It's likely that Fable 3 will ship initially without F# source map support (we're trying to compensate that with more readable JS output) but the infrastructure to generate them is still in place. As Fable 3 is a "pure" dotnet app, we need a dotnet library to generate the source maps but we couldn't find any that meets our needs. The easiest way is probably to translate the sourcemap generator of Mozilla's JS library to dotnet (either F# or C#). It'd be great if the community could help with that.