Add RPC protocol compat check.

src/Aspire.Cli/Backchannel/AppHostBackchannel.cs

@@ -106,6 +106,19 @@ public async Task ConnectAsync(Process process, string socketPath, CancellationT
         var stream = new NetworkStream(socket, true);
         var rpc = JsonRpc.Attach(stream, target);
 
+        var capabilities = await rpc.InvokeWithCancellationAsync<string[]>(
+            "GetCapabilitiesAsync",
+            Array.Empty<object>(),
+            cancellationToken);
+
+        if (!capabilities.Any(s => s == "baseline.v0"))
+        {
+            throw new AppHostIncompatibleException(
+                $"AppHost is incompatible with the CLI. The AppHost must be updated to a version that supports the baseline.v0 capability.",
+                "baseline.v0"
+                );
+        }
+
         _rpcTaskCompletionSource.SetResult(rpc);
     }
 
@@ -145,4 +158,20 @@ public async Task<string[]> GetPublishersAsync(CancellationToken cancellationTok
             yield return state;
         }
     }
+
+    public async Task<string[]> GetCapabilitiesAsync(CancellationToken cancellationToken)
+    {
+        using var activity = _activitySource.StartActivity();
+
+        var rpc = await _rpcTaskCompletionSource.Task.ConfigureAwait(false);
+
+        logger.LogDebug("Requesting capabilities");
+
+        var capabilities = await rpc.InvokeWithCancellationAsync<string[]>(
+            "GetCapabilitiesAsync",
+            Array.Empty<object>(),
+            cancellationToken).ConfigureAwait(false);
+
+        return capabilities;
+    }
 }

src/Aspire.Cli/Backchannel/AppHostIncompatibleException.cs

@@ -0,0 +1,9 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Aspire.Cli.Backchannel;
+
+internal sealed class AppHostIncompatibleException(string message, string requiredCapability) : Exception(message)
+{
+    public string RequiredCapability { get; } = requiredCapability;
+}

src/Aspire.Cli/Commands/NewCommand.cs

@@ -70,7 +70,7 @@ public NewCommand(DotNetCliRunner runner, INuGetPackageCache nuGetPackageCache)
         }
         else
         {
-            return await PromptUtils.PromptForSelectionAsync(
+            return await InteractionUtils.PromptForSelectionAsync(
                 "Select a project template:",
                 validTemplates,
                 t => $"{t.TemplateName} ({t.TemplateDescription})",
@@ -84,7 +84,7 @@ private static async Task<string> GetProjectNameAsync(ParseResult parseResult, C
         if (parseResult.GetValue<string>("--name") is not { } name)
         {
             var defaultName = new DirectoryInfo(Environment.CurrentDirectory).Name;
-            name = await PromptUtils.PromptForStringAsync("Enter the project name:",
+            name = await InteractionUtils.PromptForStringAsync("Enter the project name:",
                 defaultValue: defaultName,
                 cancellationToken: cancellationToken);
         }
@@ -96,7 +96,7 @@ private static async Task<string> GetOutputPathAsync(ParseResult parseResult, st
     {
         if (parseResult.GetValue<string>("--output") is not { } outputPath)
         {
-            outputPath = await PromptUtils.PromptForStringAsync(
+            outputPath = await InteractionUtils.PromptForStringAsync(
                 "Enter the output path:",
                 defaultValue: pathAppendage ?? ".",
                 cancellationToken: cancellationToken
@@ -114,7 +114,7 @@ private static async Task<string> GetProjectTemplatesVersionAsync(ParseResult pa
         }
         else
         {
-            version = await PromptUtils.PromptForStringAsync(
+            version = await InteractionUtils.PromptForStringAsync(
                 "Project templates version:",
                 defaultValue: VersionHelper.GetDefaultTemplateVersion(),
                 validator: (string value) => {

src/Aspire.Cli/DotNetCliRunner.cs

@@ -468,6 +468,22 @@ private async Task StartBackchannelAsync(Process process, string socketPath, Tas
                     // We don't want to spam the logs with our early connection attempts.
                 }
             }
+            catch (AppHostIncompatibleException ex)
+            {
+                logger.LogError(
+                    ex,
+                    "AppHost is incompatible with the CLI. The AppHost must be updated to a version that supports the {RequiredCapability} capability.",
+                    ex.RequiredCapability
+                    );
+
+                // If the app host is incompatable then there is no point
+                // trying to reconnect, we should propogate the exception
+                // up to the code that needs to back channel so it can display
+                // and error message to the user.
+                backchannelCompletionSource.SetException(ex);
+
+                throw;
+            }
 
         } while (await timer.WaitForNextTickAsync(cancellationToken));
     }

src/Aspire.Cli/ExitCodeConstants.cs

@@ -14,4 +14,5 @@ internal static class ExitCodeConstants
     public const int FailedToBuildArtifacts = 6;
     public const int FailedToFindProject = 7;
     public const int FailedToTrustCertificates = 8;
+    public const int AppHostIncompatible = 9;
 }

src/Aspire.Cli/Utils/PromptUtils.cs → src/Aspire.Cli/Utils/InteractionUtils.cs

@@ -1,11 +1,12 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Aspire.Cli.Backchannel;
 using Spectre.Console;
 
 namespace Aspire.Cli.Utils;
 
-internal static class PromptUtils
+internal static class InteractionUtils
 {
     public static async Task<string> PromptForStringAsync(string promptText, string? defaultValue = null, Func<string, ValidationResult>? validator = null, CancellationToken cancellationToken = default)
     {
@@ -42,4 +43,10 @@ public static async Task<T> PromptForSelectionAsync<T>(string promptText, IEnume
 
         return await AnsiConsole.PromptAsync(prompt, cancellationToken);
     }
+
+    public static int DisplayIncompatibleVersionError(AppHostIncompatibleException ex)
+    {
+        AnsiConsole.MarkupLine($"[red bold]:thumbs_down:  The app host is not compatible. {ex.Message}[/]");
+        return ExitCodeConstants.AppHostIncompatible;
+    }
 }

src/Aspire.Hosting/Backchannel/AppHostRpcTarget.cs

@@ -138,4 +138,31 @@ public async Task<string[]> GetPublishersAsync(CancellationToken cancellationTok
         var publishers = e.Advertisements.Select(x => x.Name);
         return [..publishers];
     }
+
+#pragma warning disable CA1822
+    public Task<string[]> GetCapabilitiesAsync(CancellationToken cancellationToken)
+    {
+        // The purpose of this API is to allow the CLI to determine what API surfaces
+        // the AppHost supports. In 9.2 we'll be saying that you need a 9.2 apphost,
+        // but the 9.3 CLI might actually support working with 9.2 apphosts. The idea
+        // is that when the backchannel is established the CLI will call this API
+        // and store the results. The "baseline.v0" capability is the bare minimum
+        // that we need as of CLI version 9.2-preview*.
+        //
+        // Some capabilties will be opt in. For example in 9.3 we might refine the
+        // publishing activities API to return more information, or add log streaming
+        // features. So that would add a new capability that the apphsot can report
+        // on initial backchannel negotiation and the CLI can adapt its behavior around
+        // that. There may be scenarios where we need to break compataiblity at which
+        // point we might increase the baseline version that the apphost reports.
+        //
+        // The ability to support a back channel at all is determined by the CLI by
+        // making sure that the apphost version is at least > 9.2.
+
+        _ = cancellationToken;
+        return Task.FromResult(new string[] {
+            "baseline.v0"
+            });
+    }
+#pragma warning restore CA1822
 }