[BUG] Claude Code is using older models in the background

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 am on Anthropic's Pro plan and Claude Code is advertised to use Sonnet on this subscription. Nevertheless, it turns out that Sonnet is switched to Haiku in the background without notification. See screenshot from my recent session:

<img width="1246" height="249" alt="Image" src="https://github.com/user-attachments/assets/a1045f2c-f46f-4f72-97f9-75330271138c" />

What Should Happen?

Claude Code should use the model configured in /models or at least be more transparent about model switching.

Error Messages/Logs

> /pr-comments is fetching PR comments… 285 think deeply and evaluate whether they make sense
  ⎿ API Error: 400 
    {"type":"error","error":{"type":"invalid_request_error","message":"'claude-3-5-haiku-20241022' does
     not support thinking."},"request_id":"req_011CT1E5qiHVNyfW1N4fxfcd"}

> /model 
╭───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│                                                                                                                                                                               │
│  Select Model                                                                                                                                                                 │
│  Switch between Claude models. Applies to this session and future Claude Code sessions. For custom model names, specify with --model.                                         │
│                                                                                                                                                                               │
│   ❯ 1. Sonnet   Sonnet 4 for daily use ✔                                                                                                                                      │
│                                                                                                                                                                               │
│  Want Opus 4.1? Run /upgrade to upgrade to Max

Steps to Reproduce

1. Run Claude Code on Pro plan
2. Prompt it to think deeply about a problem
3. API Error surfaces saying that Haiku doesn't support thinking

Since this is happening in the background, the problem might not be constantly reproducible

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

v1.0.110

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_