[TUI] /model in Plan mode updates Default globally and changes Medium to High

What version of Codex CLI is running?

codex-cli 0.146.0

What platform are you using?

Windows, Codex CLI TUI.

What issue are you seeing?

Changing the model with /model while Plan mode is active does not preserve separate Default- and Plan-mode settings.

Starting state:

• Default mode: gpt-5.6-terra, Medium
• Plan mode entered with Shift+Tab

After running /model in Plan mode and selecting gpt-5.6-sol, Medium, the resulting state is:

• Default mode: gpt-5.6-sol, Medium
• Plan mode: gpt-5.6-sol, High

The requested Sol/Medium selection is applied to Default mode, while Plan mode retains or introduces a High reasoning override. This makes it impossible through the observed flow to configure Default as Terra/Medium and Plan as Sol/Medium.

Steps to reproduce

1. Start a Codex CLI session in Default mode using gpt-5.6-terra with Medium reasoning.
2. Press Shift+Tab to enter Plan mode.
3. Run /model while Plan mode is active.
4. Select gpt-5.6-sol with Medium reasoning.
5. Cycle between Default and Plan modes and inspect the model/reasoning shown for each.

Expected behavior

Either:

• the selection made while Plan mode is active is saved as the Plan-mode override, leaving Default mode at Terra/Medium; or
• the UI clearly asks whether the selection applies to the Plan override or the global Default setting, and respects that choice.

Expected final state for this reproduction:

• Default mode: Terra/Medium
• Plan mode: Sol/Medium

Actual behavior

• Default mode changes to Sol/Medium.
• Plan mode becomes Sol/High.

Additional context

Related issue #14379 states that /model in Plan mode should offer an “Apply to Plan mode override” path. The behavior above suggests that the scope is ambiguous, the Plan override is not being saved, or a separate Plan reasoning override is unexpectedly replacing Medium with High.

Reproduce the flow in the Windows Codex CLI TUI: enter Plan mode, run /model, choose gpt-5.6-sol with Medium reasoning, and switch modes to inspect the resulting settings. Trace the /model handling and mode-specific model and reasoning state; done means Default remains Terra/Medium while Plan uses Sol/Medium, or the UI clearly applies the chosen scope.