SDK

Answer-First: Anthropic released Claude Sonnet 4.5 along with open-sourcing the Agent SDK infrastructure, setting a new benchmark for autonomous coding agents and context-managed execution. Tech Radar, April 27, 2026: Claude Sonnet 4.5 and the Agent SDK — The Best Coding Model Just Open-Sourced Its Infrastructure [!NOTE] Dating and status correction. Claude Sonnet 4.5 and the Claude Agent SDK were announced by Anthropic on September 29, 2025 (Anthropic announcement), not in the week of this radar entry — this piece is a retrospective architecture analysis, not a launch-week report. Anthropic has since shipped Claude Sonnet 5, so treat model-ranking claims below as reflecting the Sonnet 4.5 generation rather than the current frontier. The Agent SDK architecture analysis remains applicable. ...