Accept dev localhost resource service URLs

src/Aspire.Hosting/Dashboard/DashboardServiceHost.cs

@@ -138,7 +138,7 @@ void ConfigureKestrel(KestrelServerOptions kestrelOptions)
                 kestrelOptions.Listen(IPAddress.Loopback, port: 0, ConfigureListen);
                 _logger.LogDebug("Resource service endpoint not configured. Listening on {Scheme}://127.0.0.1:<random>.", scheme);
             }
-            else if (uri.IsLoopback)
+            else if (IsLocalResourceServiceEndpoint(uri))
             {
                 // Listen on the requested localhost port.
                 kestrelOptions.ListenLocalhost(uri.Port, ConfigureListen);
@@ -178,6 +178,28 @@ internal static string ResolveScheme(Uri? configuredUri, bool allowUnsecuredTran
         return allowUnsecuredTransport ? "http" : "https";
     }
 
+    /// <summary>
+    /// Determines whether the resource service endpoint is scoped to the local machine.
+    /// </summary>
+    internal static bool IsLocalResourceServiceEndpoint(Uri uri)
+    {
+        if (uri.IsLoopback)
+        {
+            return true;
+        }
+
+        var host = uri.Host.EndsWith(".", StringComparison.Ordinal)
+            ? uri.Host[..^1]
+            : uri.Host;
+
+        // RFC 6761 reserves "localhost." and names under it for loopback resolution:
+        // https://www.rfc-editor.org/rfc/rfc6761#section-6.3. Uri.IsLoopback only
+        // recognizes "localhost" itself, but Aspire-generated polyglot AppHosts can use
+        // scoped names like "myapp.dev.localhost".
+        return string.Equals(host, KnownHostNames.Localhost, StringComparison.OrdinalIgnoreCase)
+            || host.EndsWith(".localhost", StringComparison.OrdinalIgnoreCase);
+    }
+
     /// <summary>
     /// Gets the URI upon which the resource service is listening.
     /// </summary>

tests/Aspire.Cli.EndToEnd.Tests/SmokeTests.cs

@@ -64,6 +64,48 @@ await auto.WaitUntilAsync(s =>
         await auto.WaitForSuccessPromptAsync(counter);
     }
 
+    [CaptureWorkspaceOnFailure]
+    [Fact]
+    public async Task CreateAndRunPolyglotAppHostWithDevLocalhostUrls()
+    {
+        var repoRoot = CliE2ETestHelpers.GetRepoRoot();
+        var strategy = CliInstallStrategy.Detect(output.WriteLine);
+
+        var workspace = TemporaryWorkspace.Create(output);
+
+        using var terminal = CliE2ETestHelpers.CreateDockerTestTerminal(repoRoot, strategy, output, mountDockerSocket: true, workspace: workspace);
+
+        var counter = new SequenceCounter();
+        var auto = new Hex1bTerminalAutomator(terminal, defaultTimeout: TimeSpan.FromSeconds(500));
+        await using var terminalRun = CliE2ETestHelpers.StartRun(terminal, workspace, auto, counter, output, TestContext.Current.CancellationToken);
+
+        await auto.PrepareDockerEnvironmentAsync(counter, workspace);
+        await auto.InstallAspireCliAsync(strategy, counter);
+
+        const string projectName = "PolyglotDevLocalhost";
+        await auto.AspireNewAsync(projectName, counter, template: AspireTemplate.ExpressReact, useDevLocalhost: true);
+
+        await auto.RunCommandFailFastAsync($"cd {projectName}", counter);
+        await auto.RunCommandFailFastAsync("grep -F 'ASPIRE_RESOURCE_SERVICE_ENDPOINT_URL' aspire.config.json && grep -F 'polyglotdevlocalhost.dev.localhost' aspire.config.json", counter);
+
+        await auto.TypeAsync("aspire run");
+        await auto.EnterAsync();
+
+        await auto.WaitUntilAsync(s =>
+        {
+            if (s.ContainsText("Capability Error") ||
+                s.ContainsText("ASPIRE_RESOURCE_SERVICE_ENDPOINT_URL must contain a local loopback address"))
+            {
+                throw new InvalidOperationException("Polyglot AppHost failed to start with a *.dev.localhost resource service endpoint.");
+            }
+
+            return s.ContainsText("Press CTRL+C to stop the AppHost and exit.");
+        }, timeout: TimeSpan.FromMinutes(3), description: "Press CTRL+C message for polyglot AppHost with *.dev.localhost URLs");
+
+        await auto.Ctrl().KeyAsync(Hex1bKey.C);
+        await auto.WaitForSuccessPromptAsync(counter);
+    }
+
     [CaptureWorkspaceOnFailure]
     [Fact]
     public async Task LatestCliCanStartStableChannelAppHost()

tests/Aspire.Hosting.Tests/Dashboard/DashboardServiceHostTests.cs

@@ -22,4 +22,21 @@ public void ResolveScheme_ReturnsExpectedScheme(string? uriString, bool allowUns
 
         Assert.Equal(expectedScheme, scheme);
     }
+
+    [Theory]
+    [InlineData("https://localhost:5001", true)]
+    [InlineData("https://localhost.:5001", true)]
+    [InlineData("https://127.0.0.1:5001", true)]
+    [InlineData("https://[::1]:5001", true)]
+    [InlineData("https://myapp.dev.localhost:5001", true)]
+    [InlineData("https://myapp.dev.localhost.:5001", true)]
+    [InlineData("https://example.com:5001", false)]
+    [InlineData("https://localhost.example.com:5001", false)]
+    [InlineData("https://example-localhost:5001", false)]
+    public void IsLocalResourceServiceEndpoint_ReturnsExpectedResult(string uriString, bool expectedResult)
+    {
+        var result = DashboardServiceHost.IsLocalResourceServiceEndpoint(new Uri(uriString));
+
+        Assert.Equal(expectedResult, result);
+    }
 }