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

> Source: <https://firerun.io/gemini-2-5-reroute-dec-1-firebase-apps-2026/>
> Published: 2026-11-07 00:00:00+00:00

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

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](https://aistudio.google.com/docs/changelog)).

## 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.