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Vibe codingHardvc-g270
Subject Ai code reviewLevel Senior–Staff~20 minCommon in Algorithms & data structures interviewsIndustries Software development

Question

An AI built this stock-signal backtest validation using k-fold CV. Backtest Sharpe is excellent; live trading loses money. Explain the leakage the shuffle introduces.

python
from sklearn.model_selection import cross_val_score, KFold # df sorted by date; features include rolling returnsX = df[feature_cols].valuesy = df['next_day_up'].valueskf = KFold(n_splits=5, shuffle=True, random_state=0)score = cross_val_score(clf, X, y, cv=kf).mean()print('CV accuracy:', score)
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Treat the AI’s output as a draft to verify, not an answer to trust. Name the specific flaw and the input that triggers it, say how you’d catch it — tests, edge cases, reading critically — and how you’d re-prompt or decompose to get it right.

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