Fable 4 + .NET 10 SDK: "Project and references (0 source files) parsed in ..."

[cr3wdayt5p]

Hi

I just installed the new .NET SDK version: 10.0.100

But this appears to break the compilation of my Fable 4 project. It finds no source files and gives the following success message:

Build succeeded in 0.5s
Project and references (0 source files) parsed in 4586ms

(but clearly it did something behind the scenes since it took 4+ seconds)

If I pin the SDK to version 9.0.305 then everything works fine.

Question: Is Fable 4 not compatible with .NET 10 SDK?
I have tested with both version 4.24 and 4.25. Same result.

I have also tested with Fable 5 version 5.0.0-alpha.14. This version does not have the issue. It finds all the source files, and start the compilation as expected (but in my case it fails due to some of the new null safety features).

The project in question has a .NET 8 TFM:

<TargetFramework>net8.0</TargetFramework>

But bumping this to either net9.0 or net10.0 does not change the result (still 0 source files).

The project builds without issues with dotnet build.

Workaround

Pin the SDK to version to some .NET 9 version with global.json, e.g.:

{
    "sdk": {
        "version": "9.0.305",
        "rollForward": "latestFeature"
    }
}

Machine config

• OS: macOS 26.1
• .NET SDK versions: 9.0.305 and 10.0.100
• Fable versions: 4.24, 4.25 and 5.0.0-alpha.14.

[MangelMaxime]

Hello,

Can you please try to run Fable 4 with --test:MSBuildCracker flag?

but in my case it fails due to some of the new null safety features)

If you have time, we would be interested in learning more about this (in a new discussion or issue if possible to keep things organized). Is it related to F# directly or to how Fable handle it?

[cr3wdayt5p]

Can you please try to run Fable 4 with --test:MSBuildCracker flag?

Yes, thanks, the --test:MSBuildCracker flag mostly solves the issue.
It works as expected for our release build 👍

But for our debug build it partly fails. It appears the new cracker does not run with the FABLE_COMPILER flag set to true.
We have this in our .fsproj file:

<ItemGroup Condition="'$(Configuration)'=='Debug' and '$(FABLE_COMPILER)'!='true'">
    <!-- Here we reference Expecto, etc. for testing with `dotnet test` in CI. -->
</itemGroup>

This used to work fine with the old cracker and .NET 8 and 9 SDKs.

but in my case it fails due to some of the new null safety features)

If you have time, we would be interested in learning more about this (in a new discussion or issue if possible to keep things organized). Is it related to F# directly or to how Fable handle it?

Yes, I will add this to my todo list to investigate further. But a quick look hinted that it might be related to using let! in the option CE from FsToolkit.ErrorHandling.


Update: It is related FsToolkit.ErrorHandling. We used an old version (4.18.0) but bumping it to the latest 5.1.0 fixed the issues. But still somewhat strange that the code (with FsToolkit.ErrorHandling, 4.18.0) compiled fine with dotnet build but not with Fable 5.0.0-alpha.14.

[cr3wdayt5p]

I can see that the new MSBuildCracker sets /p:Fable=True (https://github.com/fable-compiler/Fable/blob/main/src/Fable.Compiler/MSBuildCrackerResolver.fs#L112).

So if I update the conditions in my project files from:

<ItemGroup Condition="'$(Configuration)'=='Debug' and '$(FABLE_COMPILER)'!='true'">

to:

<ItemGroup Condition="'$(Configuration)'=='Debug' and '$(FABLE_COMPILER)'!='true' and '$(Fable)'!='True'">

then my debug builds works as well.

Maybe we should consider adding /p:FABLE_COMPILER=true to the new cracker to match the behavior of the old cracker? (simpler upgrade for others who have conditionals in their project files based on the old properties)

[MangelMaxime]

Maybe we should consider adding /p:FABLE_COMPILER=true to the new cracker to match the behavior of the old cracker? (simpler upgrade for others who have conditionals in their project files based on the old properties)

This is indeed an error it should be FABLE_COMPILER

[MangelMaxime]

@cr3wdayt5p Fable 4.27.0 should fix the MSBuid condition.

You should be able to use FABLE_COMPILER now

[cr3wdayt5p]

Thanks, it indeed works with 4.27.0 and --test:MSBuildCracker.

But how about the original question?

