21st

21st is a Y Combinator-backed company that infrastructure and UI building blocks for the agentic internet. The company participated in the Winter 2026 batch of Y Combinator.

21st.dev is the largest React component registry for AI applications, used by 1.4M developers (200K MAU).

We provide the building blocks to create AI agents and agent-powered apps: UI components, agent templates, and an SDK with built-in UI, tools, memory, observability, and managed infrastructure.

21st was part of Y Combinator's Winter 2026 cohort, has a team of approximately 3 employees, and is actively operating.