21tunnel is a developer tool that provides secure tunnels for AI agents and localhost services. It allows you to mint a master key once, then delegate scoped, short-lived child keys to your AI assistant for autonomous operation. With instant cascade revoke, you can kill all associated tunnels and keys with one click. It supports HTTP, TCP, and SSH protocols, offers custom domains, edge authentication, and a live request inspector. Built for AI agents like Claude Code, Cursor, Aider, and Devin, it also serves as an open-source ngrok alternative.