Your AI coding agent needs a public URL for the preview it just built. Most setup guides tell you to paste your API token into the agent's environment. That token can open unlimited tunnels, never expires, and could be exposed if the agent leaks it. There is a safer pattern that has been used across the industry for years.
Full disclosure: I work with 21tunnel, one of the tunneling services that implements this pattern. While the examples use 21tunnel, the security model applies to any tunneling service.
AI coding agents like Claude Code, Cursor, Aider, and Devin increasingly need to expose localhost so developers can test webhooks, preview applications, or share work with teammates. The real challenge isn't tunneling—it's securely delegating permissions to an autonomous agent.
TL;DR
Don't give your AI agent a permanent API token.
