Oxpecker fable plugin not working with fable 4.25.0+

[Lanayx]

When I'm trying to run command from any of the Oxpecker.Solid projects after updating from 4.24 to 4.25:

 dotnet fable --noCache

I get the error

Loaded Oxpecker.Solid.SolidComponentAttribute from ..\..\src\Oxpecker.Solid.FablePlugin\bin\Debug\net6.0\Oxpecker.Solid.FablePlugin.dll
Started Fable compilation...
Compiled 12/12: src\Components\TodoList.fs                    
Fable compilation finished in 4006ms

Unhandled exception. System.IO.FileNotFoundException: Could not load file or assembly 'FSharp.Core, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.
File name: 'FSharp.Core, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at System.ModuleHandle.ResolveType(QCallModule module, Int32 typeToken, IntPtr* typeInstArgs, Int32 typeInstCount, IntPtr* methodInstArgs, Int32 methodInstCount, ObjectHandleOnStack type)
   at System.ModuleHandle.ResolveTypeHandle(Int32 typeToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext)
   at System.Reflection.RuntimeModule.ResolveType(Int32 metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments)
   at System.Reflection.CustomAttribute.FilterCustomAttributeRecord(MetadataToken caCtorToken, MetadataImport& scope, RuntimeModule decoratedModule, MetadataToken decoratedToken, RuntimeType attributeFilterType, Boolean mustBeInheritable, ListBuilder`1& derivedAttributes, RuntimeType& attributeType, IRuntimeMethodInfo& ctorWithParameters, Boolean& isVarArg)
   at System.Reflection.CustomAttribute.IsCustomAttributeDefined(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, RuntimeType attributeFilterType, Int32 attributeCtorToken, Boolean mustBeInheritable)
   at System.Reflection.CustomAttribute.IsDefined(RuntimeType type, RuntimeType caType, Boolean inherit)
   at System.RuntimeType.IsDefined(Type attributeType, Boolean inherit)
   at System.Diagnostics.StackTrace.ToString(TraceFormat traceFormat, StringBuilder sb)
   at System.Diagnostics.StackTrace.ToString(TraceFormat traceFormat)
   at System.Exception.get_StackTrace()
   at Fable.Cli.Main.compilationCycle@1118-8.Invoke(FSharpResult`2 _arg3) in /home/mmangel/Workspaces/Github/fable-compiler/Fable/fable4/src/Fable.Cli/Main.fs:line 1128
   at Fable.Cli.Main.compilationCycle@1115-6.Invoke(Tuple`2 _arg2) in /home/mmangel/Workspaces/Github/fable-compiler/Fable/fable4/src/Fable.Cli/Main.fs:line 1116
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvokeNoHijackCheck[a,b](AsyncActivation`1 ctxt, b result1, FSharpFunc`2 userCode) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 528
   at Microsoft.FSharp.Control.AsyncPrimitives.PostOrQueueWithTrampoline@932.Invoke(Unit unitVar0) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 932
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 112
--- End of stack trace from previous location ---
   at Microsoft.FSharp.Control.AsyncResult`1.Commit() in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 454
   at Microsoft.FSharp.Control.AsyncPrimitives.QueueAsyncAndWaitForResultSynchronously[a](CancellationToken token, FSharpAsync`1 computation, FSharpOption`1 timeout) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 1139
   at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronously[T](CancellationToken cancellationToken, FSharpAsync`1 computation, FSharpOption`1 timeout) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 1166
   at Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T](FSharpAsync`1 computation, FSharpOption`1 timeout, FSharpOption`1 cancellationToken) in /home/dev/Projects/fsharp/src/FSharp.Core/async.fs:line 1515
   at Fable.Cli.Entry.Run@297-3.Invoke(Unit _arg5) in /home/mmangel/Workspaces/Github/fable-compiler/Fable/fable4/src/Fable.Cli/Entry.fs:line 391
   at Fable.Cli.Entry.main@517-1.Invoke(Tuple`2 _arg2) in /home/mmangel/Workspaces/Github/fable-compiler/Fable/fable4/src/Fable.Cli/Entry.fs:line 523
   at Fable.Cli.Entry.main(String[] argv) in /home/mmangel/Workspaces/Github/fable-compiler/Fable/fable4/src/Fable.Cli/Entry.fs:line 515

