Anthropic released Claude Opus 4 and Claude Sonnet 4 after focusing on mid-range Sonnet variants previously.Opus 4 is designed for complex, long-running tasks that can operate autonomously for hours.Anthropic revived the Opus line due to the increasing demand for agentic AI applications.Opus, Sonnet, and Haiku are the three AI model 'size' names offered by Claude, introduced in March 2024.