[Bug] Silent model switch from Sonnet 4.5 to Sonnet 4.6 despite explicit user selection

Bug Description
Model switched silently mid-session despite explicit model selection

What happened:
I explicitly selected sonnet at the start of a session — not auto model select. Mid-session, Claude Code silently switched to sonnet[1m] without prompting or notifying me.

Error that appeared in the conversation immediately before the switch:

claude-sonnet-4-6 is temporarily unavailable, so auto mode cannot determine the safety of Bash right now. Wait briefly and then try this action again.

Expected behavior: If the selected model is temporarily unavailable, Claude Code should surface an error or wait/retry — not silently switch to a different, more expensive model.

Actual behavior: Model changed from claude-sonnet-4-6 (sonnet) to claude-sonnet-4-6[1m] mid-session without any prompt or notification, consuming more tokens than intended and overriding an explicit user choice.

Environment:

• Platform: macOS (darwin 25.5.0)
• Shell: zsh
• Claude Code CLI

Environment Info

• Platform: darwin
• Terminal: tmux
• Version: 2.1.146
• Feedback ID: 267e5e55-5440-4bb4-a257-4d3dfa737fbe

Errors

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