Change the model string and drop thinking_budget and sampling parameters before Dec. 1, or Gemini 2.5 requests get rerouted to Gemini 3 models.
Any app that still calls gemini-2.5-pro or gemini-2.5-flash through the Gemini Developer API will have those requests rerouted to Gemini 3 models on Dec. 1, with different billing and request rules. Google announced the change in its Nov. 2 Gemini API release notes (Gemini API changelog, Nov. 2, 2026).
What Google Said #
Both models are deprecated and no longer selectable in Google AI Studio. “Effective December 1, 2026, all API requests to gemini-2.5-pro will automatically route to” gemini-3.8-flash, Google said, and billing follows Gemini 3.8 Flash rates. Requests to gemini-2.5-flash route to gemini-3.5-flash-lite at Flash-Lite rates.
Google listed four breaking changes for both models:
- Function calling and
thought_signaturevalidation thinking_levelreplacesthinking_budget- Sampling parameters are removed
- Prefilled model turns are removed
Google’s migration advice: change the model string before Dec. 1, test function calling and thought_signature handling, swap thinking_budget for thinking_level, and strip sampling parameters and prefilled turns. It linked a migration guide for each target model.
Firebase repeated the notice in its Nov. 2 release notes, and said some settings will return 400 errors after the switch.
The Take #
A silent reroute is a worse deal than a shutdown. A shutdown errors loudly on day one. A reroute keeps returning 200s from a different model with different pricing, and requests that use a setting the new model rejects fail with a 400.
Firebase AI Logic apps that name a 2.5 model in client code are the likeliest to get caught. A shipped mobile build keeps its hardcoded model string until users update. Server code and Genkit flows change in one deploy.
Grep every repo for gemini-2.5 and for thinking_budget, temperature, top_p and top_k sent to those models. Ship the new model string in a test build now, not on Nov. 30. For mobile apps, the update lag means the work should start this week.