Once it is out, can you tell what it is doing? If you can't, you shipped it blind.
Observability
Also called o11y, Monitoring
How well you can work out what a running system is doing from the outside, using what it emits, including for problems you didn't predict.
It decides whether you find out from your signals or from your users. It also changes what an agent can do: if it can read logs and traces, it can check its own work, and you aren't the one relaying symptoms.
Say it to your agent
“Before you ship this, give me a way to see it in production: log one JSON line per request with route, status, duration and release, and tell me the exact query that would show the new code failing. Run that query on staging and show me its output.”