21st
21st is a YC Winter 2026 (W26) company. Based in San Francisco, CA, USA. Team of 3.
About 21st
Summary
21st builds a registry of React UI components, agent templates, and an SDK for developers building AI-powered applications. The library ships over 12,000 components, covering marketing blocks, UI primitives, and shadcn themes, each delivered as a prompt that any AI coding tool can consume directly. It replaces the work of hand-coding common interface patterns and wiring up agent infrastructure from scratch.
Business model
A free tier plus paid plans, as shown on the site's pricing page.
Go-to-market
Developers, from solo makers to product teams at large technology companies, adopt it self-serve by copying component prompts into their existing AI coding environments such as Claude Code, Codex, or Lovable.
From the Y Combinator listing
Infrastructure and UI building blocks for the agentic internet
21st.dev is the largest React component registry for AI applications, used by 2M developers (370K MAU). We provide the building blocks to create AI agents and agent-powered apps: UI components, agent templates, and an SDK with built-in UI, tools, memory, observability, and managed infrastructure.
Facts
- B2B / B2B -> Engineering, Product and Design
- San Francisco, CA, USA
- 3
- Active
- Early
Founders (2)
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