Community
$0
Forever
- Full component registry access
- shadcn-style CLI install
- Browse and contribute components
21st.dev — Agent Frameworks Platform
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
- 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)
Plans & pricing
What's included in each plan, and how the tiers compare.
Agent SDK (preview)
Contact
- 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
Best 21st.dev alternatives & competitors
Top companies in Agent Frameworks you can use instead of 21st.dev.
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Compare 21st.dev
Side-by-side comparisons with other tools in this category.
Best integrations for 21st.dev
Companies from adjacent layers in the AI stack that work well with 21st.dev.
Last verified: April 29, 2026