GPT-5.5 has been launched on Codex, but is not currently available.

BlockBeats News, April 22, GPT-5.5 has appeared in the model selector of OpenAI Codex, located at the top of the list.

After selecting GPT-5.5 and submitting the request, Codex returned a 400 error, stating "The 'gpt-5.5' model is not supported when using Codex with a ChatGPT account," indicating that the model is currently unavailable. OpenAI has not yet issued any official announcement or documentation regarding GPT-5.5.