21st: Infrastructure and UI building blocks for the agentic internet

We're in YC

21st is backed by Y Combinator. Excited for what's ahead!

Home

UI Components

Built-in UI, chat history, spend limits, sandbox execution, and observability. Works with Claude and OpenAI.

E2B Sandboxes

Isolated VM per session

Streaming

Token-by-token via SSE

Auth & Limits

Tokens, rate limits, spend caps

Observability

Session replay & traces