Complete Mediation
Every access goes through the check, with no path around it.
One of the protection principles Saltzer and Schroeder set out in 1975, and it still holds unchanged.
The idea is simple and uncomfortable: if a single route skips the check, all the others become decoration. Locking the front door while the window stays open does not help — whoever wants in uses the window, and whoever got stopped at the door learns to use it too.
In practice it shows up in an irritating way: you try the same thing three different ways and get blocked all three times. Living through it, it feels like obstinacy. Looking at the design, it is the principle working.
Read the article: Prompt injection already had a name in 1988