Custom Agents are sometimes complicated because of the flexibility that we provide for our customers to do in depth vulnerability assessment.  Therefore we try to make the templates that we ship as flexible as possible.  But sometimes you just want to perform a simple check and the template is a little overwhelming.  We have one client that wanted a simple agent to look through all of their web pages to ensure that their privacy statement was on every session and create a vulnerability if it was missing.  I have created and attached a very simple agent that does a similar task and does it in only a few lines of code.  It simply goes through every session and if it finds a response status code of 500 (server error), it flags it as vulnerable.  This agent should be a good starting point for your simple agent needs.