Gemini 3.1 Flash TTS returns HTTP 403 after first successful request on paid Tier 1 Prepay account

Gemini API
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1

I am using Google AI Studio Generate Speech with gemini-3.1-flash-tts-preview.

Issue

  • The project is shown as Tier 1 / Prepay.
  • The Prepay credit balance is positive.
  • Cloud Billing shows a Paid account and recorded the cost of the first successful generation.
  • The first TTS generation succeeded on August 9, 2026.
  • Every subsequent generation returns: Http response at 400 or 500 level, error: , http status code: 403
  • The problem reproduces with a one-sentence Japanese prompt and default voice/settings.
  • A newly created API key linked to the same project returns the same 403.
  • Usage is within the displayed limits: RPM 1/10, TPM 355/10K, RPD 1/100.
  • No warning or appeal banner appears in AI Studio or Google Cloud Console.
  • The API keys were created in AI Studio and are restricted to the Gemini API by default.

Expected behavior

TTS generation should continue while the paid Tier 1 project has a positive Prepay balance and usage remains within limits.

Actual behavior

All Generate Speech attempts after the first successful request fail with HTTP 403.

Google Cloud Billing Support confirmed that this technical issue is outside their scope and directed me to report it here.

Could the Google team check whether the paid project/account has been restricted by mistake or whether there is a billing/access synchronization issue? I can provide the project ID privately to authorized Google staff, but I am not posting it publicly.

2

Hi,
Can you share your project ID and Billing account ID with us via DM?

3

Hi Payal, thank you for your reply. I tried to send the requested Project ID and Billing account ID via DM, but the forum returned: “Sorry, you cannot send a personal message to that user.” Could you please enable DMs from my account or send me a DM first, so I can reply privately with the requested details?

4

Hi @Musubi, could you please share the error log with us and the exact 403 error you are getting?

5

Hi @Payal_Sharma2,

I reproduced the issue again on August 18, 2026 at approximately 09:48 UTC in Google AI Studio.

Model: gemini-3.1-flash-tts-preview
Voice: Kore
Input: A single short Japanese sentence

The exact error displayed in AI Studio is:

Http response at 400 or 500 level, error: , http status code: 403

The request fails even with this very short input. My account is still on the paid Tier 1 prepaid plan. AI Studio does not display any more detailed response body or request ID.

Please let me know if you need any additional diagnostic information or if there is a safe way to obtain the request ID without exposing the API key.