21st (YC Winter 2026)

TL;DR: 21st is a Y Combinator Winter 2026 startup Infrastructure and UI building blocks for the agentic internet, currently active with a team of 3.

Infrastructure and UI building blocks for the agentic internet

YC ID
31123
YC Batch
Winter 2026
Status
Active
Team size
3
Industry
B2B, Engineering, Product and Design
YC profile
https://www.ycombinator.com/companies/21st

Last verified:

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.

Frequently asked questions

Is 21st a YC (Y Combinator) company?

Yes. 21st was funded by Y Combinator in the Winter 2026 batch. Infrastructure and UI building blocks for the agentic internet

What does 21st do?

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.

What YC batch was 21st in?

21st was part of Y Combinator's Winter 2026 batch.

How big is 21st?

21st has a team of 3 people according to Y Combinator's directory.

Is 21st still active?

21st is currently listed as Active with a team of 3 in Y Combinator's directory.

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