Gemini 3 Flash fails to call UnityMCP tools in VSCode Copilot Chat agent mode (HTTP, request body error)

[Labrador52]

Hi,
I encountered a problem when using VSCode Copilot Chat (agent mode) with the Gemini 3 Flash model to call UnityMCP tools.

Environment:

Editor: VSCode
Copilot Chat agent mode
Model: Gemini 3 Flash (default settings)
UnityMCP: Installed and running, HTTP connection
OS: Windows
Steps to reproduce:

Start UnityMCP server (HTTP mode).
In VSCode Copilot Chat, select Gemini 3 Flash as the model.
Ask the agent to start Unity editor play mode (e.g., "Start play mode" or similar).
The request fails with the following error:
Notes:

The same operation works correctly with GPT-4.1 model.
No custom model settings, just default Gemini 3 Flash.
The goal was simply to let the agent trigger play mode in Unity.
Expected:
Gemini 3 Flash should be able to call UnityMCP tools just like GPT-4.1.

Actual:
Request fails with 400 invalid request body.

I noticed other users have reported similar issues, so I am submitting this issue to provide another example for reference.

[Labrador52]

I have tried running MCP multiple times with GPT-4.1, and the success rate is 100%. I haven't tested other models yet, so for my current needs, switching to GPT-4.1 is just a minor inconvenience. However, I plan to use UnityMCP much more in the near future, as I find it extremely powerful and it greatly reduces the complexity of my workflow.

[dsarno]

Is this potentially just a problem with Gemini's servers? I can't get any Gemini models to do anything right now.

I find it extremely powerful and it greatly reduces the complexity of my workflow.

Awesome!!

[dsarno]

Yeah I just did a bunch of Gemini 3 Flash commands through (Google's) Antigravity, and they work fine -- but those same calls fail in Cursor and Rider / Copilot.

I don't think this is related to the mcp package or networking.

[Labrador52]

Since UnityMCP isn't causing this, I'll look into my VSCode Copilot settings as the potential cause. Much appreciated!

[msanatan]

Please check, I tested the last release with Antigravity and Gemin 3 Flash, and it's probably my default model for Unity MCP. But share your findings! It's not the first report, and we can't replicate. On my end, I'm almost always impressed by Gemini 3 flash lol