21st

Active

Infrastructure and UI building blocks for the agentic internet

Winter 2026Founded 20243 peopleSan Francisco, CA, USA

21st.dev ↗LinkedIn ↗X ↗See on the Idea Map B2B momentum

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AI insightcan contain mistakes

AI Application Component RegistryAPI/InfraDevelopers building AI agents and agentic appsMedium competition

Moat

Large developer community (370K MAU); comprehensive component library; SDK with built-in infrastructure.

Key risk

Component quality and maintenance burden; competition from cloud providers; developer fragmentation.

Why now

Agentic app development is exploding; teams need pre-built infrastructure (UI, memory, observability).

Competitors

npm, Component libraries, UI libraries (React, Vue), cloud provider SDKs

About

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.

From their website

as of Jun 7, 202621st.dev ↗

Marketplace

21st provides a platform of community-made UI components and building blocks for web interfaces, centered on React. It catalogs and enables discovery, copying, and remixing of UI components contributed by designers and developers. The offering emphasizes a large library of ready-to-use blocks and components.

A library/marketplace of React UI components published to the 21st.dev Community. Users can discover components (e.g., Buttons, Forms, Cards, AI Chats, Pricing Sections), copy or remix them, and integrate them into their projects. The site enumerates components by categories and types, suggesting a CMS-like catalog and remixable code snippets for UI construction.

Who it’s for: Frontend teams, UI/UX designers, and developers looking for modular React UI components and building blocks to assemble apps and websites.

Features

large library of React UI components
ability to copy and remix components
categorized components (buttons, forms, nav, etc.)
community-contributed with themes and authors
search and filtering of components
live catalogs of components and variants
pricing sections and UI blocks

community contributions and a large catalog; active curation and search/browsing features

Founders · 2

Serafim KorablevCEO / co-founder

One of the first full-time vibe-coders (since October 2023). Co-founder of 21st.dev, a UI components registry with over 1m users. Previously co-founded Rork.com (then a web app builder for Next.js), the first Telegram-native memecoin launchpad (multi-million exit), and Via, a cross-chain crypto routing protocol with $1.5B GTV backed by Naval, Safe, Sandeep, and others. Passionate about design and basketball.

LinkedIn ↗X ↗

Sergey BunasFounder

Google🎓 Cambridge

CS dropout. Senior Software Engineer at Deel (W19). Built Perplexity before Perplexity (Suggesty, 2022 — #1 Product Hunt, featured in Google's internal newsletter). Solo-built a Figma competitor pre-AI (Stage, 10K users). Cambridge AI hackathon winner (solo)

LinkedIn ↗X ↗

Launch

Launched on Y Combinator · Mar 2026

View launch post ↗

The production layer for shipping frontier AI agents inside your product - sandboxing, streaming, billing, UI, and observability out of the box.

21st.dev launches 21st Agents SDK, a production layer for frontier AI agents. It lets developers define an agent in TypeScript, deploy with a single command, and manage a hosted, sandboxed agent with streaming, billing, observability, and a UI, targeting teams building AI agents (Claude Code, Codex, and others) into products.