A systematic debugging method

Goal

Find the cause of a defect by controlled experiments rather than by guessing and re-running, and leave behind a record that prevents repeating the search.

Prerequisites

A reproducible failure, or at least a precise description of the symptom, and the ability to run the code with instrumentation (debugger, logging, tests).

Steps

1. Reproduce. Reduce the failing case to the smallest input and shortest path that still fails. If it cannot be reproduced, gather more observations before theorising.
2. Observe precisely. Write down the exact error, stack trace, inputs, versions and environment. Distinguish what was seen from what is assumed.
3. Form a hypothesis that explains all observations, and derive a prediction: "if this is the cause, then setting X will change the outcome to Y".
4. Test the prediction with one change at a time: a breakpoint or targeted log statement, a modified input, a bisection over commits or over the input.
5. If the prediction fails, record the ruled-out hypothesis and form the next one; if it holds, confirm by removing the cause and watching the symptom disappear.
6. Fix, add a regression test, and write the cause into the commit message.

Expected result

A cause that explains every observation, a fix that removes the symptom, and a test that would fail if the cause returned.

Limits and test basis

Heisenbugs (timing, memory corruption) may change under observation; use low-impact instrumentation and statistical reproduction. The method is general practice; the cited tools illustrate two of its steps.