App: expose mobile device emulation in the in-app browser DevTools

What version of Codex is running?

26.415.40636

What platform is your computer?

macOS 26.3.1 (arm64)

What problem are you seeing?

The desktop app's in-app browser can open an inspect/DevTools panel, but unlike Chrome DevTools there doesn't appear to be any supported way to switch into mobile device emulation / responsive mode.

I could not find:

• a "Toggle Device Toolbar" button in the DevTools UI
• - a documented menu item or shortcut in the Codex app
• - - any browser UI action for viewport presets or device emulation

This makes it hard to debug responsive/mobile-only layout issues from inside Codex, even though the in-app browser is otherwise very useful for frontend review.

Steps to reproduce

1. Open a thread in Codex desktop.
2. 2. Open the in-app browser and navigate to a page.
3. 3. Open the inspect panel / DevTools from the browser.
4. 4. Look for a mobile device toggle (Chrome's "Toggle Device Toolbar") or any viewport preset control.

Expected behavior

There should be a supported way to test responsive/mobile layouts from the in-app browser, for example:

• full DevTools device emulation / responsive mode
• - a toolbar button like Chrome's "Toggle Device Toolbar"
• - - viewport presets (iPhone, Pixel, iPad, etc.)
• - - - or at minimum manual width/height + DPR + user-agent override

Actual behavior

Inspect works, but I only see desktop-style DevTools with no obvious or documented mobile emulation entry point.

Why this matters

A lot of Codex app browser workflows involve frontend review and iteration. Being able to switch to a mobile viewport without leaving the app would make the in-app browser much more useful for responsive debugging.

Additional context

If full device emulation is not planned yet, it would still help to document that explicitly and/or expose a simpler responsive viewport mode as an interim step.