The OSI model, find the layer the fault lives on

A service ships with a contract, GET /health on 127.0.0.1:8080 answers 200 with body ok. It is broken twice, at two different layers, and the lower fault hides the upper one. Use the OSI model the way operators actually use it, to route each symptom to a layer, pick the tool that probes that layer, and fix what you find.