The RC 2 version of Xcode 26.3 does indeed distribute a new version of Codex that enables the gpt-5.3-codex model.
I would guess that's why they waited before publishing 26.3 to the App Store, and it also answers the question as to how their model update strategy might play out
@stroughtonsmith I think codex 5.3 has the new response interaction tool (prompting the user to pick a choice) that requires the agent to actually expose such tool. Might also be reason for delaying until RC2 and tool availability.
This has been discussed as part of OpenCode adoption of Codex5.3. Might be similar for Xcode
Steve Troughton-Smith
Awax