Claude Sonnet 4 is Anthropic's balanced mid-tier model released alongside Opus 4 in May 2025, designed to combine strong coding and reasoning capabilities with computational efficiency. It achieves state-of-the-art 72.7% on SWE-bench while offering significantly lower cost and faster response times than Opus models. Key strengths include autonomous codebase navigation, reduced error rates in agent-driven workflows, and high reliability in following intricate instructions, making it a versatile choice for both routine and complex development tasks.