GPT-5.5 Codex reasoning-token clustering may be leading to degraded performance
github.com
July 4, 2026
3 min read
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51/100
Summary
GPT-5.5 Codex exhibits a pattern where reasoning output tokens cluster at 516, 1034, and 1552. This clustering may correlate with degraded performance on complex tasks.
Key Takeaways
- GPT-5.5 exhibits a clustering anomaly in reasoning tokens, with a significant number of responses terminating at exactly 516 tokens, and additional spikes at 1034 and 1552 tokens.
- The overall reasoning-token intensity for GPT-5.5 has decreased from February to June 2026, while the frequency of exact-516 clustering has increased sharply.
- GPT-5.5 accounts for 19.3% of all responses but 82.0% of exact-516 events, indicating a disproportionate clustering behavior compared to other models.
- The observed token clustering does not align with expected behavior for complex tasks, suggesting potential issues with reasoning-budget or truncation mechanisms in GPT-5.5.
Community Sentiment
Positives
- Some users are exploring alternatives like GLM 5.2 to avoid reliance on inconsistent performance from Codex and Claude, indicating a search for better options.
- Commenters are sharing their experiences of switching between Codex and Claude, highlighting the competitive landscape and encouraging others to find the best fit for their needs.
Concerns
- Users are reporting significant drops in Codex's performance, claiming that the once-reliable quality has evaporated, leading them to switch to alternatives.
- There's a pervasive sentiment that performance degradation is a business decision rather than a technical issue, suggesting distrust in the motives behind the changes.
- Commenters are expressing frustration over the lack of consistent performance and the expectation that users should tolerate the ups and downs of non-deterministic systems.
