[MODEL] Fable 5.1 defaults to using Bash on windows no matter how many times I correct it.

Preflight Checklist

• [x] I have searched existing issues for similar behavior reports
• [x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Claude ignored my instructions or configuration

What You Asked Claude to Do

My global CLAUDE.md and project CLAUDE.md both direct the agent to use Powershell and native Read/Edit/Write tools only.

What Claude Actually Did

Fable 5.1 opens every session using Bash, sed and other commands until I ask it not to. Long conversations sometimes drift back to using Bash.

Expected Behavior

Claude should detect a windows environment and use only powershell and native tools. Claude should ESPECIALLY follow CLAUDE.md instructions to use powershell and native tools.

Files Affected

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

start a new session with Fable 5.1 on Windows and monitor the tool and command line use. Every new session starts with the model using Bash until corrected.

Claude Model

Other

Relevant Conversation

Impact

Medium - Extra work to undo changes

Claude Code Version

2.1.258

Platform

Anthropic API

Additional Context

_No response_