[BUG] Context window uses more than available somehow?

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?

Using standard context window (NOT 1M). And it seems that claude code is somehow ignoring the non 1M window, but still using it for calculations?

version: 2.1.159

<img width="694" height="348" alt="Image" src="https://github.com/user-attachments/assets/cf076705-bf79-4d10-8546-2f3fbc363a3b" />

What Should Happen?

Context should auto-compact before it hots 100%

Error Messages/Logs

Steps to Reproduce

Run claude with CLAUDE_CODE_DISABLE_1M_CONTEXT, and use opus 4.8, use the session until it gets to 100% size.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.159

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

Other

Additional Information

_No response_