21st.dev is a web-based platform that provides a large registry of reusable UI components, templates, and agent-oriented building blocks for developers building modern web and AI-powered applications. It combines a community-driven component marketplace with AI-assisted generation tools, allowing users to discover, remix, and generate production-ready interface elements.

The platform focuses heavily on React-based ecosystems, particularly Next.js and Tailwind CSS, and is designed for “design engineers” who want to ship interfaces faster without building every component from scratch. It includes prebuilt UI elements such as chat interfaces, dashboards, forms, navigation patterns, and full landing page sections.

Beyond static components, 21st.dev also positions itself around the emerging “agentic internet,” offering an Agents SDK that helps developers build AI agents with built-in UI, state handling, observability, sandbox execution, and deployment infrastructure. This expands its scope from UI component sharing into full AI application infrastructure.

The system works through a combination of a searchable registry, CLI/IDE integrations, and AI prompts that can generate multiple UI variants. Developers can browse community-made components, install them directly into projects, or use AI to create customized versions tailored to specific product needs.

21st.dev is positioned as both a design engineering marketplace and an AI development infrastructure layer, bridging UI design systems with AI agent workflows in a single ecosystem.

Key features include:

• Large registry of React + Tailwind UI components
• AI-powered generation of UI component variants
• Community marketplace for reusable interface blocks
• CLI and IDE integrations for instant installation
• Agent SDK for building AI-powered applications
• Prebuilt templates for chat apps, dashboards, and tools • Support for observability, sandboxing, and agent workflows

Common use cases include:

• Building SaaS interfaces
• Generating UI components with AI
• Accelerating frontend development
• Creating AI chat and agent interfaces
• Designing design systems
• Prototyping modern web applications

21st.dev is developed by 21st Labs, a startup focused on building infrastructure for AI-native applications and component-driven frontend development.