CLI fixes: MCP error message and help display

src/Aspire.Cli/Commands/RootCommand.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.CommandLine;
+using System.CommandLine.Help;
 
 #if DEBUG
 using System.Globalization;
@@ -121,13 +122,19 @@ public RootCommand(
         Options.Add(WaitForDebuggerOption);
         Options.Add(CliWaitForDebuggerOption);
 
-        // Handle standalone 'aspire --banner' (no subcommand)
+        // Handle standalone 'aspire' or 'aspire --banner' (no subcommand)
         this.SetAction((context, cancellationToken) =>
         {
             var bannerRequested = context.GetValue(BannerOption);
-            // If --banner was passed, we've already shown it in Main, just exit successfully
-            // Otherwise, show the standard "no command" error
-            return Task.FromResult(bannerRequested ? 0 : 1);
+            if (bannerRequested)
+            {
+                // If --banner was passed, we've already shown it in Main, just exit successfully
+                return Task.FromResult(0);
+            }
+
+            // No subcommand provided - show help
+            new HelpAction().Invoke(context);
+            return Task.FromResult(0);
         });
 
         Subcommands.Add(newCommand);

src/Aspire.Cli/Mcp/McpErrorMessages.cs

@@ -13,7 +13,7 @@ internal static class McpErrorMessages
     /// </summary>
     public const string NoAppHostRunning =
         "No Aspire AppHost is currently running. " +
-        "To use Aspire MCP tools, you must first start an Aspire application by running 'aspire run' in your AppHost project directory. " +
+        "To use Aspire MCP tools, you must first start an Aspire application by running 'aspire run --detach' in your AppHost project directory. " +
         "Once the application is running, the MCP tools will be able to connect to the dashboard and execute commands.";
 
     /// <summary>

tests/Aspire.Cli.Tests/Mcp/ExecuteResourceCommandToolTests.cs

@@ -34,6 +34,7 @@ public async Task ExecuteResourceCommandTool_ThrowsException_WhenNoAppHostRunnin
             () => tool.CallToolAsync(null!, CreateArguments("test-resource", "resource-start"), CancellationToken.None).AsTask()).DefaultTimeout();
 
         Assert.Contains("No Aspire AppHost", exception.Message);
+        Assert.Contains("--detach", exception.Message);
     }
 
     [Fact]

tests/Aspire.Cli.Tests/Mcp/ListConsoleLogsToolTests.cs

@@ -28,6 +28,7 @@ public async Task ListConsoleLogsTool_ThrowsException_WhenNoAppHostRunning()
             () => tool.CallToolAsync(null!, arguments, CancellationToken.None).AsTask()).DefaultTimeout();
 
         Assert.Contains("No Aspire AppHost", exception.Message);
+        Assert.Contains("--detach", exception.Message);
     }
 
     [Fact]

tests/Aspire.Cli.Tests/Mcp/ListResourcesToolTests.cs

@@ -22,6 +22,7 @@ public async Task ListResourcesTool_ThrowsException_WhenNoAppHostRunning()
             () => tool.CallToolAsync(null!, null, CancellationToken.None).AsTask()).DefaultTimeout();
 
         Assert.Contains("No Aspire AppHost", exception.Message);
+        Assert.Contains("--detach", exception.Message);
     }
 
     [Fact]