GPT-5.5 Codex Hits Rough Patch with Sharp Performance Decline
gpt-5 openai reasoning
| Source: Mastodon | Original article
GPT-5.5 Codex experiences severe performance issues. Incorrect results occur 40% of the time due to a specific reasoning cluster.
The GPT-5.5 Codex, once a reliable tool, is now experiencing severe performance degradation. A specific 516-token reasoning cluster is causing incorrect results 40% of the time, a reproducible failure mode that developers suspect is linked to OpenAI's rumored cost-cutting measures. This issue is not isolated, as numerous users have reported similar problems on OpenAI's community forums and issue trackers since May 2026, with some noting a decline in performance over time.
The degradation of GPT-5.5 Codex matters because it affects the reliability and trustworthiness of the model, particularly for paid users who rely on it for daily development work. As users consider renewing their subscriptions, the model's decreased performance may influence their decisions. This is not the first time GPT-5.5 has faced criticism, with previous reports of degradation and silent downgrades to earlier models.
As the situation unfolds, it is essential to watch for OpenAI's response to these issues and any potential fixes or updates to address the performance degradation. Users will be looking for assurances that the model can be relied upon for critical tasks, and developers will be monitoring the situation to see if the problems can be resolved without compromising the model's capabilities.
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