At the core, they're really very simple [1]. Run LLM API calls in a loop with some tools.

From there, you can get much fancier with any aspect of it that interests you. Here's one in Bash [2] that is fully extensible at runtime through dynamic discovery of plugins/hooks.

[1] https://ampcode.com/notes/how-to-build-an-agent

[2] https://github.com/wedow/harness