Free Trial credit ($300) not applied to Gemini API usage — charged from Prepay balance instead

Gemini API
1

Hello,

I recently linked a Google Cloud Billing account (with an active $300 Free Trial credit) to my project in AI Studio in order to use the Gemini API (image generation with gemini-3.1-flash-image-preview and gemini-3-pro-image-preview).

After setting up a Prepay balance (minimum $25/€25 purchase) to resolve a “prepayment credits are depleted” error, I’ve confirmed that my Gemini API usage is being deducted entirely from this $25 Prepay balance, and NOT from my $300 Free Trial credit, which remains untouched.

This was not clear to me when I linked billing — I reasonably expected the Free Trial credit to cover Gemini API usage like it does for other Google Cloud services. I’ve seen other users report this exact same issue on this forum (e.g. “Free Trial Credits Not Applied to Gemini API Usage via AI Studio”).

Could you please clarify:

  1. Is this the intended, permanent behavior, or a known bug being worked on?
  2. Is there any way to apply my remaining Free Trial credit toward Gemini API costs?
  3. If not, could this limitation be documented more clearly at the point where users link a billing account in AI Studio, to avoid this confusion for others?
2

It’s intended and even documented in Google cloud side ( I know I’ve seen it )

You have to use agent platform (vertex ai) if you want to spend the $300