DebugMidBackendAuto-graded

Fix a test that fails randomly in CI

Separates engineers who find the real cause of an unpredictable failure from those who hide it behind a retry.The contributor-leaderboard suite is green on your laptop and red about half the time in CI, and re-running the job makes it pass — which is exactly how a pipeline rots. Exactly one test is flaky, and it is flaky for a real reason: an anti-pattern in the code under test, not bad luck. Find the cause and make the suite deterministic enough to pass fifty runs out of fifty. How you are allowed to fix it is the point: you may not skip the test or weaken its assertion.

25 min

python

Start lab

You're done when

The suite passes repeatedly — determinism holds across many consecutive runs, not one lucky pass.
The fix lands in the source under test, addressing the real non-determinism rather than the test that exposed it.

What it measures

Debugging and profilingTracking an intermittent failure to its actual cause when the reproduction itself is unreliable.

Automated testingUnderstanding that a flaky test is usually reporting a real defect, and that silencing it destroys the signal.

Fixing the cause, not the symptomResisting the skip or the loosened assertion, which is what the constraint here is designed to detect.

Your environment

Terminal + editorA full shell in the browser

Claude CodePre-authenticated, no key setup

Isolated microVMYours alone, wiped afterwards

Start this lab

25 min

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