Xcode/Codex - Your request couldn't be completed

I’m experiencing the same authentication issue with Codex in Xcode and wanted to share my setup and a temporary workaround that works for me.

Account

• ChatGPT Pro (recently upgraded from Plus to Pro)

Environment

• macOS 26.5.1 (25F80)

• Xcode 26.5 (17F42)

• Xcode automatically installs Codex 0.129.0-alpha.9 from Settings

Issue

In Xcode → Settings → Intelligence → Codex → Sign in with ChatGPT account :

1. A browser window opens for authentication.

2. I sign in successfully.

3. The browser shows:

“Signed in to Codex. You may now close this page.”

1. I close the browser window and return to Xcode.

2. Xcode still shows “Not signed in”.

I have tried:

• Safari

• Chrome

• Clearing browser cache and cookies related to ChatGPT/OpenAI

• Removing related entries from Keychain Access on my Mac and signing in again

None of these resolved the issue.

Temporary Workaround

I found a workaround from this GitHub issue: Codex 0.129.0-alpha.9 breaks Xcode Coding Assistant with decode error #23109

Instead of using the version that Xcode installs automatically (0.129.0-alpha.9), I manually installed Codex 0.120.0.

I also tested:

• 0.133.0

• 0.139.0

but only 0.120.0 worked for me.

The downside is that GPT-5.5 is not available in Codex 0.120.0 , so I am currently using GPT-5.4.

I’ve been waiting a couple of days for a fix, and so far this is the only workaround that has worked for me. Hopefully it helps others who are seeing the same behavior.