Reproduction first

Reproduction first

A fix with no reproduction is a wish. You change a line, the symptom is absent this run, and you never learn whether you fixed the defect or merely failed to provoke it. A repro that fails on command turns debugging into a loop with a red light that you drive to green: build that light before you touch anything.

Method

1. Harvest the exact conditions from the report. Input values, build version, OS, locale, environment variables, data state, and the click-by- click sequence. Ask for the failing input itself, not a prose summary of it.
2. Reproduce once in the crudest way that works. Full app, real data, whatever it takes to watch the failure with your own eyes. You cannot minimize a failure you have never actually seen happen.
3. Nail down every source of nondeterminism. Seed the RNG, freeze the clock with a library like freezegun or libfaketime, fix timezone and locale, disable retries. If it fails one run in twenty, wrap it in a loop of 100 and confirm a stable failure rate before trusting any change.
4. Shrink the input by halving. Delete half the rows, half the config, half the steps, and rerun. If it still fails, cut again; if it recovers, the half you removed held the trigger, so restore it and cut the other side.
5. Amputate dependencies until only the fault is left. Swap the database for a literal, the network call for a recorded fixture, the framework for a plain function. The target is the smallest script that still fails.
6. Freeze the repro as a failing test. Encode the expected output as an assertion. The reproduction is now permanent, runs in seconds, and becomes the regression guard the instant it flips green.
7. Confirm it fails for the right reason. Read the assertion message. A test that trips on a typo in its own setup rather than on the bug will lie to you the moment you rely on it.

Checks

• Can you make the failure appear ten times out of ten from a cold start?
• Is the reproduction small enough to paste into a ticket in full?
• Does the failing message name the real defect, not a fixture mistake?

Boundaries

Some faults need production scale or data you cannot mirror locally. Capture the tightest repro you can, write down what stays unreproducible, and lean on heisenbugs tactics for the timing-dependent remainder.