Is Fable 4 not compatible with .NET 10 SDK?

(i.e. with the legacy cracker)

Should Fable 4 default to --test:MSBuildCracker on .NET 10 SDK?

Or should it display an error message that it is not compatible .NET 10 SDK (unless --test:MSBuildCracker is used)?

Both would be better than "silently" failing with "Project and references (0 source files) parsed [...]" 😊

Or should this issue just continue to be open until search engines (and AI agents) have indexed it?

[halcwb]

Same problem here and struggling to get the command syntax right. This is the original command to compile and run:

dotnet fable --test:MSBuildCracker watch -o output -s -e .jsx --run npx vite

So, I tried adding the --test:MSBuildCracker flag but without success.

[MangelMaxime]

dotnet fable watch --test:MSBuildCracker -o output -s -e .jsx --run npx vite

I think you need to move the flag to after the watch command

[MangelMaxime]

Should Fable 4 default to --test:MSBuildCracker on .NET 10 SDK?

Fable 5, already use the MSBuild cracker by default and we should change the cracker only on a major version in theory because it is could be a breaking change. It does things a little differently.

I still need to look into why the Buildalyzer has troubles with .NET 10.

[MangelMaxime]

After looking into Buildalyzer project our issue seems related to:

Buildalyzer/Buildalyzer#319

It has been fixed 1 month ago but not released.

I think to unblock ourself, I will look into building Buildalyzer ourself and use it from a local source folder waiting for them to release the fix.

I am hesitant to defaulting to MSBuildCracker because some people reported some hiccups when testing the first version of Fable 5.

[halcwb]

dotnet fable watch --test:MSBuildCracker -o output -s -e .jsx --run npx vite

I think you need to move the flag to after the watch command

I tried that at first, but that didn't work.

This is what I get:

dotnet fable watch --test:MSBuildCracker -o output -s -e .jsx --run npx vite
Fable 4.27.0: F# to JavaScript compiler
Minimum @fable-org/fable-library-js version (when installed from npm): 1.11.0

Thanks to the contributor! @ctaggart
Stand with Ukraine! https://standwithukraine.com.ua/

Parsing Client.fsproj...
Retrieving project options from cache, in case of issues run dotnet fable clean or try --noCache option.
Project and references (0 source files) parsed in 118ms

And then just nothing.

[MangelMaxime]

This is strange only the position of watch should matters:

Fable/src/Fable.Cli/Entry.fs

Lines 555 to 578 in c16b716

	match commands with
	| [ "--help" ] -> return printHelp ()
	| [ "--version" ] -> return Log.always Literals.VERSION
	| [ "clean"; dir ] -> return clean args language dir
	| [ "clean" ] -> return clean args language rootDir
	| [ "watch"; path ] ->
	return! Runner.Run(args, language, rootDir, runProc, verbosity, fsprojPath = path, watch = true)
	| [ "watch" ] -> return! Runner.Run(args, language, rootDir, runProc, verbosity, watch = true)
	| [ "precompile"; path ] ->
	return! Runner.Run(args, language, rootDir, runProc, verbosity, fsprojPath = path, precompile = true)
	| [ "precompile" ] -> return! Runner.Run(args, language, rootDir, runProc, verbosity, precompile = true)
	| [ path ] ->
	return!
	Runner.Run(
	args,
	language,
	rootDir,
	runProc,
	verbosity,
	fsprojPath = path,
	watch = args.FlagEnabled("--watch")
	)
	| [] -> return! Runner.Run(args, language, rootDir, runProc, verbosity, watch = args.FlagEnabled("--watch"))
	| _ -> return! Error "Unexpected arguments. Use `fable --help` to see available options."

The flag is position independent:

Fable/src/Fable.Cli/Entry.fs

Line 271 in c16b716

let useMSBuildForCracking = args.FlagOr("--test:MSBuildCracker", false)

[cr3wdayt5p]

@halcwb I don't think there is anything wrong with the command.
I copied the command and ran it on my project without any issues:

dotnet fable watch --test:MSBuildCracker -o output -s -e .jsx --run npx vite

It finds all project files and compiles them.

Machine Info

• OS: macOS 26.1
• .NET SDK: 8.0.416
• Fable: 4.27.0

I would suggest clearing out the output folder manually (delete it), and maybe running dotnet clean (or manually deleting the artifact folder(s)).