[BUG] Default Sonnet Model in Claude Code v.2.0.27 with VertexAI is wrong and failing all requests after upgrading from earlier versions

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?

After upgrading to latest version (2.0.27) all our users are having failures due to the following error:
⎿  API Error: 404 {"error":{"code":404,"message":"Publisher Model projects/claude-code/locations/us-east5/publishers/anthropic/models/claude-sonnet-4-5-20250929 not found.","status":"NOT_FOUND"}}

The workaround was to push everyone with a config file property of "ANTHROPIC_DEFAULT_SONNET_MODEL": "claude-sonnet-4-5@20250929"

Seems like the default model name in the new 2.0.27 version is wrong (claude-sonnet-4-5-20250929 instead of claude-sonnet-4-5@20250929).

What Should Happen?

Change the default model to claude-sonnet-4-5@20250929

Error Messages/Logs

API Error: 404 {"error":{"code":404,"message":"Publisher Model `projects/claude-code/locations/us-east5/publishers/anthropic/models/claude-sonnet-4-5-20250929` not found.","status":"NOT_FOUND"}}

Steps to Reproduce

Upgrade to latest version and make sure you don't have any "ANTHROPIC_DEFAULT_SONNET_MODEL" set in your env / config files.
The issue should reproduce.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.0.2

Claude Code Version

2.0.27 (Claude Code)

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_