---
title: "What the Codex performance report found | Ian Nuttall"
description: "Codex degradation report findings"
canonical: "https://ian.is/post/codex-performance-findings"
language: "en"
---

# What the Codex performance report found

Nov 2025 / 146,358 views / [via X](https://x.com/iannuttall/status/1984531393062240611)

Codex degradation report TLDR:

\- The investigation found no single root cause; rather a mix of behavior shifts and small bugs, with several fixes already shipped and more on the way.

\- Older hardware underperformed in evals and was removed, and improved load balancing is rolling out to reduce latency.

\- Compaction was used more often, and repeated compactions hurt accuracy; recursive summaries were blocked and a warning was added to keep chats focused.

\- The \`apply\_patch\` flow could delete and then recreate files after failed diffs, which is risky if interrupted; mitigations are in progress and a longer‑term model fix is planned.

\- There was no fleet‑wide latency regression, but some sessions escalated retries and timeouts inefficiently; handling of long‑running and interactive tasks is being improved.

\- A constrained‑sampling bug produced out‑of‑distribution tokens, including occasional mid‑sentence language switches in under 0.25% of sessions; a fix is rolling out.

\- The Responses API added extra newlines around tool definitions; this had no measurable performance impact.

\- Evals show CLI 0.45 maintained quality while reducing tokens by roughly 10%; neither \`--search\` nor recent prompt changes caused regressions.

\- Infra analysis found low auth‑cache hit rates adding about 50 ms to tail latency; working‑directory error rates were unchanged.

\- The guidance is to prefer minimalist toolsets and shorter, targeted chats, and a dedicated real‑world performance team is being formed and hiring.

[![](https://ian.is/media/tweets/1984465716888944712/1953337039510003712/c68110b9ada0593beecd5b7a8fc70fe4e6e6b698dc2eb96e6d4265faed94d626.jpg)**Tibo**@thsottiaux](https://x.com/thsottiaux)

> We promised unprecedented transparency for Codex and to take the reports of degradation seriously, despite seeing incredible growth week over week.
>
> Here is our report and what we have found over the last seven days
> https://docs.google.com/document/d/1fDJc1e0itJdh0MXMFJtkRiBcxGEFtye6Xc6Ui7eMX4o/edit?usp=sharing

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