[BUG] VS Code sidebar chat: API Error 400 - messages[1].role unknown variant system

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The VS Code sidebar chat panel always returns API Error 400. When I send any message in the sidebar chat, I get: "Failed to deserialize the JSON body into the target type: messages[1].role: unknown variant system, expected user or assistant"

The terminal claude command works fine with the same subscription account.

What Should Happen?

The sidebar chat panel should send messages and receive responses normally.

Error Messages/Logs

API Error: 400 Failed to deserialize the JSON body into the target type: messages[1].role: unknown variant `system`, expected `user` or `assistant` at line 1 column 697

Steps to Reproduce

1. Install Claude Code VS Code extension (latest version)
2. Login with Claude.ai subscription via OAuth
3. Open the sidebar chat panel
4. Type any message (e.g. "你好") and press Enter
5. Error appears immediately: API Error 400 messages[1].role unknown variant system

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.143

Platform

Other

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_