Codex 'gpt-5.5 Model Is Not Supported' Error: The Fix

If Codex now answers with The 'gpt-5.5' model is not supported when using Codex with a ChatGPT account, the cause is almost certainly not your plan: OpenAI retired GPT-5.5 from Codex for ChatGPT sign-in on October 14, 2026, at noon PDT (19:00 UTC). Anything that still names gpt-5.5 - a config default, a script, a scheduled task, an old thread - now fails. The fix is to name a model your account can use, and on a Plus plan or above that is gpt-6.1-sol.

Last updated: October 16, 2026

The wording of that error is not new, which is why old search results mislead. It appeared in April for a different reason, and it is now back with a deadline behind it. Here is how to tell which one you have, and what to change.

What OpenAI says changed#

The Codex models page carries the retirement notice, and the workspace model availability page repeats it for admins. Read together:

• What retired: GPT-5.5, in Codex, for users signed in with ChatGPT. The OpenAI API and Codex sessions authenticated with an API key are not affected, and the models page says GPT-5.5 stays available there.
• Replacement on Plus, Pro, Business, Enterprise and Edu: GPT-6.1 Sol, gpt-6.1-sol, when available to your account.
• Replacement on Free and Go: GPT-6 Luna, gpt-6-luna, in the desktop app, when available.
• Where to look: workspace defaults, saved model settings, managed configurations, custom agents, scheduled tasks, and any script that selects a model.
• Enterprise and Edu catch: GPT-6.1 Sol is off by default until an administrator enables it, and the admin page is explicit that changing a default does not grant model access. If your admin has not switched it on, picking it will not work.

This is the third such cut this season. The same page lists gpt-5.4 and gpt-5.4-mini as retired from ChatGPT sign-in on August 31 (replace with gpt-6-sol and gpt-6-luna), and gpt-5.3-codex-spark as retired on September 14. If you only fixed things last time, re-grep: the pattern is that OpenAI retires an older model from the ChatGPT-login path on a date, and anything pinned to it breaks that day.

The fix, in order#

1. Change the default. Set the model in your Codex config.toml, which the docs say is shared by the desktop app, the CLI and the IDE extension:

   TOML

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   model = "gpt-6.1-sol"
   

2. Switch inside a session. In an interactive CLI session, /model switches models and reasoning effort.

3. Pin it per command. The docs show both forms:

   Terminal

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   codex --model gpt-6.1-sol
   codex exec -m gpt-6.1-sol "Review the current changes"
   

4. Sweep for stragglers. Search your repos and dotfiles for the old id - grep -rn "gpt-5.5" . is enough to find hard-coded values in CI workflows, scripts, custom agent definitions and managed config. The ones that bite are the unattended ones, like the headless runs in our Codex exec in CI guide, because nobody is watching a terminal when they fail.

5. Free or Go plan? Use gpt-6-luna in the desktop app instead. The models page lists Free and Go as not included in the GPT-6.1 Sol launch rollout.

If you would rather keep GPT-5.5, the documented route is API-key authentication, where model access follows your API organization and project. That moves you from plan allowances to per-token billing, so it is a cost decision, not just a config change. The numbers for both paths are in the Codex usage limits and pricing breakdown.

Which error do you actually have?#

Same string, three different causes:

The long-thread trap#

The nastiest version is not a bad config. openai/codex#54226, opened October 15 by a Plus user on Codex Desktop for Windows (version 26.1007.71326), reports that a long-running thread created on GPT-5.5 fails with Error running remote compact task: The 'gpt-5.5' model is not supported when using Codex with a ChatGPT account - even after switching to GPT-5.6 Luna or GPT-6 Astra. The reporter tried restarting, forking the thread and editing the stored model setting in the session file, and none of it cleared the error. The issue is open, and Codex's own triage bot flagged a likely duplicate, #54030, which is titled around a GPT-5.5 to GPT-6.1 Sol compaction fallback failing.

I could not reproduce this here, so treat it as a user report, not a confirmed mechanism. The practical read: compaction appears to reuse the model a thread started with, so very long GPT-5.5 threads are the ones at risk. If a thread matters, ask the agent to write its state to a file in the repo before you hit the limit, then start a new thread on a current model and point it at that file.

What people are actually saying#

On the Codex GitHub tracker, the two open compaction reports ask for the same thing: saved threads and compaction should fall back to a supported model instead of surfacing a retired id.

The other reaction is about churn. A r/codex thread this week asks whether GPT-5.6 Sol is going to be retired soon, which is a fair worry given three retirements in two months. The counter-case is that the retirements come with replacements: the Sol tier that replaced GPT-5.5 is, by OpenAI's own description on the models page, near GPT-6 Astra at a lower cost, and we covered the vendor-reported figures in the GPT-6.1 Sol release guide. Whether it is a better fit for your repo is something to measure on your own tasks, not take from a launch page.

The demand signal is real but thin. In Google Trends, the exact error sentence shows as a Breakout rising query under "codex" over the past 7 days, yet it barely registers against a steady neighbour: a one-hour reading of 11 against an average of 62 for "claude code skills". Expect the long tail of this problem to be people pasting the error into search, not a mass event.

The Monday checklist#

• Grep for gpt-5.5, gpt-5.4, gpt-5.3-codex and gpt-5.2 across repos, CI and dotfiles.
• Set model = "gpt-6.1-sol" (or gpt-6-luna on Free and Go) and confirm with a one-line codex exec.
• On a team plan, ask the admin to enable GPT-6.1 Sol before you roll the default.
• For irreplaceable long threads, export the state to a file now.
• If you run Codex against a gateway or a local model instead, none of this applies, but the endpoint must speak the Responses API; see Codex custom model providers.

Continue Reading#

• GPT-6.1 Sol Release Guide - pricing, the Astra safety hold and how the replacement model performs
• Codex Usage Limits and Pricing - plan allowances, credits and what API-key billing costs instead
• Codex Exec in CI - the unattended runs most likely to break on a retired model id
• Codex Custom Model Providers - pointing Codex at a gateway or local model instead of ChatGPT sign-in
• ChatGPT Free vs Go vs Plus for GPT-6 - which plan gets which model

Sources#