Claude Agent SDK
SDK for building agents on the Claude Code harness, rebranded from Claude Code SDK in late September 2025. June 2026 releases add hierarchical subagent spawning, Dynamic Workflows (fanning out tens to hundreds of parallel subagents), rubric-driven Outcomes, and fallback model chains, enabling "ultracode" mode with up to 1,000 subagents per execution; supports Python and TypeScript.