Support per-TUI instance model selection

What variant of Codex are you using?

CLI

What feature would you like to see?

Currently, the selected model appears to be reloaded between sessions, so changing the model in one TUI instance can affect new sessions created in another already-running TUI instance.

I do not think this behavior is useful. A TUI instance should read config.toml when it starts, then keep its model selection local for the lifetime of that TUI. New sessions inside the same TUI should use that TUI’s current model, not a model changed elsewhere.

A possible implementation would be an opt-in mode such as codex --local, where model changes stay local to that TUI instance and are not written back to the global config.

This would allow multiple TUI instances to use different models independently, for example, one terminal using 5.4-mini for lighter tasks and another using 5.5 for more complex work.

Additional information

similar issue: #14540