To reproduce - download Oxpecker repository

1. Go to this folder https://github.com/Lanayx/Oxpecker/tree/main/examples/EmptySolid
2. Run dotnet tool update fable
3. Observe fable build fails

Basically just reiterating issue Lanayx/Oxpecker#77

[MangelMaxime]

I don't know where the problem can be coming from because the only impactful commits between the 2 version is:

ae2463b

[Lanayx]

Some crazy thoughts:

1. Stacktrace starts from FSharpAsync.RunSynchronously which was removed in 4.25 that might or might not be an issue
2. Next location has System.Exception.get_StackTrace() in the middle, so it might be that some exception occured and then another exception was thrown during stacktrace collection that overrode original one.
3. Global json change is also suspicious

[MangelMaxime]

We can eliminate the first one in theory because

1. Stacktrace starts from FSharpAsync.RunSynchronously which was removed in 4.25 that might or might not be an issue

The changes regarding RunSynchronously are Fable related, if you use RunSynchronously in your F# code then at compilation Fable is going to generate an error at compilation instead of generating something like Async.runSynchronously in your JavaScript which would fails at runtime

Here what is failing is the Fable compiler running in .NET not its compilation result

[dbrattli]

It seems that Fable 4.25.0 may have .NET 9 compatibility issues. It ships with FSharp.Core 8.0.200-dev but .NET 9 expects FSharp.Core 9.0.0.0, causing assembly loading failures.

The fix already exists in main branch (commit 418fb1f). Recommend fix is e.g. releasing Fable 4.26.0 with the F# 9.0 FCS updates backported.

I have tested this locally and can reproduce the build error with 4.25.0. The error is gone when cherry picking 418fb1f and making a new build.

What I did to test:

# In Fable
git checkout -b release/4.26.0 4.25.0
git cherry-pick 418fb1fdc
# Resolve conflicts (target framework, version)

dotnet pack src/Fable.Cli/Fable.Cli.fsproj -c Release -o ./local-packages
dotnet tool uninstall -g fable
dotnet tool install -g fable --version 4.25.0-fixed --add-source ./local-packages

# In Oxpecker
fable watch --exclude Oxpecker.Solid.FablePlugin --noCache --extension .jsx --run vite

# This works on my machine. Doing the same without the cherry-pick, fails

[MangelMaxime]

Thanks for the investigation @dbrattli

I am working on making a PR to fix the issue, this is a bit tricky because I also need to revert some improvements which were also made in that commit for Fable 5 only. But that's should be do-able :)

[MangelMaxime]

Tips: In case you want to use a local version of Fable you can also use ./build.sh package to a create a local package. It will give you a package that you can install on top of any existing Fable version no need for manual uninstall (because the version will always be higher)

Local packages created.

Use the following commands to install them:

- Fable.Cli: dotnet tool update fable --version 4.999.0-local-build-20250713-18613 --add-source /home/mmangel/Workspaces/Github/fable-compiler/Fable/fable4/temp/packages
- Fable.Core: dotnet add package Fable.Core --version 4.999.0-local-build-20250713-18613 --source /home/mmangel/Workspaces/Github/fable-compiler/Fable/fable4/temp/packages

[MangelMaxime]

@Lanayx Fable 4.26.0 should fix the issue

[Lanayx]

@MangelMaxime Unfortunately, is still doesn't work with Oxpecker.Solid plugin

Could not scan C:/Users/DavidFaivre/.nuget/packages/oxpecker.solid.fableplugin/0.7.0/lib/net6.0/Oxpecker.Solid.FablePlugin.dll for Fable plugins, skipping this assembly. Original error: The exception has been reported. This intern
al exception should now be caught at an error recovery point on the stack. Original message: The type 'AssemblyCompanyAttribute' is required here and is unavailable. You must add a reference to assembly 'System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.)

[MangelMaxime]

@Lanayx Does it work with the EmptySolid from Oxpecker repo directly?

If yes, could it be that a new release of the plugin is needed to update its dependencies ?

[Lanayx]

@MangelMaxime I reproduced the issue in clean folder downloaded from github as zip file, however didn't reproduce in a repo folder, which is pretty weird. I'll try to dig deeper tomorrow morning.

[Lanayx]

@MangelMaxime Just verified several times in clean folders that EmptySolid doesn't work neither with plugin built from sources (develop branch) nor downloaded from nuget (main branch) because of the aforementioned error. In the repo is didn't reproduce first, but after git clean -xfd it reproduced there as well.