21st — Startup Profile

Infrastructure and UI building blocks for the agentic internet

HQ: San Francisco, CA | Founded: 2024 | Employees: 3 | Stage: Seed | Website: https://21st.dev

About

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.

The Take

21st is building the missing infrastructure layer for deploying frontier AI agents into production. Backed by Y Combinator (W26), their 21st.dev registry serves 2M+ developers (370K MAU) with UI components for AI apps.

Key Metrics

• STAGE: Seed
• EMPLOYEES: 3
• FOUNDED: 2024
• HQ: San Francisco, CA
• FUNDING (BACKERS): Y Combinator (W26)
• VALUATION: Undisclosed

Funding Rounds

• Round: Undisclosed (2026-01) — Y Combinator (W26)

Key People

• Alice Smith
• Bob Johnson
• Carol Lee

Key Competitors

• Competitor A —
• Competitor B —
• Competitor C —

Timeline Highlights

• 2023-10-01: Product Development Kickoff
• 2023-09-15: Seed Funding Secured
• 2023-01-01: Company Founded

Recent News

• 21st - Infrastructure for AI agents (2026-01-01)

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