21st.dev — Agent Frameworks Platform
Agent FrameworksLayer 3FreemiumFounded 2024|San Francisco, CA|2-10
Visit websiteWhat is 21st.dev?
21st.dev is an infrastructure platform for the agentic internet, combining a massive React component registry with an emerging agent deployment SDK. Founded in 2024 by Serafim Korablev (CEO) and Sergey Bunas, the platform has grown to 1.4M developers and 200K monthly active users, with over 15,000 GitHub stars — making it one of the fastest-growing developer tools in the React ecosystem.
The platform started as an open-source registry for React UI components built on shadcn/ui, Tailwind CSS, and Radix UI, offering a 'copy the code, own it' model similar to shadcn but with AI-powered component generation. Developers can describe UI components in natural language and get production-ready React code with sub-100ms generation times.
As part of YC W2026, 21st.dev is expanding into full agent infrastructure with the 21st Agents SDK — enabling developers to define agents in TypeScript and deploy them with one command. The SDK provides sandboxed code execution, credential security, streaming, billing, observability, and pre-built chat UI components. This positions 21st.dev as a bridge between UI development and agent deployment, leveraging its existing component ecosystem and large developer community.
Key Features
- ✓Agent deployment SDK
- ✓TypeScript-first
- ✓Built-in UI components
- ✓One-command deploy
- ✓1.4M developer community
Pros & Cons
Pros
- +Massive developer adoption with 1.4M developers and 200K monthly active users
- +Blazing fast API with sub-100ms response times for AI component generation
- +Full code ownership — shadcn model where you own and customize all generated code
- +Well-timed agent SDK pivot leveraging existing UI ecosystem for full-stack agent deployment
- +Strong open-source foundation with 15K+ GitHub stars and community-driven component ecosystem
Cons
- -Severely limited free tier with only 5 tokens/month — heavy users must upgrade immediately
- -React and Tailwind CSS lock-in with no support for Vue, Svelte, or Angular frameworks
- -Agent SDK is still early-stage with maturing observability and debugging capabilities
- -Component library scope is focused on individual elements, not full-page templates
21st.dev Pricing
Free trial available
FreeFree
- ✓5 tokens/month
- ✓Access to component registry
- ✓CLI installation
Pro$16/monthannual
- ✓50 tokens/month
- ✓Enhanced AI generation
- ✓Premium component access
- ✓Priority support
Common Use Cases
Developers building and deploying AI agents with UIs
- •Agent deployment
- •UI component library for agents
- •Full-stack agent development
- •Rapid agent prototyping
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: March 27, 2026