You’re right, the model list for Automations in the desktop app is currently more limited than on the web. The web app uses a newer model picker that exposes the full set of options, including the fast mode toggle.
An updated picker for the desktop app is in development.
In the meantime, you can create or edit the automation on the Cursor website (Dashboard > Automations) and choose the non-fast Grok 4.6 there. The automation will run with whatever model you save, even if the desktop picker can’t select it yet.