[BUG] Claude uses Opus instead of Sonnet4.5 for the first 25% on the Sonnet4.5 setting

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?

When using Claude Code, it uses Opus instead of Sonnet4.5 for the first 25% of usage, even when the Sonnet4.5 (base recommended) setting is on.

This is evidenced when after 25% of usage, it says "25% usage up, switching to Sonnet4.5".

This :

1. Means there is no way to use Sonnet 4.5 in Claude code without first exhausting 25% of the usage.
2. May help explain the recent degraded model performance of the last 2 weeks (including poor effort and instruction following).

What Should Happen?

Claude should use Sonnet 4.5 only, and the legacy Hybrid mode should not be incorrectly activated.

This should be implemented ASAP as a priority fix.

Error Messages/Logs

Steps to Reproduce

1. Use latest version of Claude Code
2. Choose recommended version (Sonnet 4.5)
3. Use up 25% of usage, with poor model performance.
4. Receive message that 'due to 25% usage completion, model is reverting back to Sonnet4.5".
5. Enjoy great model performance.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.37

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_