Anthropic released Claude Sonnet 4.5, elevating the Sonnet tier to flagship status for agents, coding, and computer use. It was the strongest model in the world for building complex agents at launch, according to Anthropic's evaluations.

Sonnet 4.5 emphasized sustained autonomous work: better planning, more reliable tool use, and stronger performance on long-horizon tasks. It shipped with an upgraded Claude Code and new agent-oriented API features.

The release showed Anthropic's tier strategy evolving — the 'middle' model had become the one most developers actually built on.