21st provides infrastructure and UI building blocks for the agentic internet. Its product, 21st.dev, is a React component registry serving roughly 2 million developers and 370K monthly active users, offering UI components, agent templates, and an SDK with built-in tools, memory, observability, and managed infrastructure for AI agents. 21st is part of Y Combinator's Winter 2026 batch.
21st
ActiveInfrastructure and UI building blocks for the agentic internet
Total raised
$500K
1 round
Stage
Seed
Jan 2026
Team
1-10
since 2026
Pricing
—
Founded
2026
San Francisco, United States
Agent-ready
—
21st.dev React component registry with reusable UI building blocks
Library of agent templates to bootstrap agentic applications
SDK with built-in tools for AI agents
Memory primitives for persisting agent state and context
Observability for monitoring agent behavior
Managed infrastructure so teams avoid running their own backend
Serves a large developer community (reported ~2M developers, ~370K MAU)
Focused on UI and infrastructure for the agentic internet
12/100
Early
MCP server
Public API
Webhooks
OAuth 2.0
SDKs
No public agent surfaces detected yet.
Jan 2026 Seed $500K ● Y Combinator
Capital network
$500K raised ·1 backer·10 network links
- Backers1
- Shared portfoliocompanies these backers also fund
- Extended networkfunds that co-invest alongside them
Meshy
AI 3D model generator for game dev and creators
AI Developer Tools3D Generation
Higharc
AI-powered connected homebuilding platform
AI DesignAI Real Estate
Raspberry AI
AI Design
Scenario
Creative AI infrastructure for generating game-ready art assets at scale
AI DesignAI Gaming
Vizcom
AI Design
qbiq
AI that generates optimized floor plans, 3D tours and CAD models in minutes
AI DesignAI Construction
- What is 21st.dev?
- It is 21st's React component registry and agent platform, offering UI building blocks, agent templates, and an SDK with tools, memory, observability, and managed infrastructure for building agentic applications.
- How large is 21st's user base?
- The company reports serving roughly 2 million developers with around 370,000 monthly active users on the 21st.dev registry.
- What does the SDK include?
- The SDK provides built-in tools, memory, and observability, backed by managed infrastructure, so developers can build and run AI agents without assembling those pieces themselves.
- Is 21st only for UI components?
- No. It started as a React UI registry but now also provides agent templates and infrastructure aimed at building applications for the agentic internet.
Discussion
Sign in to join the discussion.
Sign inExplore more around 21st
Contextual paths to related AI startups, deals and rankings.