Claude Opus 5: The System Card

Zvi Mowshowitz reports on Claude Opus 5, Anthropic's newly released model positioned as a cost-effective alternative to Fable 5. On most practical tasks, Opus 5 matches Fable's capabilities while running faster and costing half as much, with notable improvements in agentic coding and long-horizon knowledge work. Anthropic deliberately trained Opus 5 away from cyber offense techniques, leaving it weaker than its flagship Mythos model at chaining complex exploits—though still stronger than its predecessor. The release also includes refined safety classifiers that trigger eighty-five percent less often than Fable's, enabling engineers to find vulnerabilities in source code while continuing to block binary-level analysis.

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