21st

Infrastructure and UI building blocks for the agentic internet

Winter 2026ActiveB2BEngineering, Product and DesignDeveloper ToolsB2BOpen SourceAPIAISan Francisco, CA, USA

Company

21st.dev is the largest React component registry for AI applications, used by 1.4M developers (200K 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.

Verdict

High Signal

Market Opportunity

B2B developer infrastructure for AI-native applications is a massive and rapidly expanding market — every company building AI apps needs UI components and agent infrastructure. ICP is clear: dev teams building AI agents and AI-powered apps. Monetization path includes SDK usage, managed infrastructure, and agent templates — recurring revenue model with strong expansion potential as AI agent adoption accelerates.

High Signal

Founder Signal

Serafim has two exits (Gas Pump acquired by BabyDoge Nov 2024, Cutly acquired Nov 2024) plus Via Protocol with $1.5B GTV backed by Naval and others — serial founder with real outcomes. Sergey has 8 months at Deel building UI design systems, solo-built a Figma competitor with 15K users and 90 paying teams (Stage), and led frontend at X5 Retail Group across 19K+ stores — a genuinely experienced technical founder. Complementary CEO/CTO split with both having shipped real products at scale.

Medium Signal

Competition

Competitors include shadcn/ui (free, dominant open-source component library), v0 by Vercel (AI-generated UI), and Retool/Glide for agent UIs. On the agent SDK side, LangChain, LlamaIndex, and emerging players compete. However, 21st's moat is the combination of a large, established developer community (1.4M users) with an agent-specific SDK layered on top — a distribution advantage competitors without this community would struggle to replicate quickly.

High Signal

Product

1.4M total developers with 200K MAU on a React component registry is real, verifiable scale. Sergey's LinkedIn confirms 700K+ monthly visitors and 10K+ GitHub stars. Product is live with a full SDK, agent templates, documentation, and sandbox execution — not vaporware.

OverallA Tier

21st has genuinely impressive traction — 1.4M developers is a real distribution moat that most YC companies would kill for, and the founders have multiple exits and verified product-shipping credentials. The pivot from pure component registry to agent infrastructure SDK is smart and timely, leveraging existing developer trust to expand into a higher-value market. The main risk is competitive pressure from Vercel (v0), shadcn, and well-funded agent infra players who could commoditize the component layer. The 21st SDK needs to prove it can monetize at scale beyond the free component registry, but the foundation is unusually strong for a W26 company.

Active Founders

Serafim Korablev

CEO / 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.

Sergey Bunas

Founder

CS dropout. Built UI systems 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)

21st

TierA Tier

BatchWinter 2026

Team Size3

StatusActive

LocationSan Francisco, CA, USA