[Bug]: Gemini models via Github Copilot return 400 when prompted

[JsSusenka]

Bug Description

Good morning! Since the pricing change yesterday when GitHub migrated to usage-based pricing, I've wanted to try some cheaper models. Unfortunately, all Gemini models seem to return a 400 Bad Request error when prompted. I would like to note that I am currently using Anthropic models without any problems. The prompt only fails when using Gemini models.

Steps to Reproduce

1. Select Gemini 3.1-pro-preview as a model
2. Prompt the model

Expected Behavior

The prompt should function as it does with other models.

Actual Behavior

The prompt fails with a 400 Bad Request error.

Forge Version

2.13.4

Operating System & Version

macOS 26.5

AI Provider

Other

Model

gemini-3.1-pro-preview, gemini-3-flash-preview, gemini-3.5-flash

Installation Method

curl -fsSL https://forgecode.dev/cli | sh

Configuration