SAN FRANCISCO, September 23 — Anthropic introduced Claude Opus 5.5 on Tuesday, the first model in its new Claude 5.5 family and the company’s first release since it publicly called for pacing the frontier of AI development. The company says Opus 5.5 performs at the level of Claude Fable 5.1 on most work while costing 40% less to run than Opus 5. It is a major step up from Opus 5, with early testers reporting large jumps on complex work.
One tester completed a 680,000-line code migration in less than a day — work that would have taken an engineering team weeks. In a separate test to cut load times across every page of a web app, Opus 5.5 succeeded 39 of 40 times, while Opus 5 made smaller improvements that altered the app’s behavior.
Another tester had several Claude models build a game from a single prompt; Opus 5.5 scored higher than any other model on graphics and polish.
Safety testing and safeguards
On Anthropic’s automated behavioral audit, described as its most comprehensive alignment test, Opus 5.5 is the strongest-performing model the company has tested to date. It achieves the best scores of any model on that audit, which tests Claude across thousands of simulated scenarios. The model is much less likely than recent models to take hard-to-reverse actions or act outside given boundaries, and it is more resistant than Opus 5 to prompt injection.
Anthropic broadened alignment testing to cover longer tasks, impossible tasks, and scenarios modeled on real incidents, though the model still has limits. The model was tested before release by external evaluators, including Frontier Design and METR.
Full details are in the Opus 5.5 System Card. Because Opus 5.5 is comparable to Claude Mythos 5.1 in biology and cybersecurity, Anthropic is deploying it with safeguards similar to those on Claude Fable 5.1. Vetted organizations can apply today to the Life Sciences Verification Program to use Opus 5.5 for biology research.
In the coming weeks, Anthropic will expand access to its Cyber Verification Program, allowing verified cybersecurity practitioners to use Opus 5.5 for their work.
Cost, speed and clearer writing
Opus 5.5 requires less compute to serve than Opus 5. Anthropic’s tests show that at default settings it will cost 40% less than Opus 5 on typical workloads. Input and output tokens are priced at $4 and $20 per million, 20% less than Opus 5.
Cache reads, which make up the majority of agentic and coding work costs, are $0.20 per million tokens, 60% less than Opus 5. Opus 5.5 also generates output more than 30% faster than Opus 5.
In addition to the price drop, Anthropic is increasing five-hour usage limits on Pro, Max, Team, and seat-based Enterprise plans, and providing subscription users a rate limit reset that can be saved and used whenever they choose. Early testers found Opus 5.5’s writing clearer and easier to follow than prior models, addressing common feedback about Opus 5.
It puts the most important information up front, and one early tester said, “it writes the way I do.” Anthropic says this makes the model’s work easier to follow and check, which is a safety benefit as well as a practical one. On Anthropic’s benchmarks, Claude Opus 5.5 leads in agentic coding, computer use, and knowledge work.
Claude Sonnet 5.5 and Claude Haiku 5.5 will follow in the coming weeks, with many of the same improvements to performance, efficiency, and safety.
