[BUG] `opusplan` uses claude-sonnet-4-5 even when claude-sonnet-4-6 is available

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?

If I set opusplan as my default model in settings.json and/or set /model opusplan within a Claude session, it sets claude-sonnet-4-5@20250929.

What Should Happen?

The documentation suggests it should be setting claude-sonnet-4-6, which is what I get if I select Sonnet from the /model dropdown.

Error Messages/Logs

Steps to Reproduce

1. Run claude
2. /model opus

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.49

Platform

Google Vertex AI

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_