21st.dev — Agent Frameworks Platform
Agent FrameworksLayer 3Free community registry (paid SDK/agent tiers in development)What is 21st.dev?
21st.dev is the largest React component registry for the agentic internet — an open-source, community-driven marketplace of shadcn/ui-based React + Tailwind components, blocks, and hooks. It functions like 'npm for design engineers,' letting developers browse and install reusable interface elements directly into their projects.
Beyond components, 21st is pivoting into a broader infrastructure play — a full agent deployment SDK and AI Agent Registry. The Y Combinator-backed company positions itself as 'infra and UI building blocks for the agentic internet,' targeting the wave of AI-driven applications that need both UI primitives and agent runtime tooling.
Used by 1.4M+ developers with ~200K monthly actives. The component registry is free; component blocks include 3D, charts, dashboards, marketing pages, and full app shells. Strong fit for AI-driven workflows where teams need to ship UI fast on top of LLM-powered backends.
Key Features
- ✓Largest React component registry — shadcn/ui + Tailwind based
- ✓Open-source and community-driven — npm-like CLI install
- ✓Components, blocks, hooks, and full marketing/app templates
- ✓AI Agent Registry pivoting toward full agent deployment SDK
- ✓200K monthly actives, 1.4M+ developers using it
Pros & Cons
Pros
- +Largest registry of high-quality, free shadcn/ui components
- +Smooth React + Next.js integration
- +Active community contribution model
- +Strong fit for AI-driven workflows
- +YC-backed with credible pivot toward agent infrastructure
Cons
- -Quality varies across community-contributed components
- -Pricing for the agent SDK side is still unclear
- -shadcn/ui-only — limited if your stack uses a different UI kit
- -Newer than incumbent component libraries (MUI, Chakra)
21st.dev Pricing
Free trial available
- ✓Full component registry access
- ✓shadcn-style CLI install
- ✓Browse and contribute components
- ✓Agent deployment infrastructure (in development)
- ✓AI Agent Registry access
- ✓Early-access program
Common Use Cases
React developers and AI app builders who want shadcn/ui-quality components and emerging agent deployment infrastructure
- •Rapid React + Tailwind UI prototyping for AI apps
- •shadcn/ui component discovery and installation
- •Marketing site assembly from pre-built blocks
- •AI-generated dashboards and chart UIs
- •Future: agent-runtime deployment via 21st SDK
Using 21st.dev with Respan
21st.dev's Agents SDK enables developers to deploy AI agents with built-in UI components. Respan can provide observability for the LLM calls powering these agents, tracking performance, costs, and output quality across deployments.
- ✓Monitor LLM calls made by agents deployed through 21st.dev SDK via Respan
- ✓Track agent performance and costs across 21st.dev deployments in Respan dashboards
- ✓Evaluate agent output quality using Respan evaluations for 21st.dev-powered agents
- ✓Optimize agent LLM spending using Respan cost analytics
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Last verified: April 29, 2026