[BUG] Explore mode not working with vertex

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?

I use vertex AI, and the explore tool returns:

API Error: 404 {"error":{"code":404,"message":"Requested entity was not found.","status":"NOT_FOUND"}}

Looking at the tool it looks like this specifies model : sonnet? I have sonnet 4.5 enabled in vertex, and I can use that successfully with normal use.

What Should Happen?

It should at the very least tell you which model it tried to use when it failed and which region it tried it in? Then I'd be able to quickly figure this out on my own.

In addition I sort of suspect that this is using some alias for "sonnet" that should resolve to sonnet-4.5@something on the vertex side that isn't, but that's a theory..

Error Messages/Logs

⎿  Response:
       API Error: 404 {"error":{"code":404,"message":"Requested entity was not found.","status":"NOT_FOUND"}}


It would be *really* good if these error messages can include the model it actually asked for. Figuring this out on the GCP side is non trivial. I had a similar issue before I found ANTHROPIC_SMALL_FAST_MODEL. 

Not to mention on vertex some of these models are deployed to some regions..

Steps to Reproduce

export CLAUDE_CODE_USE_VERTEX=1
export CLOUD_ML_REGION="global"
export ANTHROPIC_MODEL='claude-opus-4-5@20251101'
export ANTHROPIC_SMALL_FAST_MODEL='claude-haiku-4-5@20251001'
export ANTHROPIC_DEFAULT_HAIKU_MODEL='claude-haiku-4-5@20251001'

run claude code and ask any question about the codebase.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.73

Platform

Google Vertex AI

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_