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Analysis of 1,400 sessions suggests Opus 5.5 reasoning performance degraded on November 3

·2026.11.10 09:00

Key point

A test on a real bug showed Opus 5.5 high failed to find a fix after 3 hours, while xhigh resolved it in minutes.

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Details

  • Analysis of 1,400 Claude Code sessions by Gemini 4 Argon indicates a sharp change in Opus 5.5 reasoning behavior occurred on November 3, despite the model name remaining unchanged.
  • The amount of reasoning performed by Opus 5.5 in 'high' mode dropped significantly after the date change.
  • In a practical test on a real bug, Opus 5.5 high jumped into action too quickly, became confused, and failed to find a fix after 3 full hours.
  • Opus 5.5 xhigh took a moment to think but found a fix after a few minutes, suggesting the previous advice to 'use high for everything' no longer applies.
anthropicclaude-codemodel-evaluationopus-5.5gemini-4-argon
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