument CreateProjectFile(IEnumerable<IntegrationReference> integratio
         }
 
         var channels = await _packagingService.GetChannelsAsync(cancellationToken);
-        var configuredChannelName = AspireConfigFile.Load(_appPath)?.Channel
+        var configuredChannelName = requestedChannel
+            ?? AspireConfigFile.Load(_appPath)?.Channel
             ?? AspireJsonConfiguration.Load(_appPath)?.Channel;
 
         // Resolve channel sources and add them via RestoreAdditionalProjectSources
@@ -417,9 +419,10 @@ private XDocument CreateProjectFile(IEnumerable<IntegrationReference> integratio
     public async Task<AppHostServerPrepareResult> PrepareAsync(
         string sdkVersion,
         IEnumerable<IntegrationReference> integrations,
-        CancellationToken cancellationToken = default)
+        CancellationToken cancellationToken = default,
+        string? requestedChannel = null)
     {
-        var (_, channelName) = await CreateProjectFilesAsync(integrations, cancellationToken);
+        var (_, channelName) = await CreateProjectFilesAsync(integrations, cancellationToken, requestedChannel);
         var (buildSuccess, buildOutput) = await BuildAsync(cancellationToken);
 
         if (!buildSuccess)

src/Aspire.Cli/Projects/GuestAppHostProject.cs

@@ -224,9 +224,10 @@ private string GetPrepareSdkVersion(AspireConfigFile config)
         IAppHostServerProject appHostServerProject,
         string sdkVersion,
         List<IntegrationReference> integrations,
+        string? requestedChannel,
         CancellationToken cancellationToken)
     {
-        var result = await appHostServerProject.PrepareAsync(sdkVersion, integrations, cancellationToken);
+        var result = await appHostServerProject.PrepareAsync(sdkVersion, integrations, cancellationToken, requestedChannel);
         return (result.Success, result.Output, result.ChannelName, result.NeedsCodeGeneration);
     }
 
@@ -235,15 +236,22 @@ private string GetPrepareSdkVersion(AspireConfigFile config)
     /// </summary>
     /// <returns><see langword="true"/> if the code was generated successfully; otherwise, <see langword="false"/>.</returns>
     internal async Task<bool> BuildAndGenerateSdkAsync(DirectoryInfo directory, CancellationToken cancellationToken)
+    {
+        var config = LoadConfiguration(directory);
+        return await BuildAndGenerateSdkAsync(directory, config, cancellationToken);
+    }
+
+    private async Task<bool> BuildAndGenerateSdkAsync(DirectoryInfo directory, AspireConfigFile config, CancellationToken cancellationToken)
     {
         var appHostServerProject = await _appHostServerProjectFactory.CreateAsync(directory.FullName, cancellationToken);
 
-        // Step 1: Load config - source of truth for SDK version and packages
-        var config = LoadConfiguration(directory);
+        // Step 1: Use the supplied config as the source of truth. Update uses an
+        // in-memory config here so a failed generation does not leave
+        // aspire.config.json pinned to versions the current CLI cannot run.
         var integrations = await GetIntegrationReferencesAsync(config, directory, cancellationToken);
         var sdkVersion = GetPrepareSdkVersion(config);
 
-        var (buildSuccess, buildOutput, _, _) = await PrepareAppHostServerAsync(appHostServerProject, sdkVersion, integrations, cancellationToken);
+        var (buildSuccess, buildOutput, _, _) = await PrepareAppHostServerAsync(appHostServerProject, sdkVersion, integrations, config.Channel, cancellationToken);
         if (!buildSuccess)
         {
             if (buildOutput is not null)
@@ -355,7 +363,7 @@ public async Task<int> RunAsync(AppHostProjectContext context, CancellationToken
                 async () =>
                 {
                     // Prepare the AppHost server (build for dev mode, restore for prebuilt)
-                    var (prepareSuccess, prepareOutput, channelName, needsCodeGen) = await PrepareAppHostServerAsync(appHostServerProject, sdkVersion, integrations, cancellationToken);
+                    var (prepareSuccess, prepareOutput, channelName, needsCodeGen) = await PrepareAppHostServerAsync(appHostServerProject, sdkVersion, integrations, config.Channel, cancellationToken);
                     if (!prepareSuccess)
                     {
                         return (Success: false, Output: prepareOutput, Error: "Failed to prepare app host.", ChannelName: (string?)null, NeedsCodeGen: false);
@@ -848,7 +856,7 @@ public async Task<int> PublishAsync(PublishContext context, CancellationToken ca
             var sdkVersion = GetPrepareSdkVersion(config);
 
