I have found a fix. Go to Settings --> Components in Xcode, uninstall the Codex component fully by deleting it. Then, after I waited for about 10 seconds, go in to the Intelligence menu, click Get on Codex, then sign in normally. That resolved the issue -- must've been a stale install error with Xcode updates which persisted across versions due to the Codex component remaining installed.

above solution didn't work for me, unfortunately. I filed a bug report with FB22776692

Getting the same issue, updated to MacOS 26.5 to try to fix it, but once Xcode auto-updated to 26.5, codex was unable to send prompts. I signed out to try and fix it, but then I couldn't sign back in. I tried clearing the cache, signing in and out of ChatGPT on Xcode, restarting multiple times and there is just no way to authenticate. It perpetually says 'Not Signed in', despite the auth pop-up on Safari window authenticating successfully, and going back to localhost "you can close this window now". Tested Claude agent signing in and out, as well as ChatGPT (Chat) in Xcode and they both work no problem. Was working in 26.4, no problem, without restarting the auto-update broke codex functionality. After Mac restart & update from MacOS 26.4 to 26.5, still broken (as well as updating Xcode CLI tools). Think this is a glitch on Xcode side. This seems like the inverse to the thread's problem for me -- Codex working fine in Xcode version 26.4, but broken once updated to version 26.5