[BUG] [Google Vertex] Opus 4.6 model name is not valid for the count_tokens API in Google 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?

Claude Code sends an incompatible model ID to Google Vertex's count-tokens API for Opus 4.6, causing a 400 error.

The model name needs to be claude-opus-4-6 (which does NOT match old Vertex naming schemes), but Claude code sends claude-opus-4-6@default (which does match old Vertex naming schemes).

I know this is more a problem with Vertex, but thought I'd raise here as well.

What Should Happen?

Claude Code should use model names that work with Google Vertex.

Error Messages/Logs

Steps to Reproduce

  TOKEN=$(gcloud auth print-access-token)
  BASE="https://MY_REGION-aiplatform.googleapis.com/v1/projects/MY_PROJECT/locations/MY_REGION/publishers/anthropic/models"

  # Works: model with date tag
  curl -s "$BASE/count-tokens:rawPredict" \
    -H "Authorization: Bearer $TOKEN" \
    -H "Content-Type: application/json" \
    -d '{"model":"claude-opus-4-5@20251101","messages":[{"role":"user","content":"hello"}]}'
  # => {"input_tokens":8}

  # Works: model without any tag
  curl -s "$BASE/count-tokens:rawPredict" \
    -H "Authorization: Bearer $TOKEN" \
    -H "Content-Type: application/json" \
    -d '{"model":"claude-opus-4-6","messages":[{"role":"user","content":"hello"}]}'
  # => {"input_tokens":8}

  # FAILS: model with @default tag
  curl -s "$BASE/count-tokens:rawPredict" \
    -H "Authorization: Bearer $TOKEN" \
    -H "Content-Type: application/json" \
    -d '{"model":"claude-opus-4-6@default","messages":[{"role":"user","content":"hello"}]}'
  # => 400 "claude-opus-4-6@default is not supported"

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.37

Platform

Google Vertex AI

Operating System

Other Linux

Terminal/Shell

iTerm2

Additional Information

_No response_