**Title:** GitHub Copilot App BYOK fails with OpenAI-compatible endpoint whil...

**Title:** GitHub Copilot App BYOK fails with OpenAI-compatible endpoint while VS Code works

I'm using an OpenAI-compatible BYOK endpoint from OpenCode Go.

The same endpoint, API key, and model work normally when configured in VS Code, but fail when configured in the GitHub Copilot App.

The Copilot App returns HTTP 400 errors such as:

```text
Failed to deserialize the JSON body into the target type:
messages[1]: unknown variant `image_url`, expected `text`
```

and:

```text
invalid_request_error:
Extra inputs are not permitted,
field: `messages[2].refusal`,
value: None
```

Steps to reproduce:

1. Configure the OpenCode Go OpenAI-compatible endpoint in VS Code.
2. Use the model normally — requests succeed.
3. Configure the same endpoint, API key, and model in GitHub Copilot App using BYOK.
4. Send a request.
5. GitHub Copilot App returns HTTP 400.

Expected behavior:

The same OpenAI-compatible endpoint should work in GitHub Copilot App as it does in VS Code.

Actual behavior:

Requests from GitHub Copilot App contain fields/message formats that the endpoint rejects, while VS Code requests succeed.

I'm not sure whether this is caused by the Copilot App request payload or an incompatibility in the endpoint, but the issue is reproducible specifically with GitHub Copilot App.

---
| Field | Value |
| --- | --- |
| App version | 1.1.10 |
| OS | Windows 10.0.26300 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 9 |

从 /chat BYOK 请求路径开始，将 GitHub Copilot App 的 payload 与针对相同 endpoint、密钥和模型的正常 VS Code 请求进行比较。重现 HTTP 400 响应，并确定 app 是否发送了 image_url 和 refusal 字段；当 endpoint 能够接受 app 的请求且不再出现这些反序列化错误时，即表示完成。

由索引模型根据 Issue 内容生成。