21st

21st provides infrastructure and UI building blocks for developers creating AI agents. The platform offers an SDK that handles the complex backend requirements of AI agent deployment, including chat history management, spend limits, and sandbox execution environments. It works with popular AI models like Claude and OpenAI, providing a foundation for building agentic applications.

The platform stands out by offering built-in UI components that developers can use to create chat interfaces quickly. It includes features like isolated virtual machines for each session through E2B sandboxes, token-by-token streaming via server-sent events, and comprehensive observability tools including session replay and tracing. This eliminates the need for developers to build these infrastructure components from scratch.

21st is particularly valuable for developers and engineering teams who want to focus on their agent's logic rather than infrastructure concerns. It suits use cases like building AI-powered chat applications, creating automated workflow agents, or developing specialized copilots for tasks such as email management, code explanation, or form automation. The included agent templates demonstrate practical applications across various domains from GitHub analysis to Slack monitoring.