Anthropic has introduced Claude Opus 5.5, the first model in its new Claude 5.5 family. According to the company, the model performs at the level of Claude Fable 5.1 on most work while costing 40% less to run than Opus 5. It marks the company's first release since it called for pacing the frontier, and it was tested before release by external evaluators including Frontier Design and METR.
On Anthropic's automated behavioral audit—described as its most comprehensive alignment test—Opus 5.5 is the strongest-performing model tested to date. The company says it is much less likely than recent models to take hard-to-reverse actions or act outside given boundaries, and more resistant than Opus 5 to prompt injection. Alignment testing was broadened to cover longer tasks, impossible tasks, and scenarios modeled on real incidents, though Anthropic notes the model still has limits.
Performance gains were reported across complex engineering work. One early tester completed a 680,000-line code migration in less than a day—work Anthropic says would have taken an engineering team weeks. In a web-app load-time optimization test, Opus 5.5 succeeded 39 of 40 times, while Opus 5 made smaller improvements that also altered the app's behavior. In an internal test translating HAProxy from C into Rust, both Opus 5.5 and Fable 5.1 passed nearly all regression tests, but Opus 5.5 finished in 9.5 hours versus 12 and cost 51% less.
Pricing reflects reduced compute requirements. Input and output tokens are $4 and $20 per million—20% less than Opus 5—while cache reads are $0.20 per million, 60% less. Opus 5.5 also generates output more than 30% faster than Opus 5. A fast mode is available in Claude Code and the Claude Platform with up to 2.5x speed at $8 per million input tokens and $40 per million output tokens. Anthropic is also increasing five-hour usage limits on Pro, Max, and Team plans and providing subscription users a saveable rate limit reset.
Because Opus 5.5 is comparable to Claude Mythos 5.1 in biology and cybersecurity, it is deployed with safeguards similar to those on Claude Fable 5.1. Vetted organizations can apply to the Life Sciences Verification Program for biology research, and access to the Cyber Verification Program will expand in coming weeks. Claude Sonnet 5.5 and Claude Haiku 5.5 are expected to follow with many of the same improvements.