5 open source auth libraries that actually handle AI agents (2026)

I hit this problem a few months back. About 50 agents, each talking to different MCP tool servers, and I couldn't answer a basic question: which agent did what, and who said it could? Most auth libraries didn't help. They were built for humans logging into web apps. Sessions, cookies, redirect flows. My agents don't have browsers. They need their own tokens, scoped permissions, and when one spins up a sub-agent, I need some way to track that chain. Without it you're just staring at logs guessin