Codex adds device toolbar for responsive testing

This is exactly the kind of boring-looking feature that matters: it turns Codex from a code generator into a more credible build-test-iterate environment for UI work.

• –Responsive testing is one of the biggest pain points in agent-assisted frontend development, so a device toolbar removes a lot of manual context switching.
• –The feature makes Codex more useful for reproducing visual bugs, checking breakpoints, and validating component behavior across screen sizes.
• –It also pushes Codex closer to a true end-to-end dev loop, where the agent can write code and immediately inspect how it renders.
• –For teams building consumer web apps, this is more practical than flashy: it saves time on the last mile of QA, where AI tools usually fall short.