Gemini 2.5 Calls Reroute to Gemini 3 on Dec. 1 and Can Break Apps

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_signature validation
• thinking_level replaces thinking_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.