Claude Opus 5: The System Card
Zvi (Don't Worry About the Vase) TheZvi ● Covered by 15 sources
Anthropic released Claude Opus 5, a model positioned between Opus 4.8 and their larger Mythos 5, claiming comparable performance to Fable 5 at half the price and faster speed. Key concrete improvements include reducing prompt injection attack success rates from 5.5% to 2.0% on the IPI benchmark, cutting safety classifier false positives from 42% to 5% in FrontierBench, and achieving 69% multi-turn appropriate response rates for self-harm queries versus 58% for Fable. The model now permits vulnerability analysis in source code while maintaining blocks on binary analysis, and shows improved agentic safety and alignment compared to previous versions, though it lacks the full multi-step exploit capability of Mythos 5.
Why it matters
Claude Opus 5 is trying to be the best of both worlds. On many practical tasks, Opus 5 is pitched as straight up as good or better than Fable 5, while being faster, at half the price. Most tasks do … Continue reading →
Also covered by
- Latent Space — [AINews] Claude Opus 5: Fable-level performance at Opus price (half Fable)
- Simon Willison — Quoting Boris Cherny
- Simon Willison — Introducing Claude Opus 5
- MarkTechPost — Meet the New Claude Opus 5: Frontier-Class Agentic Coding and Computer Use at Unchanged Opus Pricing
- Ars Technica — Anthropic's Opus 5 is about token efficiency, not a capability leap
- The New Stack — Opus 5 costs a third of the price — and that’s actually the problem
- AWS Machine Learning — Introducing Claude Opus 5 on AWS: Anthropic’s most capable Opus model
- Anthropic News — Introducing Claude Opus 5
- TechCrunch AI — Anthropic launches Opus 5
- The New Stack — Anthropic’s Opus 5 is almost Fable 5
- Sakana AI — Announcing Fugu-Ultra v1.1 and Claude Code interface for Fugu
- The Verge — Anthropic releases Opus 5 with ‘close’ to Fable 5’s capabilities
- The Batch — Testing Mythos and Fable, Moving Beyond SWE-bench, Nvidia's Open Contender
- The Batch — Mythos Begets Fable, Cursor's Composer 2.5, Agents Building Agents
