F# tooling doesn't work in VS

The failure appears in the Visual Studio project-system stack traces, at CPSProjectFactory.CreateProjectContextAsync and RoslynProjectFactory.CreateAsync, for the Mobile.iOS.fsproj and Fable.SignalR.fsproj projects. Start by reproducing it on VS Insiders 12308.306 and checking the two linked threads (dotnet/fsharp#14727 and #18766) for an existing diagnosis. Done means a root cause is identified, either in this repository or in the Visual Studio component it depends on.

Today's VS update completely broke F# tooling

=====================
13. 10. 2026 21:38:53
Project: Mobile.iOS.fsproj
LimitedFunctionality
System.Runtime.InteropServices.COMException: Error HRESULT E_FAIL has been returned from a call to a COM component.
   at Microsoft.VisualStudio.Shell.Interop.IVsTaskBody.DoWork(IVsTask pTask, UInt32 dwCount, IVsTask[] pParentTasks, Object& pResult)
   at Microsoft.VisualStudio.Services.VsTask.<>c__DisplayClass89_0.<GetCallbackForMultipleParent>b__0(Task`1[] _)
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.VisualStudio.Services.VsTask.RethrowException(AggregateException e)
   at Microsoft.VisualStudio.Services.VsTask.InternalGetResult(Boolean ignoreUIThreadCheck)
   at Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.CPS.CPSProjectFactory.<CreateProjectContextAsync>d__11.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.ProjectSystem.VS.LanguageServices.RoslynProjectFactory.<CreateAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.ProjectSystem.LanguageServices.RoslynProjectUpdateSubscription`1.<>c__DisplayClass38_1.<<OnEvaluationUpdateAsync>g__CreateProjectAsync|4>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.ProjectSystem.LanguageServices.RoslynProjectUpdateSubscription`1.<>c__DisplayClass38_0.<<OnEvaluationUpdateAsync>g__ProcessInitialEvaluationDataAsync|1>d.MoveNext()
===================
13. 10. 2026 21:38:53
Project: Fable.SignalR.fsproj
LimitedFunctionality
System.ArgumentException: Value does not fall within the expected range.
   at Microsoft.VisualStudio.Shell.Interop.IVsTaskBody.DoWork(IVsTask pTask, UInt32 dwCount, IVsTask[] pParentTasks, Object& pResult)
   at Microsoft.VisualStudio.Services.VsTask.<>c__DisplayClass89_0.<GetCallbackForMultipleParent>b__0(Task`1[] _)
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.VisualStudio.Services.VsTask.RethrowException(AggregateException e)
   at Microsoft.VisualStudio.Services.VsTask.InternalGetResult(Boolean ignoreUIThreadCheck)
   at Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem.CPS.CPSProjectFactory.<CreateProjectContextAsync>d__11.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.ProjectSystem.VS.LanguageServices.RoslynProjectFactory.<CreateAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.ProjectSystem.LanguageServices.RoslynProjectUpdateSubscription`1.<>c__DisplayClass38_1.<<OnEvaluationUpdateAsync>g__CreateProjectAsync|4>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.ProjectSystem.LanguageServices.RoslynProjectUpdateSubscription`1.<>c__DisplayClass38_0.<<OnEvaluationUpdateAsync>g__ProcessInitialEvaluationDataAsync|1>d.MoveNext()

https://github.com/dotnet/fsharp/issues/14727 https://github.com/dotnet/fsharp/issues/18766#issuecomment-4809493946

Related information

• VS Insiders [12308.306]