21st.dev

21st.dev is a platform that provides a curated library of open-source UI components and AI agent templates, alongside a dedicated Agents SDK for rapidly building and deploying AI-powered applications. Developers can browse, copy, and integrate ready-made React/Tailwind components directly into their projects, saving significant design and development time. The platform also offers an Agents SDK with built-in infrastructure including sandboxed execution, streaming, authentication, spend limits, and observability. Backed by Y Combinator, 21st.dev targets frontend developers and AI engineers who want to ship faster.

• UI Component Library: Browse hundreds of community-contributed React components (chat UIs, inputs, animations, etc.) and copy them directly into your project.
• AI Agent Templates: Pre-built agent templates for common use cases like email agents, web scrapers, note takers, and monitoring agents — ready to deploy with minimal configuration.
• Agents SDK: A first-party SDK that provides built-in chat history, token streaming via SSE, spend caps, rate limits, and session replay/traces for AI agents.
• E2B Sandbox Execution: Each agent session runs in an isolated VM via E2B, ensuring secure and reproducible code execution.
• Magic Chat & 1Code: Integrated AI-powered tools for generating and editing UI components through natural language prompts.
• Observability: Session replay and distributed traces give developers full visibility into agent behavior and performance.
• Works with Claude & OpenAI: The Agents SDK supports both Anthropic Claude and OpenAI models out of the box.
• Community Contributions: Developers can publish their own components to the 21st.dev community and gain visibility through usage metrics.