It is the whole operation in miniature, with every link in the chain simple enough to follow.
A spare Raspberry Pi runs an open-source uptime monitor, privately, checking three websites once a minute. When a site fails or recovers, the message goes to the same front-door workflow that handles everything else, and arrives on my phone. Delivery was confirmed end to end.
Detection is deterministic. The agent's role is routing and presentation, which is exactly the role it should have in a monitoring chain.
Part of a larger story: The agent operation: a front door, specialists and gates.