21st
21st is a Y Combinator-backed platform providing Agents SDK, open-source UI components, and developer tools for the agentic internet. It offers built-in chat UI, sandbox execution, spend limits, observability, and works with Claude and OpenAI.
🚀 Agents SDK: The fastest way to build and deploy AI agents with built-in UI, chat history, spend limits, sandbox execution via E2B, and session observability. Works with Claude and OpenAI out of the box.
🎨 Open-Source UI Components: A curated marketplace of React and Tailwind components inspired by shadcn/ui. Browse, copy, and ship polished interfaces without starting from scratch.
⚡ Agent Templates: Ready-to-use starter templates including Next.js chat apps, Python terminal clients, Go SDK integrations, GitHub repo analyzers, Slack monitors, and browser automation agents.
🔧 Developer-First Architecture: Token-by-token SSE streaming, auth and rate limits, isolated VM sandboxes per session, and full session replay with traces for debugging and optimization.
🌟 Backed by Y Combinator: A well-funded platform trusted by developers building production AI agents, with an active open-source community and growing component ecosystem.
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