In Xcode 26.3 Apple introduced deep agentic coding support, integrating with popular agentic coding tools from OpenAI and Anthropic. Using the Codex agent allows us to benefit from the productivity gains of agentic coding, right within the IDE that we are used to using every day. However, this integration can breakdown with inscrutable errors through no fault of our own. In this article we will see how to resolve the situation and get back up and running.
When attempting to use Codex in Xcode, you may have been presented with an error like this:
Codex encountered an error: The 'gpt-5.3-codex' model is not supported when using Codex with a ChatGPT account.
What is particularly galling about this error is that the model in question will be selectable in the Xcode Settings. Moreover, choosing other models available in the Settings will produce the same error.
To resolve this error, we must specify the model and reasoning effort that we wish to use in our config.toml file, found at ~/Library/Developer/Xcode/CodingAssistant/codex/config.toml. Simply add the following two lines, substituting the model value with the name of the one you wish to use:
model = "gpt-5.5"
model_reasoning_effort = "medium"Terminate, and restart Xcode. Your Codex session will pick up the model you specified, and if you have access to that model, you will be back up and running.