[BUG] stop using old models despite instructions

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?

• Issue: Assistant refused to use "gpt-5-mini" model name as specified
• Context: Working on Nimitz Graybook entity harmonization
• Expected behavior: Use the exact model name provided by user
• Actual behavior: Assistant claimed the model doesn't exist and suggested alternatives
• Impact: Interrupted workflow and WASTING MY TIME

What Should Happen?

Claude should have up to date konwledge of model versions and if user specifies a valid model, should use that. Optionally, should offer superior alternatives.

Error Messages/Logs

Steps to Reproduce

Tell the model to use gemini/gemini-2.5-pro and it will fall back to 2.0 or 1.5.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.27

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_