Sub-Skill: Debugging

Purpose: Systematic techniques for reproducing failures, isolating variables, and finding root causes without guessing. Complements skills/tdd/SKILL.md — a failing test is the best reproduction case.

Rules

Reproduction

1. Reproduce before fixing. Before changing any code, confirm you can trigger the failure on demand. A fix applied to an unreproduced bug is a guess, not a solution. Reference: ERR-2026-018
2. Capture the minimal reproduction. Reduce the failing scenario to the smallest input, fewest dependencies, and shortest code path that still exhibits the symptom. Smaller reproductions expose the cause faster and prevent regression.
3. Write the reproduction as a test. Encode the reproduction case as an automated test before touching production code. This locks in the failure signal and prevents silent regression. See skills/tdd/SKILL.md.

Isolation

1. Change one variable at a time. Never modify multiple suspects simultaneously. Each change must be independently observable so you know which variable caused the change in behaviour.
2. Use binary search on the call stack. When the failure source is unknown, bisect: confirm the bug is present at the midpoint of the execution path, then recurse into the half that contains it. Avoid reading every line top-to-bottom.
3. Prefer structured logging over print-debugging. Add log statements at decision boundaries with structured key-value pairs (not free-form strings). Structured output is grep-able, diffable, and removable without side effects.

Hypothesis and Root Cause

1. State a falsifiable hypothesis before each experiment. Write down: "I believe X causes Y because Z. If I change X, Y should disappear." Run the experiment. If the hypothesis is wrong, update your model before the next experiment.
2. Distinguish symptom from cause. The error message or stack trace is the symptom. The root cause is the incorrect assumption, missing guard, or wrong state that produced it. Never stop at the symptom — trace back to the decision that allowed the bad state to exist.
3. Do not fix symptoms in isolation. Patching the symptom without addressing the root cause produces a second bug that hides the first. Ensure the fix makes the root cause impossible, not just the observed symptom unlikely.

Verification

1. Verify the fix with the reproduction test. After applying the fix, confirm the reproduction test now passes and no previously passing tests regressed. A fix that breaks other tests has introduced a new bug.
2. Ensure the fix is observable. After merging, confirm the symptom is gone in the target environment using the same signal (log line, metric, test) that first revealed the bug. Never declare a bug fixed without an observable confirmation.

See also

• skills/tdd/SKILL.md — write the reproduction as a failing test first
• skills/code-quality/SKILL.md — narrow exception catches to avoid masking bugs (ERR-2026-016)
• skills/error-log/SKILL.md — errors that motivated rules in this skill

Notes

• Binary search applies to time too. Use git bisect to find the commit that introduced a regression; do not read the full diff manually.
• Logging is temporary. Remove or gate debug log statements behind a flag before merging. Permanent debug noise degrades signal quality for the next investigation.