Understanding Gemini API Billing: What Happens to Free Usage?

As I explore the Gemini API, I’m contemplating the implications of enabling billing, particularly since the free tier no longer grants access to professional models.

This brings me to a few important inquiries regarding the billing system:

1. Does Enabling the Paid Tier Affect Free Usage?
   Once I switch to the paid tier, is there any allowance for free usage? For instance, will I continue to have access to features within the limits of the free tier without incurring additional charges, or will everything become billable the moment I activate my billing account?

Navigating these questions is crucial for planning my usage effectively and managing costs associated with the Gemini API. Any insights from fellow users or official guidelines would be greatly appreciated!