/systematic-debugging — Root Cause Before Fix

Orientation

Use when: root cause is unknown and premature fixes keep failing -- enforces observe -> hypothesize -> verify before touching code. Don't use when: root cause is already known (use /marshal to implement the fix); the bug is shallow and the stack trace is enough (use /do).

Protocol

Phase 1: OBSERVATION & REPRODUCTION

1. Read the error message, stack trace, or bug description
2. Reproduce the issue:
   • If it's a type error: run typecheck and read the full error
   • If it's a runtime error: identify the triggering conditions
   • If it's a behavioral bug: document expected vs actual behavior
3. Isolate the failing component/function:
   • What file? What function? What line?
   • What are the inputs when it fails?
   • Does it fail consistently or intermittently?

Output: A clear problem statement: "{Component} does {X} when it should do {Y}, triggered by {condition}"

Phase 2: HYPOTHESIS & VERIFICATION

1. Formulate up to 3 hypotheses for WHY the bug exists:
   • H1: {most likely cause} — because {evidence}
   • H2: {second candidate} — because {evidence}
   • H3: {third candidate} — because {evidence}
2. For each hypothesis, define a verification step:
   • Add a console.log / diagnostic read / breakpoint
   • Check a specific value at a specific point
   • DO NOT change any logic yet — only observe
3. Run the verification:
   • Which hypothesis was confirmed?
   • Which were eliminated?
   • If none confirmed: formulate new hypotheses with the new information

CRITICAL: Do not skip this phase. Do not "just try" a fix. Verify first.

Phase 3: ROOT CAUSE ANALYSIS

Once a hypothesis is confirmed:

1. Explain WHY the bug happens, not just WHERE:
   • Trace the data flow backward from the symptom to the source
   • Identify the specific incorrect assumption or logic error
   • Document the causal chain: "A calls B with X, B assumes X > 0, but A passes -1 when {condition}"
2. Check for related occurrences:
   • Is this pattern used elsewhere? Could the same bug exist in similar code?
   • Is there a systemic issue (e.g., missing validation at a boundary)?

Output: Root cause statement: "The bug occurs because {cause}. This happens when {trigger}."

Phase 4: IMPLEMENTATION

1. Write a failing test case that reproduces the bug (if test framework exists)
2. Apply the minimal fix — change only what's necessary to resolve the root cause
3. Verify the fix:
   • Test case now passes
   • Typecheck passes
   • No regressions in related functionality
4. If the root cause analysis revealed related occurrences, fix those too

Emergency Stop Rule

If a fix fails TWICE: STOP.

Do not try a third guess. The root cause analysis was wrong. Either:

• Go back to Phase 2 with new hypotheses
• Ask the user for more context about the system's intended behavior
• Check if there's a higher-level architectural issue

Three failed fixes in a row means you're guessing, not debugging.

What This Skill Prevents

• Shotgun debugging — changing random things until the error goes away
• Symptom fixing — patching the output without understanding the cause
• Fix cascades — one bad fix creating three new bugs
• Silent regressions — fixing one path while breaking another

Contextual Gates

Disclosure: "Debugging [symptom]. Phases 1-3 read-only; will confirm before applying fixes in Phase 4." Reversibility: amber — Phase 4 applies targeted fixes to source files; undo with git checkout on modified files. Phases 1-3 are read-only. Trust gates:

• Any: investigation (Phases 1-3).
• Familiar (5+ sessions): applying fixes (Phase 4).

Quality Gates

• A clear problem statement exists before any hypothesis is formed
• At least one hypothesis was verified (not assumed) before the fix was written
• The fix addresses the root cause, not just the symptom
• Typecheck passes after the fix with no new errors
• If related occurrences were found, they were fixed or documented

Fringe Cases

Bug is intermittent: Document the triggering conditions as precisely as possible. Reproduce it at least once before forming hypotheses. If it can't be reproduced, stop at Phase 1 and ask for more context.

Two fix attempts have already failed: Invoke the Emergency Stop Rule. Return to Phase 2 with new hypotheses. Do not try a third guess without re-reading the relevant code.

No test framework exists: Skip the "write a failing test" step in Phase 4. Verify the fix manually and document how to reproduce the original bug for future reference.

Error is in a dependency or generated file: Document the root cause but do not modify the dependency. Propose a workaround in the consuming code instead.

Exit Protocol

---HANDOFF---
- Bug: {problem statement}
- Root cause: {one-line cause}
- Fix: {what was changed}
- Verified: {typecheck + tests passing}
- Related: {any similar patterns found and fixed}
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