[Hot Design] Hot design fails to launch

[Lamparter]

See Lamparter/Elapsed#27 for the original report.

I've recently acquired a Uno Platform Studio pro subscription but have noticed that Uno Platform Studio and Hot Reload are no longer working on my Uno Platform project, Elapsed.
They both seem to run into some kind of generic error as they are activated:

fail: Uno.Diagnostics.UI.DiagnosticsOverlay+DiagnosticElement[0]
      Failed to get preview for Hot Reload.
      System.InvalidOperationException: Cannot resolve the hot-reload client.
         at Uno.UI.HotDesign.Client.HotReload.HotReloadStatusView.Create(IDiagnosticViewContext ctx)
         at Uno.UI.HotDesign.Client.HotReload.OnAssemblyLoading.<Initialize>g__ViewFactory|0_0(IDiagnosticViewContext ctx)
         at Uno.Diagnostics.UI.DiagnosticViewManager`2.GetView(IDiagnosticViewContext context) in C:\a\1\s\src\Uno.UI\Diagnostics\DiagnosticViewManager.TView.TState.cs:line 38
         at Uno.Diagnostics.UI.DiagnosticView`2.Uno.Diagnostics.UI.IDiagnosticView.GetElement(IDiagnosticViewContext context) in C:\a\1\s\src\Uno.UI\Diagnostics\DiagnosticView.TView.TState.cs:line 45
         at Uno.Diagnostics.UI.DiagnosticsOverlay.DiagnosticElement.CreateValue() in C:\a\1\s\src\Uno.UI.Toolkit\Diagnostics\DiagnosticsOverlay.DiagnosticElement.cs:line 28
03-23 17:06:19.434+00:00 E/Uno.Diagnostics.UI.DiagnosticsOverlay+DiagnosticElement: Failed to get preview for Hot Reload. System.InvalidOperationException: Cannot resolve the hot-reload client.
   at Uno.UI.HotDesign.Client.HotReload.HotReloadStatusView.Create(IDiagnosticViewContext ctx)
   at Uno.UI.HotDesign.Client.HotReload.OnAssemblyLoading.<Initialize>g__ViewFactory|0_0(IDiagnosticViewContext ctx)
   at Uno.Diagnostics.UI.DiagnosticViewManager`2.GetView(IDiagnosticViewContext context) in C:\a\1\s\src\Uno.UI\Diagnostics\DiagnosticViewManager.TView.TState.cs:line 38
   at Uno.Diagnostics.UI.DiagnosticView`2.Uno.Diagnostics.UI.IDiagnosticView.GetElement(IDiagnosticViewContext context) in C:\a\1\s\src\Uno.UI\Diagnostics\DiagnosticView.TView.TState.cs:line 45
   at Uno.Diagnostics.UI.DiagnosticsOverlay.DiagnosticElement.CreateValue() in C:\a\1\s\src\Uno.UI.Toolkit\Diagnostics\DiagnosticsOverlay.DiagnosticElement.cs:line 28

InvalidOperationException: Cannot resolve the hot-reload client.
at Uno.UI.HotDesign.Client.HotReload.HotReloadStatusView.Create(...)

This happens no matter what configuration I use on my project and the bug still occurs if run without attaching the debugger in Visual Studio.

Through a lot of testing, I have managed to find the commit in my project where this issue first appeared: c28d7d1.
The most significant change introduced in that commit was changing my MSBuild project's configurations to include Release and Preview instead of Release and Debug (I renamed the Debug profile to Preview).
I would be surprised if that's the issue, but that's all I can think of.

Which tools are affected?

Hot Design®, Hot Reload

Renderer

• Skia
• Native

Affected platforms

Desktop - net10.0-desktop (on Windows), Android - net10.0-android

Uno.Sdk version

6.5.31

IDE version

Microsoft Visual Studio 2026 Insiders [11612.153]

Uno Platform Extension version

unoplatform.uno-platform-addin-2022 [6.5.9.9]

[Lamparter]

It looks like a potential workaround is to rename the Preview configuration to Debug. However, that doesn't fix the issue, especially considering the app's entire build system and CD integration are built around the configuration being called Preview.

[nickrandolph]

@Lamparter so just to confirm, switching the configuration back to Debug does work? If so, we can investigate further.

[Lamparter]

@nickrandolph if you set the MSBuild configurations to anything BUT Release|Debug, Uno studio will not work; yes