1 min lesson
The fifth class is the AI-native one
Connect "The fastest debugging question is 'which layer owns this'?" to the decision you would make.
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The fifth class is the AI-native onemodel regression
The class a non-AI QA background will miss entirely: a model regression. A model upgrade or a backend routing change silently shifts behavior, diff quality or latency while the app version is unchanged. Nothing crashes. Users just feel that “it got dumber,” and your only tell is a behavior change correlated with a model swap rather than a release.
Every failure report should be sorted into client, context/retrieval, agent-orchestration, network or model. The fastest debugging question is “which layer owns this?” A hallucinated API is model/generation; an ignored file is context/retrieval; a streaming stall is network or backend. The layer tells you which logs to pull and which team to hand it to.
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Full explanation
Which Layer Owns the Bug
Interactive diagram. Tab through its regions; each focused region shows its detail in the panel below.
Name the layer before you investigate - it points you at the right logs and the right team.
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Optional practice
Practice: The fifth class is the AI-native one
QA user says Cursor “ignored the file I had open and gave me a totally generic answer.” Which failure class is most likely and what do you check first?