Excellent offline, mediocre live, no error anywhere
A churn model scores 0.91 AUC on a held-out set built with a point-in-time-correct feature store query. Deployed behind an API, its live decisions perform at roughly the level of the previous heuristic — a large, consistent gap with the offline number.
Feature values logged at serving time are within their expected ranges. No nulls above the usual rate. The model artefact's hash matches the one evaluated offline. Input schema validation passes on every request.
Training features were computed in a Spark job over the warehouse. Serving features come from a Redis online store populated by a separate streaming job.
The artefact is identical and the inputs validate. Give me your ranked hypotheses for the offline-online gap, and for each the comparison you would run to confirm it.
Choose how you want to be interrogated
The same case under two modes is two different exercises. If you are unsure, RCA drill is the one that trains root-cause analysis most directly.
RCA drill
A symptom, and nothing else. Find the cause and prove it.
Trains: Hypothesis generation, ranking by prior, and naming the test that discriminates.
Red team
You propose. It attacks. It is trying to make you wrong.
Trains: Defending a position under pressure, and noticing when you should concede.
Socratic
Questions only. It will never tell you the answer.
Trains: Deriving from first principles instead of retrieving from memory.
Design review
Present an architecture. Justify every trade-off you made.
Trains: Stating what you optimised and what you gave up, on purpose.
Viva
Rapid escalating questions. Ends in a graded verdict.
Trains: Answering under time pressure without hedging.
Integration
Problems that span several domains at once. The hardest mode.
Trains: Reasoning across boundaries, where the cause is in a different system to the symptom.
Before you start: you begin locked. The Mentor will give you nothing — not a nudge, not a category — until you state a specific position and the mechanism you think produces it. Four genuine attempts unlock the resolution. There is no shortcut.