Sensor
Also called Feedback control
A control that acts after the agent does, observing what it produced so it can correct itself: a test run, a type check, a screenshot, a review.
Sensors are what make a guide trustworthy. Without one, you can't tell whether the agent followed the instruction or only said so.
Say it to your agent
“You have a guide saying all database access goes through src/db/. Now add the sensor: a test that fails if any file outside src/db/ imports the database client.”