21st
Infra and UI building blocks for the agentic internet
Pricing information
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What is 21st?
21st is a React component registry and development resource for AI applications and the agentic internet. It brings together reusable UI building blocks, agent templates, and an SDK for assembling agents and agent-powered applications, making it relevant to frontend developers and design engineers who want a more structured starting point for product work.
The platform connects interface construction with several parts of an agent application workflow: developers can begin with components or templates, then use the SDK to organize UI, tools, memory, observability, and managed infrastructure. It is a useful fit for teams turning AI interactions into product interfaces, especially during early product construction, UI reuse, and agent-app prototyping; the practical fit still depends on a project’s React-based stack and its own engineering requirements.
Key features of 21st
React UI Registry
A centralized registry of React interface building blocks for AI applications, giving developers reusable foundations for product UI work instead of starting each interface from scratch.
Agent Templates
Templates aimed at creating agents and agent-powered applications, providing a starting structure that teams can continue developing once the product direction is clear.
Application SDK
An SDK that brings UI, tools, memory, and observability into one development layer, helping teams organize the interaction and operational pieces of an agent application.
Managed Infrastructure
The described SDK also includes managed infrastructure as part of the agent-application building blocks, which can reduce repeated preparation of supporting foundations during development.