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Meta·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
May 2026

Summary

Meta MLE debug round, no AI tools allowed. One focused problem about a broken recommendation function. Pretty contained scope but the constraint of no assistance made it feel higher stakes than it probably was.

Questions Asked (1)

Q1

A `valid_recommend(user, candidates)` function is supposed to validate a friend recommendation list for a given user, but an existing test is failing. Find the bug and fix it with a minimal, targeted change.

Algorithms & Data StructuresRoot Cause Analysis
Author's notes

The fix itself is small, like a one-liner, but I spent way too long second-guessing myself because I kept looking for something more complex.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the expected behavior of valid_recommend and the test's failure mode, then systematically trace the code to isolate the discrepancy. Form a hypothesis about the root cause, verify it with a minimal reproduction, and apply a targeted fix that addresses the bug without altering unrelated logic.

Pro tip: Demonstrate production maturity by discussing how you'd add a regression test and consider edge cases like empty candidate lists or duplicate recommendations, showing you think beyond the immediate fix.

1. Understand the contract and failure

Review the function's docstring, type hints, and the failing test to clarify expected inputs, outputs, and the exact assertion that fails. Run the test to observe the error message and stack trace.

2. Trace and isolate the bug

Step through the function with the test's inputs, either mentally or with a debugger, to identify where the actual behavior diverges from the expected. Pay attention to boundary conditions, data types, and control flow.

3. Form and verify a hypothesis

Articulate a specific root cause (e.g., off-by-one, incorrect comparison, missing null check) and confirm it by writing a minimal reproduction or adding temporary logging. Ensure the hypothesis explains all observed symptoms.

4. Apply a minimal, targeted fix

Modify only the code necessary to correct the bug, avoiding refactoring or style changes. Re-run the failing test and the full test suite to confirm the fix and check for regressions.

5. Reflect and prevent recurrence

Consider adding a regression test for the specific bug and discuss potential edge cases that might still be uncovered. Briefly explain how you'd monitor or validate the fix in production.

Key Points to Mention

  • Reproducing the failure locally and reading the test's assertion to understand the expected vs. actual behavior.
  • Using debugging techniques such as print statements, breakpoints, or unit test isolation to pinpoint the faulty line.
  • Checking for common pitfalls: off-by-one errors, incorrect variable scope, type mismatches, and unhandled edge cases (empty lists, None values).
  • Ensuring the fix is minimal and does not introduce side effects or break other tests.
  • Adding a regression test to capture the specific scenario and prevent future regressions.
  • Communicating the root cause clearly and explaining why the fix works, demonstrating strong root cause analysis skills.

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