Claude 3.7 Sonnet, released February 24, 2025, was Anthropic's first hybrid reasoning model: a single model that could answer instantly or engage an extended-thinking mode where it reasons step by step before responding, with the thinking budget under API control. It launched alongside the first research preview of Claude Code, Anthropic's agentic command-line coding tool.

The reasoning upgrade showed up most clearly in software engineering, where Claude 3.7 Sonnet scored 62.3% on SWE-Bench Verified — up from 49.0% for its predecessor and the best published score of any model at the time. GPQA Diamond rose to 68.0%. It held the top of the Claude lineup for three months until Claude Sonnet 4 and Opus 4 arrived in May 2025.