[BUG] Opus subagents are being billed as Fable usage

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 used the environment variable to set my subagent model to opus4.8. I confirmed inside the ultra code workflow that every single model was opus4.8, and yet my usage shows that my total usage against my weekly limit and my fable usage are equivalent.

What Should Happen?

There should be more total usage than just Fable because I was using hundreds and hundreds of Opus sub-agents in every session.

Error Messages/Logs

Steps to Reproduce

1. Set the global sub-agent environment variable in your settings.json to Opus
2. Run a giant ultra code workflow
3. Check your usage.

Claude Model

Other

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.198 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

<img width="980" height="713" alt="Image" src="https://github.com/user-attachments/assets/bde93748-bb48-4d7b-8949-3cec9737ce14" />