21st YC Application

Winter 2026Active

21st went through Y Combinator in the Winter 2026 batch as a B2B company. Below: what the company does, who founded it, and its application to YC, reconstructed question by question in the real YC form.

YC batch: Winter 2026
Industry: B2BEngineering, Product and Design
Team size: 3
Status: Active
Launched: 2025

Founders

Serafim KorablevCEO / co-founder
Sergey BunasFounder

What 21st does

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.

21st's YC application, reconstructed

Y Combinator application

21st, Winter 2026 batch. The real form questions, answered from public data.

Founders2
Company7
Progress7
Idea4
Equity3
Curious2
Batch Preference1

Company

Company name

21st

Describe what your company does in 50 characters or less.

Infrastructure and UI blocks for agentic internet

What is your company going to make? Please describe your product and what it does or will do.

We're building 21st.dev, the largest React component registry tailored for AI applications. Our product includes UI components, agent templates, and an SDK that bundles UI, memory, tools, observability, and managed infrastructure. This makes it easier for developers to build AI agents and agent-powered apps without reinventing the core pieces.

Where do you live now, and where would the company be based after YC?

We live and work in San Francisco, CA, and the company will remain based here after YC.

Explain your decision regarding location.

San Francisco is where our team lives and the natural ecosystem for AI infrastructure startups. Staying here lets us tap into local engineers, early users, and investor networks.

21 more answers in the full application

Founders, Progress, Idea, Equity, Curious, Batch Preference are in the full version. Create a free account to open it in YC Insights, next to every other YC batch.

See full application

Frequently Asked Questions

Is this the real 21st YC application?

No. Y Combinator keeps submitted applications private. This is a reconstruction: the real YC form questions, answered from 21st's public data (launch notes, founder backgrounds, product description). It shows how a company like 21st would present itself to YC, not the text they submitted.