Anthropic launches Claude Opus 5.5: Fable 5.1-level performance at 40% lower cost
claudeai · x · 2026-09-23
Anthropic launched Claude Opus 5.5, the first model in its Claude 5.5 family, matching Claude Fable 5.1 on most work while costing 40% less to run than Opus 5.
Performance
- New leading model; one tester completed a 680,000-line code migration in under a day
- Succeeded in 39 of 40 attempts at cutting load times across a web app, where Opus 5 made smaller improvements that altered app behavior
- Scored highest building a game from a single prompt, on graphics and polish
Safety
- Best score to date on Anthropic's automated behavioral audit across thousands of simulated scenarios
- Less likely to take hard-to-reverse actions or exceed boundaries; more resistant to prompt injection than Opus 5
- Alignment testing broadened to longer tasks, impossible tasks, and real-incident scenarios
Externally evaluated by METR and Frontier Design pre-release; first model since Anthropic called for pacing the frontier.
Related event: Anthropic Launches Claude Opus 5.5: Stronger, 40% Cheaper, Tops AA Index(259 posts)→
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