Print proxy URL when starting Dev Proxy in detach mode (#1591)

* Initial plan

* Print proxy URL with assigned port when starting Dev Proxy in detach mode

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

* Update DevProxy/Program.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix API URL showing port 0 when using --api-port 0 in detach mode

After Kestrel binds, read the actual API address from
IServerAddressesFeature and update the state file.
The parent process now also waits for a resolved API URL
before printing the detach output.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>
Co-authored-by: Waldek Mastykarz <waldek@mastykarz.nl>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

DevProxy/Commands/DevProxyCommand.cs

@@ -1,6 +1,7 @@
 using DevProxy.Abstractions.Plugins;
 using DevProxy.Abstractions.Proxy;
 using DevProxy.Abstractions.Utils;
+using DevProxy.State;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.AspNetCore.Hosting.Server.Features;
 using System.CommandLine;
@@ -314,6 +315,13 @@ private async Task<int> InvokeAsync(ParseResult parseResult, CancellationToken c
                 var serverAddresses = _app.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>();
                 var address = serverAddresses?.Addresses.FirstOrDefault() ?? $"http://{_proxyConfiguration.IPAddress}:{_proxyConfiguration.ApiPort}";
                 _logger.LogInformation("Dev Proxy API listening on {Address}...", address);
+
+                // Update state file with the actual Kestrel API address
+                // (resolves port 0 to OS-assigned port)
+                if (IsInternalDaemon)
+                {
+                    _ = UpdateStateWithApiUrlAsync(address);
+                }
             });
             await _app.RunAsync(cancellationToken);
 
@@ -716,4 +724,14 @@ private async Task CheckForNewVersionAsync()
             );
         }
     }
+
+    private static async Task UpdateStateWithApiUrlAsync(string apiUrl)
+    {
+        var state = await StateManager.LoadStateByPidAsync(Environment.ProcessId);
+        if (state is not null)
+        {
+            state.ApiUrl = apiUrl;
+            await StateManager.SaveStateAsync(state);
+        }
+    }
 }

DevProxy/Program.cs

@@ -163,13 +163,25 @@ await Console.Error.WriteLineAsync(
             await Task.Delay(200);
 
             var state = await StateManager.LoadStateByPidAsync(process.Id);
-            if (state != null)
+            if (state is { Port: > 0 } && !string.IsNullOrEmpty(state.ApiUrl) && !state.ApiUrl.EndsWith(":0", StringComparison.Ordinal))
             {
+                Uri? apiUri = null;
+                if (!string.IsNullOrWhiteSpace(state.ApiUrl))
+                {
+                    Uri.TryCreate(state.ApiUrl, UriKind.Absolute, out apiUri);
+                }
+
+                // Build proxy URL in a way that correctly handles IPv6 hosts.
+                string hostForProxy = apiUri?.Host ?? IPAddress.Loopback.ToString();
+                var proxyUriBuilder = new UriBuilder(Uri.UriSchemeHttp, hostForProxy, state.Port);
+                var proxyUrl = proxyUriBuilder.Uri.ToString().TrimEnd('/');
+
                 if (isJsonOutput)
                 {
                     await Console.Out.WriteLineAsync(FormatJsonResultEntry(new
                     {
                         state.Pid,
+                        ProxyUrl = proxyUrl,
                         state.ApiUrl,
                         state.LogFile
                     }));
@@ -179,6 +191,7 @@ await Console.Out.WriteLineAsync(FormatJsonResultEntry(new
                     await Console.Out.WriteLineAsync("Dev Proxy started in background.");
                     await Console.Out.WriteLineAsync();
                     await Console.Out.WriteLineAsync($"  PID:       {state.Pid}");
+                    await Console.Out.WriteLineAsync($"  Proxy URL: {proxyUrl}");
                     await Console.Out.WriteLineAsync($"  API URL:   {state.ApiUrl}");
                     await Console.Out.WriteLineAsync($"  Log file:  {state.LogFile}");
                     await Console.Out.WriteLineAsync();