[BUG] Per-session isolation for effort level and model selection

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 separate terminal tabs, changing the effort level or model in one tab immediately affects all other open sessions. This is because both settings are stored globally in ~/.claude.json, which is shared across all instances. It's impossible to run simultaneous sessions with different configurations — e.g., one tab on Sonnet with low effort for quick lookups and another on Opus with max effort for a complex task. The current workflow requires manually re-setting effort/model every time you switch tabs.

What Should Happen?

Effort level and model selection should be stored per-session (in-memory) rather than written back to ~/.claude.json on every change. The global/settings.json value should serve as the default when a new session starts, but changes made within a session should not persist globally or affect other running sessions. Per-project defaults via .claude/settings.json would also be a useful complement.

Error Messages/Logs

Steps to Reproduce

1. Open two terminal tabs, each running claude in any directory.
2. In Tab 1, change the effort level (e.g., via /model → set effort to Max).
3. Switch to Tab 2 — observe that the effort level has changed there as well, without any action taken in that tab.
4. Repeat with model selection: change the model in Tab 1, observe it changes in Tab 2.

Note: The bug affects any two concurrently running Claude Code sessions. Both effort level and model selection appear to be stored globally in ~/.claude.json and written on every change, so any session mutation is immediately visible to all other sessions.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.116

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_