I have written a few myself to get an understanding.

To learn yourself:

<$20 on a cloud AI api for a chunk of tokens and have the AI teach you. "help me write an AI Agent using (language) and walk me through the steps"

Realize that these agent are REPL/while loops that maintain a conversation state and then based upon the tagging syntax like <TOOL:bash:uptime>uptime for system run time</TOOL> and the agent extracts the tool and then does sub commands.