21st.dev: Agent Frameworks

Founded 2024|San Francisco, CA|2-10 people|Free 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

Core capabilities this platform advertises.

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

Strengths and tradeoffs

What this tool does well, and the limitations to keep in mind.

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

Plans & pricing

What's included in each plan, and how the tiers compare.

Community

Forever

Full component registry access
shadcn-style CLI install
Browse and contribute components

Agent SDK (preview)

Contact

Agent deployment infrastructure (in development)

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