Claude Opus 5.5 (Adaptive Reasoning, Low Effort, Default Fallback)

Released on September 22, 2026, Claude Opus 5.5 is the flagship model of the Claude 5.5 family from Anthropic. It is designed specifically for high-stakes agentic coding, multidisciplinary research, and complex knowledge work. This model represents a strategic shift toward efficiency, offering performance comparable to the higher-tier Claude Fable 5.1 while achieving a 40% reduction in typical operational costs and a 30% increase in output speed compared to its predecessor, Claude Opus 5.

A defining technical feature of Opus 5.5 is its Adaptive Thinking architecture, which is permanently enabled to provide deeper reasoning capabilities. Users can control the depth of this reasoning via a specific effort parameter, allowing for nuanced calibration between speed and analytical rigor. The model is multimodal, accepting both text and visual inputs such as diagrams, screenshots, and financial charts, with a knowledge cutoff updated to June 2026.

Anthropic has integrated extensive safety measures into Opus 5.5, including a comprehensive behavioral audit that tests the model across thousands of simulated scenarios to prevent hard-to-reverse actions or boundary circumvention. It introduces the strongest cyber-defense capabilities of any Anthropic model to date, featuring routine bug-finding and remediation protocols. Due to its advanced capabilities in biology and cybersecurity, access to specific high-risk tasks is managed through verification programs for vetted organizations.

In benchmark evaluations, Claude Opus 5.5 leads in agentic coding and computer use, showing significant improvements in long-horizon tasks such as auditing large codebases. The model supports a massive 1,000,000-token context window, enabling it to process extensive technical documentation or entire repositories in a single request. While it is highly optimized for technical and professional workflows, it also features a more natural communication style developed to improve collaboration during extended sessions.