Gemini Bad Request with Github Copilot

[ngt-sg]

Description

No matter which Gemini model I use with Github Copilot authentication I get a Bad Request: Bad Request error

It works for a colleague with the same Github Copilot license from our company.

I can prompt Gemini from the Copilot CLI

EDIT:
Found the issue. Gemini models fail with the Docker Gateway MCP with the Linear plugin enabled

Plugins

Tried both with and without "opencode-openai-codex-auth" installed

OpenCode version

1.1.19

Steps to reproduce

1. Authenticate with Github Copilot
2. Have Docker MCP gateway with Linear plugin enabled
3. Select any Gemini model
4. Write any prompt
5. Receive Bad Request: Bad Request error

Screenshot and/or share link

Operating System

macOS 26.1

Terminal

Ghostty and macOS Terminal

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Bug: 'Invalid request body' with github-copilot/gemini-3-pro-preview model #5161: Invalid request body with github-copilot/gemini-3-pro-preview - same Bad Request error pattern
• Could not use model "Gemini 3 Pro Preview" of Github copilot provider: "invalid request body" #4816: Bad request with Gemini 3 Pro Preview through Github Copilot
• github copilot: Gemini 3.0 invalid body request #4468: Gemini 3.0 invalid body request with Copilot (longstanding issue from v1.0.76)
• Opencode Github Agent with GitHub Copilot provider Throw AI_APICallError: Bad Request #3287: Bad Request error with GitHub Copilot provider (broader pattern)
• Copilot models respond with AI_APICallError: Bad Request after a quick chat #1282: Copilot models respond with Bad Request after use (affects Gemini and other models)

The root cause appears to be related to request body transformation when using Gemini models with Copilot authentication. Issues #5161 and #4816 are particularly similar to your case.

Feel free to ignore if none of these address your specific case.

[ngt-sg]

I found configuration causing the problem. I updated the description.

In my case it was the Docker Gateway MCP with the Linear plugin enabled. It might be the Linear MCP by itself. It works fine with the other plugins I have in the Docker MCP toolkit. But removing the Linear plugin fixed the issue on my end.

[smerrill]

I have the same issue with chrome-devtools-mcp@latest.

[smerrill]

Although interestingly starting that MCP with --no-category-emulation lets it work with GitHub-hosted Gemini 3 models. Very odd.

[NateSmyth]

@smerrill

Gemini models require nullable types to use OpenAPI 3.0 nullable keyword instead of JSON Schema union types

(from vscode copilot extension)

Had the same issue with chrome_devtools and this ended up being the reason.
--no-category-emulation disables emulate which is the only tool with nullable types

[fujunchao]

I'm having the same problem with Chrome DevTools，After configuring the same parameters, the problem is solved with --no-category-emulation。

[rekram1-node]

ah yeah ig we need to sanitize the tools more aggressively for gemini

[smerrill]

The strange thing is that this is only a problem for Copilot Gemini 3, not directly via the gemini-auth plugin.