             // Prepare the AppHost server (build for dev mode, restore for prebuilt)
-            var (prepareSuccess, prepareOutput, _, needsCodeGen) = await PrepareAppHostServerAsync(appHostServerProject, sdkVersion, integrations, cancellationToken);
+            var (prepareSuccess, prepareOutput, _, needsCodeGen) = await PrepareAppHostServerAsync(appHostServerProject, sdkVersion, integrations, config.Channel, cancellationToken);
             if (!prepareSuccess)
             {
                 // Set OutputCollector so PipelineCommandBase can display errors
@@ -1148,11 +1156,7 @@ public async Task<UpdatePackagesResult> UpdatePackagesAsync(UpdatePackagesContex
                 // Check for SDK version update (silently - it's an implementation detail)
                 try
                 {
-                    var sdkPackages = await context.Channel.GetPackagesAsync("Aspire.Hosting", directory, cancellationToken);
-                    var latestSdkPackage = sdkPackages
-                        .Where(p => SemVersion.TryParse(p.Version, SemVersionStyles.Strict, out _))
-                        .OrderByDescending(p => SemVersion.Parse(p.Version, SemVersionStyles.Strict), SemVersion.PrecedenceComparer)
-                        .FirstOrDefault();
+                    var latestSdkPackage = await context.Channel.GetLatestGuestAppHostSdkPackageAsync(directory, cancellationToken);
 
                     if (latestSdkPackage is not null && latestSdkPackage.Version != config.SdkVersion)
                     {
@@ -1237,15 +1241,13 @@ public async Task<UpdatePackagesResult> UpdatePackagesAsync(UpdatePackagesContex
         {
             config.AddOrUpdatePackage(packageId, newVersion);
         }
-        SaveConfiguration(config, directory);
-
         // Rebuild and regenerate SDK code with updated packages
         _interactionService.DisplayEmptyLine();
         var regenerateResult = await _interactionService.ShowStatusAsync(
             UpdateCommandStrings.RegeneratingSdkCode,
             async () =>
             {
-                var regenerateSuccess = await BuildAndGenerateSdkAsync(directory, cancellationToken);
+                var regenerateSuccess = await BuildAndGenerateSdkAsync(directory, config, cancellationToken);
 
                 if (!regenerateSuccess)
                 {
@@ -1260,6 +1262,8 @@ public async Task<UpdatePackagesResult> UpdatePackagesAsync(UpdatePackagesContex
             return regenerateResult;
         }
 
+        SaveConfiguration(config, directory);
+
         _interactionService.DisplayMessage(KnownEmojis.Package, UpdateCommandStrings.RegeneratedSdkCode);
 
         _interactionService.DisplayEmptyLine();

src/Aspire.Cli/Projects/IAppHostServerProject.cs

@@ -41,11 +41,13 @@ internal interface IAppHostServerProject
     /// <param name="sdkVersion">The Aspire SDK version to use.</param>
     /// <param name="integrations">The integration references (NuGet packages and/or project references) required by the app host.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
+    /// <param name="requestedChannel">The package channel to use for this prepare operation, or <see langword="null" /> to use the project configuration.</param>
     /// <returns>The preparation result indicating success/failure and any output.</returns>
     Task<AppHostServerPrepareResult> PrepareAsync(
         string sdkVersion,
         IEnumerable<IntegrationReference> integrations,
-        CancellationToken cancellationToken = default);
+        CancellationToken cancellationToken = default,
+        string? requestedChannel = null);
 
     /// <summary>
     /// Runs the AppHost server process.

src/Aspire.Cli/Projects/PrebuiltAppHostServer.cs

@@ -123,12 +123,12 @@ public string GetServerPath()
     public async Task<AppHostServerPrepareResult> PrepareAsync(
         string sdkVersion,
         IEnumerable<IntegrationReference> integrations,
-        CancellationToken cancellationToken = default)
+        CancellationToken cancellationToken = default,
+        string? requestedChannel = null)
     {
         var integrationList = integrations.ToList();
         var packageRefs = integrationList.Where(r => r.IsPackageReference).ToList();
         var projectRefs = integrationList.Where(r => r.IsProjectReference).ToList();
-        string? requestedChannel = null;
 
         try
         {
@@ -140,7 +140,7 @@ public async Task<AppHostServerPrepareResult> PrepareAsync(
             // Resolve the channel the project requests for restore (aspire.config.json#channel,
             // with a legacy .aspire/settings.json#channel fallback). This is independent of the
             // running CLI's identity hive (CliExecutionContext.IdentityChannel).
-            requestedChannel = ResolveRequestedChannel();
+            requestedChannel ??= ResolveRequestedChannel();
 
             if (projectRefs.Count > 0)
             {

src/Aspire.Cli/Resources/UpdateCommandStrings.resx

@@ -123,6 +123,9 @@
   <data name="UpdateCliAfterProjectUpdatePrompt" xml:space="preserve">
     <value>An update is available for the Aspire CLI. Would you like to update it now?</value>
   </data>
+  <data name="UpdateCliBeforeGuestProjectUpdatePrompt" xml:space="preserve">
+    <value>The selected Aspire SDK is newer than this Aspire CLI. Update the Aspire CLI now and re-run `aspire update` to update this project?</value>
+  </data>
   <data name="ChannelOptionDescription" xml:space="preserve">
     <value>Channel to update to (stable, daily)</value>
   </data>
@@ -138,6 +141,9 @@
   <data name="DotNetToolSelfUpdateMessage" xml:space="preserve">
     <value>To update the Aspire CLI when installed as a .NET tool, run:</value>
   </data>
+  <data name="ProjectUpdateSkippedAfterCliUpdateMessage" xml:space="preserve">
+    <value>Project update skipped. Update the Aspire CLI, then re-run `aspire update`.</value>
+  </data>
   <data name="MigratedToNewSdkFormat" xml:space="preserve">
     <value>Migrated to new project format: &lt;Project Sdk="Aspire.AppHost.Sdk/{0}"&gt;</value>
   </data>