[BUG] Claude Code ignores model setting and always tries Opus 4.8 first on Vertex AI

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 2.1.209 with Google Vertex AI as the provider, Claude Code always
attempts to use Opus 4.8 first, even when the model is explicitly set to
claude-opus-4-6. Opus 4.8 is not available on Vertex AI, so it fails and falls
back to 4.6 with the message:

> Opus: Opus 4.8 not available — using Opus 4.6 for this session

What Should Happen?

Claude Code should use claude-opus-4-6 directly without attempting Opus 4.8.

Error Messages/Logs

Opus: Opus 4.8 not available — using Opus 4.6 for this session

Steps to Reproduce

1. Configure Claude Code with Vertex AI in ~/.claude/settings.json
2. Set the model to claude-opus-4-6 via any of:

• "model": "claude-opus-4-6" in settings.json
• export CLAUDE_MODEL=claude-opus-4-6
• claude --model claude-opus-4-6

1. Launch Claude Code

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.209

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_