Gemini API HTTP 400 FAILED_PRECONDITION with multiple API keys

Gemini API
1

Hello,

I am experiencing persistent HTTP 400 errors when using the Gemini API.

The API worked successfully until October 9, 2026, but has consistently failed since then.

Environment:

  • Google Cloud project: Free Tier
  • Model: gemini-3.5-flash-lite
  • Gemini API: Enabled
  • Authorization API Key: Active
  • Bound service account: Active
  • Application restrictions: None

Diagnostic results:

  1. Interactions API v1
    HTTP 400 / invalid_request

  2. Interactions API v1beta
    HTTP 400 / invalid_request

  3. Models API v1beta
    HTTP 400 / FAILED_PRECONDITION

  4. GenerateContent API v1beta
    HTTP 400 / FAILED_PRECONDITION

I also created a second Authorization API Key in the same Google Cloud project, bound to a different service account.

Both API keys return HTTP 400 / FAILED_PRECONDITION when calling the minimal GenerateContent API request.

However, Gemini 3.5 Flash-Lite works successfully in Google AI Studio Playground.

Additional information:

  • The project is on the Free Tier.
  • Billing is not enabled.
  • The Gemini API is enabled.
  • Both service accounts are active.
  • No application restrictions are configured.
  • Rate limits do not appear to be exceeded.

Could someone help identify which project-level or account-level prerequisite might be causing these errors?

Is there a known issue affecting Authorization API Keys or Free Tier projects?

Thank you.