Debugging

Debugging

A disciplined debugging loop. The goal is understanding, then fixing — not pattern-matching a fix and hoping.

When to use

• "X is broken / not working / throwing"
• "This test is failing"
• "Why does this return the wrong value?"
• "Find the bug in …"

The loop

1. Reproduce  →  2. Isolate  →  3. Hypothesize  →  4. Fix  →  5. Verify  →  6. Prevent

Never skip steps 1–3. A fix without reproduction is a guess.

1. Reproduce

Get the bug to happen on demand before changing anything.

• Ask for: exact command/input, expected vs actual, stack trace, environment
• Run it yourself. If you can't reproduce locally, that is the first finding.
• Capture the minimal reproduction (smallest input that triggers it)

2. Isolate

Shrink the problem space.

• Bisect by location — comment out / short-circuit halves until you find the failing region
• Bisect by history — git bisect if it used to work
• Bisect by data — strip input down to the smallest failing case
• Read the actual stack trace top-to-bottom; don't skim

3. Hypothesize

Form a specific, testable theory of what's wrong before touching code.

• Write it down: "I think X happens because Y."
• Predict what you'd see if the theory is correct (a value, a log line, a call order)
• Verify the prediction before writing the fix — print, log, breakpoint, REPL

4. Fix

Now, and only now, change code.

• Smallest change that addresses the root cause
• If you're tempted to add defensive code "just in case," stop — that's a sign you haven't understood the bug
• Note any related code paths with the same flaw

5. Verify

Prove the fix works and didn't break anything else.

• Re-run the minimal repro → must pass
• Run the existing test suite → must pass
• If there was no test covering this bug, write one now (see step 6)

6. Prevent regression

A bug that came back is twice as expensive.

• Add a regression test that fails without the fix and passes with it
• If the bug class is recurring (e.g. null in a specific shape), consider a type/lint rule
• Note any sibling code paths that should be patched the same way

Output format

When done, report:

## Bug: <short title>

**Root cause:** one or two sentences. What was actually wrong.

**Trigger:** minimal reproduction.

**Fix:** what changed and why this is the right level to fix at.

**Verification:** tests run, new tests added.

**Related risks:** other code paths with the same shape (or "none found").

Anti-patterns

• ❌ "I think this might work, let me try it" → that's not debugging, that's slot-machining
• ❌ Adding try/catch to make an error go away without understanding it
• ❌ Fixing symptoms (the wrong value displayed) instead of cause (where it became wrong)
• ❌ Skipping the regression test because "it's obvious now"
• ❌ Changing five things at once and not knowing which one fixed it

Useful commands

# Git bisect — find the commit that introduced the bug
git bisect start
git bisect bad                       # current is broken
git bisect good <known-good-commit>
# (run repro, then git bisect good/bad until it converges)
git bisect reset

# Re-run only the failing test in watch mode (adjust per stack)
# pytest: pytest path/to/test.py::test_name -x -vv
# vitest: npx vitest run path/to/test --reporter=verbose
# phpunit: vendor/bin/phpunit --filter testName

# Show recent changes to a file
git log -p --follow path/to/file