Status line model display syncs across parallel sessions instead of being per-session

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 running multiple Claude Code sessions in parallel terminals, changing the model via /model in one session updates the status line model display in other sessions as well. Each session should maintain and display its own independently selected model.

Steps to reproduce:

1. Open two terminal windows
2. Start claude in both
3. In terminal A, run /model and select Haiku
4. In terminal B, run /model and select Opus
5. Observe the status line in both terminals — both show the same model instead of their respective selections

Expected behavior:

Terminal A status line shows "Haiku 4.5" and terminal B shows "Opus 4.6". Model selection is per-session.

Observed behavior:

Switching the model in one terminal causes the status line in the other terminal to reflect the newly selected model as well.

Investigation:

The custom status line script reads model data exclusively from stdin (JSON piped by Claude Code per invocation). The script does not read any shared config or file. This points to the issue being upstream — either the model state is stored/read globally rather than per-session, or status line refresh events broadcast shared state.

Environment:

• OS: Linux (Debian, kernel 6.12)
• Shell: bash
• Status line: custom script via settings.json → statusLine.command