Claude model in Agent mode completely non-functional in VS Code Copilot Chat - fails on all requests including simple messages

When using Claude as the selected model in Agent mode in GitHub Copilot Chat (VS Code extension), requests that trigger tool calls fail with a 400 error indicating invalid JSON format in tool call arguments. Ask mode and other models both in ask and agent modes (GPT-5, Gemini 2.5) work without issues on identical requests.
```
`Sorry, your request failed. Please try again.

Copilot Request id: 0a0a4e31-bc20-4290-982c-db7fea498b24

GH Request Id: C251:15F35E:532F5C6:5FEEC0E:6931EDD2

Reason: Request Failed: 400 {"error":{"message":"Invalid JSON format in tool call arguments","code":"invalid_tool_call_format"}}
````

tried to sign off - sign in, checked the license ( business Copilot seat), reinstalled copilot extension, updated vs code - nothing seems to help.

報告提到的是 VS Code Copilot Chat 擴充功能，而不是檔案、測試或 copilot.vim 進入點。首先確認此儲存庫是否負責發生故障的 Agent-mode 路徑，並使用 Claude 重現 400 invalid_tool_call_format 錯誤；只有在確定了一個屬於範圍內的元件，以及回歸測試或有文件記錄的 upstream 交接之後，才算完成。

由索引模型根據 Issue 內容生成。