Systematic Debugging

Guessing at fixes feels fast and is slow. Each guess that half-works hides the real cause a little deeper, and the third patch usually breaks something the first one touched. This skill makes you find out why before you change anything.

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

If you have not finished Phase 1, you cannot propose a fix. Not a "quick one to try", not "while we investigate". Suggesting a fix in your reply counts as proposing one.

The process scales. For a simple bug the four phases take a minute: read the error, confirm the cause, write the failing check, fix it. You never skip them, you just move through them quickly.

When to use

Any technical problem: test failures, production bugs, unexpected behavior, performance regressions, build or CI failures, integration issues, flaky tests.

Use it especially when:

• There is time pressure (emergencies make guessing tempting)
• One quick fix looks obvious
• You or someone else already tried a fix that did not work
• You do not fully understand the issue

Phase 1: Root cause investigation

Before attempting ANY fix:

1. Read the error completely. The whole message and stack trace, not the first line. Note file paths, line numbers and error codes. Errors often state the exact problem.

2. Reproduce it. Find the exact steps or command that trigger it, and check whether it happens every time. If you cannot reproduce it, gather more data (logs, instrumentation); do not guess.

3. Check what changed. git diff, recent commits, new dependencies, config and environment differences. If there is a known-good commit and a command that shows the bug, let git find the culprit: git bisect start <bad> <good> && git bisect run <repro-command>, then git bisect reset to return to your branch before you change anything.

4. Gather evidence across component boundaries. When the system has layers (CI → build → deploy, UI → API → database, CLI → config → child process), do not guess which layer breaks. Add temporary logging at each boundary, run once, and let the output show where good data turns bad:

   # Layer 1: CI job
   echo "API_TOKEN in job: $([ -n "${API_TOKEN:-}" ] && echo SET || echo UNSET)"
   # Layer 2: build script
   echo "config file: ${CONFIG_PATH:-<unset>}"; test -r "${CONFIG_PATH:-}" && echo readable
   # Layer 3: the service at startup
   node -e 'console.log("NODE_ENV=", process.env.NODE_ENV)'
   

   Log whether a secret is set, never its value. Then investigate the one layer the evidence points at.

5. Trace the data backward. When the error surfaces deep in a call stack, ask where the bad value came from, then what called that, and keep going until you reach the source. Fix at the source, not where it crashed. Full technique: references/root-cause-tracing.md.

Done when you can say what is wrong and why, backed by something you observed, not something you assumed.

Phase 2: Pattern analysis

1. Find working code in the same codebase that does something similar.
2. Read the reference completely. If you are following a pattern, a library's docs or an example, read every line before applying it. Skimming is how "I adapted it slightly" bugs are born.
3. List every difference between the working and the broken case, however small. Do not assume "that can't matter".
4. Check dependencies: the config, environment, versions and assumptions the broken code relies on.

Phase 3: Hypothesis and testing

1. State one hypothesis: "X is the root cause because Y." Be specific.
2. Test it with the smallest possible experiment: a log line, a debugger check, or a temporary change. One variable at a time. Revert any code change once it has answered the question; the real fix comes in Phase 4, after the failing test.
3. Check the result. Confirmed: go to Phase 4. Not confirmed: form a new hypothesis from what you learned. Do not stack another change on top of a failed one; revert it first.
4. When you don't know, say so. "I don't understand X yet" is a valid status. Research more or ask the user instead of pretending.

Phase 4: Implementation

1. Write a failing test first. The smallest reproduction you can automate: a unit test, or a one-off script if the project has no test framework. Run it and watch it fail for the reason you expect. If a test-driven-development skill is installed, follow it here.
2. Make one fix that addresses the root cause. No "while I'm here" improvements, no bundled refactors.
3. Verify. The new test passes, the existing tests still pass, and the original symptom is gone when you rerun the reproduction from Phase 1. If a verification-before-completion skill is installed, follow it before you claim success.
4. Clean up the temporary logging and scripts from Phase 1. Keep any check that would catch this bug earlier next time (references/defense-in-depth.md covers where to put them).
5. If the fix did not work: stop and count the attempts.
   • Fewer than 3: return to Phase 1 with what you just learned.
   • 3 or more: stop fixing. Go to the next section.

After three failed fixes: question the design

Signs the problem is the design, not a bug:

• Each fix reveals new shared state or coupling somewhere else
• Each fix needs a large refactor to work
• Each fix creates new symptoms in a different place

Do not attempt fix number four. Tell the user what you tried, what each attempt revealed, and why you think the pattern itself is unsound, and decide together whether to refactor or keep patching.

Red flags: stop and return to Phase 1

If you catch yourself thinking or writing any of these:

• "Quick fix for now, investigate later"
• "Let me just try changing X and see"
• "I'll make a few changes and run the tests"
• "It's probably X, let me fix that"
• "I don't fully understand it, but this might work"
• "Here are the likely problems:" followed by fixes, with no investigation
• "One more attempt" after two have failed

And if the user says things like "Is that actually happening?", "Stop guessing", "Will that show us anything?" or "Are we stuck?", you skipped the evidence. Go back to Phase 1.

Common rationalizations

| Excuse | Reality | |--------|---------| | "It's simple, I don't need the process" | Simple bugs have root causes too, and the process is fast for them. | | "It's an emergency, no time" | Systematic debugging is faster than guess-and-check thrashing. | | "I'll try this first, then investigate" | The first guess sets the pattern for every attempt after it. | | "I'll add the test after the fix works" | A fix you never saw fail proves nothing. | | "Several fixes at once saves time" | Then you cannot tell which one worked or which one broke something. | | "I can see the problem" | Seeing the symptom is not understanding the cause. |

When there is no root cause in the code

If the investigation shows the issue really is environmental, timing dependent or external (a flaky network, a third-party outage):

1. Write down what you investigated and ruled out.
2. Add proper handling: retry with backoff, a timeout, a clear error message.
3. Add logging so the next occurrence leaves evidence.

Be suspicious of this conclusion: before accepting it, ask what you have not checked yet, because an investigation that stopped early looks the same from the inside. For flaky tests, check for race conditions first; see references/condition-based-waiting.md.

Reporting back

When you finish, tell the user in a few lines:

• Root cause: what was wrong and why, and the evidence that showed it
• Fix: what you changed and where
• Proof: the test that failed before and passes now, plus the suite result
• Ruled out: hypotheses you tested and discarded, if any

Supporting files

• references/root-cause-tracing.md: trace a bad value backward through the call stack to the place it was created.
• references/defense-in-depth.md: add validation at several layers once the cause is fixed, so the same bug cannot come back silently.
• references/condition-based-waiting.md and references/condition-based-waiting-example.ts: replace arbitrary sleeps in tests with polling for the real condition.
• scripts/find-polluter.sh: run test files one by one to find which test creates an unwanted file or state. Run it from the project root with the path to this skill's copy, for example bash .claude/skills/systematic-debugging/scripts/find-polluter.sh packages/core/.git 'src/**/*.test.ts' (TEST_CMD sets the runner; default npm test).