Development tools for Zephyr on STM32Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.
That did the trick, AC6 should enable "Use active build configuration" by default i guess.
Same for the setting Window > Preferences > General > Workspace and check the "Save automatically before build".
Realy confusing if thats not enabled.