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Openai·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Apr 2026Remote

Summary

OpenAI ML Engineer interview with a pretty gnarly debugging exercise plus a full classifier pipeline task. Not a vibe-check round at all, very hands-on from the start.

Questions Asked (2)

Q1

You're given a transformer-based model with four failing unit tests. Two bugs are documented, two are new. Find and fix all four so the model trains and evaluates correctly.

Root Cause AnalysisTechnical Trade-offsAlgorithms & Data Structures
Author's notes

The two known bugs were almost a trap in how easy they felt.

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AI HintsAI Generated

Suggested Approach

Start by running the test suite to see which tests fail and read the error messages to understand the symptoms. Then systematically debug each failure, starting with the documented bugs, and use a divide-and-conquer approach to isolate the new bugs. Finally, fix each bug and re-run tests to ensure all pass and the model trains and evaluates correctly.

Pro tip: Demonstrate a methodical debugging process: reproduce the failure, form a hypothesis, test it, and fix. Also, consider edge cases like tensor shapes, data types, and device placement, which are common sources of bugs in transformer models.

1. Run tests and gather information

Execute the test suite to identify which tests fail and capture the full error messages and stack traces. Read the documentation for the two known bugs to understand their expected fixes.

2. Prioritize and fix documented bugs

Address the two documented bugs first, as their fixes are likely straightforward and may resolve some test failures. Verify each fix by running the specific failing test.

3. Debug new bugs systematically

For remaining failures, use a divide-and-conquer approach: isolate the failing component (e.g., attention, layer norm, loss function) by writing minimal reproduction scripts or adding debug prints. Check tensor shapes, data types, and device consistency.

4. Implement and validate fixes

Apply fixes for the new bugs, ensuring they align with the model's architecture and expected behavior. Re-run all tests to confirm all pass and the model trains and evaluates correctly.

5. Reflect and prevent future bugs

Summarize the root causes and consider adding unit tests or assertions to catch similar issues. Discuss trade-offs if multiple solutions exist.

Key Points to Mention

  • Systematic debugging: reproduce, isolate, hypothesize, test, fix
  • Common transformer bugs: incorrect attention masks, wrong tensor shapes, improper layer normalization, missing residual connections
  • Use of debugging tools: print statements, breakpoints, tensor shape checks, gradient checking
  • Importance of running tests after each fix to avoid regressions
  • Consideration of edge cases: variable sequence lengths, padding, device placement (CPU/GPU)
  • Trade-offs between quick fixes and robust solutions, and documenting fixes for team knowledge

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q2

Given a labeled dataset, write code to train a classifier, analyze class balance and feature distributions, and report key performance metrics.

Product Analytics & MetricsA/B Testing & Experimentation
Author's notes

Felt more comfortable here.

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AI HintsAI Generated

Suggested Approach

Start by framing the problem: clarify the dataset, target, and evaluation goals. Then outline a structured pipeline: data loading, EDA (class balance, feature distributions), preprocessing, model training with cross-validation, and evaluation using appropriate metrics. Emphasize the importance of handling class imbalance and selecting metrics aligned with business objectives.

Pro tip: Always discuss the trade-offs between different metrics (e.g., precision vs. recall) and how they relate to the product's goals. Mention that you would set up a baseline model first to establish a performance benchmark before iterating.

1. Understand the Data and Problem

Load the dataset, inspect its structure, and clarify the prediction task, target variable, and success criteria. Identify potential issues like missing values or data leakage.

2. Exploratory Data Analysis (EDA)

Analyze class balance using value counts and visualize feature distributions with histograms or box plots. Check for correlations and outliers that might affect modeling.

3. Preprocess and Split Data

Handle missing values, encode categorical variables, scale numerical features, and split data into training and validation sets (or use cross-validation). Address class imbalance with techniques like SMOTE or class weights if needed.

4. Train and Evaluate Models

Train a baseline model (e.g., logistic regression) and a more complex model (e.g., random forest or gradient boosting). Evaluate using metrics like accuracy, precision, recall, F1-score, and AUC-ROC, considering the class balance.

5. Interpret and Communicate Results

Report key metrics, feature importances, and insights from EDA. Discuss how the model performs relative to business goals and suggest next steps for improvement.

Key Points to Mention

  • Class imbalance: discuss its impact and mitigation strategies (resampling, class weights, appropriate metrics).
  • Feature distributions: use visualizations to detect skewness, outliers, and need for transformations.
  • Model selection: start simple, then try complex models; use cross-validation for reliable performance estimates.
  • Evaluation metrics: choose metrics beyond accuracy (precision, recall, F1, AUC-ROC) based on problem context.
  • Code structure: write modular, reproducible code with clear comments and functions.
  • Business alignment: tie model performance to product metrics and potential A/B testing.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.