You cannot stop decay you cannot see.
Every other principle assumes you can tell when it is being violated. The Sensor is what makes that true. It demands that a system’s boundaries, side effects, and state be observable from the outside, without attaching a debugger and without shipping a new build to find out what happened. A system you have to redeploy in order to understand is a system that is already lying to you.
Decay is silent by default. Code rots in the dark, between the moment it is written and the moment someone finally notices the symptom. The gap between those two moments is the entropy budget, and the Sensor’s whole job is to shrink it toward zero. Per-stage timing, structured logs that name the stage and the decision, a linter that fails the build on a structural violation: these are not nice-to-haves. They are the instruments that turn invisible rot into a number, and a number is something you can act on.
When the Sensor is missing, you get a Black Box: a system that “just stopped working” with no signal underneath to say why. The fix is never to guess harder or scale the cluster. It is to restore the instruments first.
The cheapest sensor of all is a gate that fails the build. Until one exists, a structural violation is invisible and free to spread. The moment it exists, that same violation becomes a number, and a number is something you can drive to zero.
Related: The Anchor preserves the why; the Sensor makes the what observable. Its enemies are the Black Box and the Silent Fallback.