Claude Sonnet 5.5 (Medium)

Claude Sonnet 5.5 is a large language model developed by Anthropic, serving as a direct upgrade to Claude Sonnet 5. Released on September 28, 2026, it is designed to offer an optimal balance of speed and intelligence for everyday professional tasks, programming workflows, and structured document creation. It functions as a lower-cost, faster complement to the heavier Claude Opus 5.5 model.

The model introduces significant performance enhancements over its predecessor, achieving major improvements in agentic coding evaluations and real-world knowledge work. It is particularly effective at well-scoped tasks such as bug fixing, feature building, and generating polished spreadsheets, slides, and summary documents. Additionally, it supports multimodal inputs including text and images, with enhanced visual understanding capabilities.

Architecture and Specifications

Claude Sonnet 5.5 features a 1 million-token context window alongside a maximum output length of 128,000 tokens on standard API requests. It maintains pricing consistent with earlier generations while executing tasks with greater efficiency and fewer total tokens. The model incorporates advanced behavioral safety configurations and cyber safeguards, ensuring reliable execution for complex enterprise environments.