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Infrastructure and UI building blocks for the agentic internet
HQ: San Francisco, CA | Founded: 2024 | Employees: 3 | Stage: Seed | Website: https://21st.dev
21st provides infrastructure and UI building blocks for the agentic internet. Founded in 2024 by Serafim Korablev (ex-Via, Rork.com) and Sergey Bunas (ex-Deel, built Suggesty), the company started with 21st.dev, the largest React component registry for AI applications used by 2M developers. They have since launched the 21st Agents SDK, which makes it easy to deploy frontier AI agents (Claude Code, Codex) in production. The SDK provides sandboxed code execution, credential security, SSE streaming, agent chat UI, usage limits, billing, and observability out of the box. It supports React, Next.js, Node, and Python. 21st is Y Combinator W26.
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