The agent will make the check pass
Given a red signal, an agent optimises for the signal. That is not a flaw in any particular tool — it is what optimisation means, and it changes which signals are worth having.
Monitoring
Everything in The Nest tagged Monitoring.
Your monitor cannot live in the thing it monitors
A health check inside your own stack goes silent at exactly the moment it had something to say. Here are the failure domains an internal check shares with the site, and what an external one sees instead.
The deploy succeeded. That is not the same as the deploy working
A green pipeline reports that a process started. It is compatible with an empty database, a missing environment variable, a stale CDN and every real route returning 500. Only a request from outside settles it.
Knowing before your customers do is the whole product
Monitoring is usually justified as insurance against downtime. The better argument is commercial: the gap between a customer telling you your site is broken and you telling them is a gap they can feel.
Diagnose before you deploy, not after
Redirect chains, headers, certificate chains, robots policy and sitemap validity are all observable on a staging hostname, before DNS points anywhere. That is where the checks should run first.
Your agent built a single-page app and the AI crawlers see an empty div
Ask an agent for a website and you will usually get a client-rendered one. The AI crawlers largely do not execute JavaScript, so what they store is an empty div — and one curl command will tell you whether that is your page.