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Anthropic Launches Claude Sonnet 5.5, Nearing Opus Performance at Half the Price

Sep 29, 2026
The Rundown AI
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Summary

Anthropic launches Claude Sonnet 5.5, a mid-tier model it says runs 30% faster, approaches Opus 5.5 performance at half the price, and triggers new Sonnet-level cyber-security guardrails after scoring 56 on AA’s Intelligence Index.

Key Points

  • Anthropic launches Claude Sonnet 5.5, a 30% faster mid-tier model that nears Opus 5.5 performance at half the price.
  • Sonnet 5.5 scores 56 on AA’s Intelligence Index, surpassing Fable 5.1 and GPT-6 Astra at 53.
  • Its cyber capabilities trigger the first Sonnet-tier guardrails, including security fallbacks previously used for Opus and Fable restrictions.

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