@hub It used the Vala language server from the @vala_lang Flatpak Sdk. Which works perfectly if you have it installed. Unfortunately most people don't, and therefore Workbench did not work correctly.

And because you cannot make a Flatpak app depend on an extension, so it gets installed automatically, the best option we took is to bundle the language server inside Workbench, so the Sdk is not required.

Enjoy the diagnostics! :)