[BUG] VS Code extension sends invalid system role in messages array

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?

When using Claude Code in VS Code, requests intermittently fail with:
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 688
It looks like the VS Code integration sends a system role inside the messages array, but the backend endpoint only accepts user or assistant.

What Should Happen?

Claude Code in VS Code should send a valid request payload that does not include invalid role values in messages, and the request should complete 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 688

Steps to Reproduce

Open VS Code with Claude Code extension enabled.
Send any normal prompt in Claude Code chat/panel.
Observe occasional immediate failure with the error above.
Downgrade extension to an older version and retry.
Same workflow works after downgrade.

Claude Model

Other

Is this a regression?

Yes, this worked in a previous version

Last Working Version

anthropic.claude-code@2.1.153

Claude Code Version

2.1.156 (Claude Code)

Platform

Other

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

This appears to be VS Code integration specific.
CLI usage works normally in comparison.
Downgrading the VS Code Claude extension restores normal behavior.