21st Agents SDK

What is 21st Agents SDK?

21st Agents SDK is a TypeScript-first framework that lets you define, deploy, and embed production-ready AI agents in minutes. It handles the infrastructure layer—sandboxed runtimes, authentication, chat UI components, and observability—so you can focus entirely on your agent's logic. Backed by Y Combinator (W26), the SDK transforms a weekend prototype into a deployed, billable agent with a single command: npx @21st-sdk/cli deploy.

Who it's for

• Solo developers who want to ship an AI agent without spending weeks on container setup, auth, and deployment pipelines.
• Product teams that need to embed a chat-based agent into their SaaS app with built-in session management, usage tracking, and tenant isolation.
• Agency builders who maintain multiple client agents and require per-agent billing, credential injection, and a unified observability dashboard.

Key features

Sandboxed runtimes

Every agent session runs in an isolated E2B sandbox with gVisor, booting in seconds. Secrets, API keys, and environment variables are injected securely at runtime, and token exchange auth protects browser clients.

Drop-in Agent UI

Pre-built React components for chat, streaming, and tool rendering work with Next.js out of the box. Each deployed agent automatically gets an API endpoint, so you can embed the UI or connect via the server SDK.

Full observability

Every step of every session is traced. A single dashboard shows cost per session, latency, errors, and token usage—no separate logging or monitoring setup required.

Code-first agent definition

Agents are defined in TypeScript with Zod schemas for tool inputs. You configure the model, system prompt, and MCP servers directly in code, then deploy from the CLI.

What stands out

"Building an agent takes a weekend. Running it in production takes months."

That one-liner captures the core promise: 21st Agents SDK eliminates the gap between prototyping and production. Instead of stitching together containers, auth providers, chat UIs, and billing systems, you get all of it from the start. The result is a deploy pipeline that takes seconds, not weeks, and a platform that scales from a single agent to a multi-tenant SaaS offering without rewriting infrastructure.

Worth checking out if…

You're building an AI agent and want to skip the months of infrastructure work. The SDK is especially valuable if you need sandboxed execution, built-in billing, and a polished chat UI—without managing servers, auth tokens, or observability stacks yourself.