Systematic Debugging — Find the Root Cause

Iron Law: NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST.

Fixing symptoms creates whack-a-mole debugging. Every fix that doesn't address root cause makes the next bug harder to find.

Related skills: review | qa | careful

Phase 1: Root Cause Investigation

Gather context before forming any hypothesis.

1. Collect symptoms: Read error messages, stack traces, reproduction steps. If not enough context, ask ONE question at a time.
2. Read the code: Trace the code path from symptom to potential cause.

3. Check recent changes:

   git log --oneline -20 -- <affected-files>

   Was this working before? What changed? A regression means the root cause is in the diff.

4. Reproduce: Can you trigger the bug deterministically? If not, gather more evidence.

Output: "Root cause hypothesis: ..." — a specific, testable claim about what is wrong and why.

Phase 2: Pattern Analysis

Check if this bug matches a known pattern:

Also check TODOS.md for related known issues and git log for prior fixes in the same area — recurring bugs in the same files are an architectural smell.

Phase 3: Hypothesis Testing

Before writing ANY fix, verify your hypothesis.

1. Confirm: Add a temporary log or assertion at the suspected root cause. Reproduce. Does evidence match?
2. If wrong: Return to Phase 1. Gather more evidence. Do not guess.
3. 3-strike rule: If 3 hypotheses fail, STOP. Ask the user:
   • A) Continue investigating with a new hypothesis
   • B) Escalate for human review
   • C) Add logging and catch it next time

Red flags — slow down:

• "Quick fix for now" — there is no "for now." Fix it right or escalate.
• Proposing a fix before tracing data flow — you're guessing.
• Each fix reveals a new problem elsewhere — wrong layer, not wrong code.

Phase 4: Implementation

Once root cause is confirmed:

1. Fix the root cause, not the symptom. Smallest change that eliminates the actual problem.
2. Minimal diff. Fewest files, fewest lines. Resist refactoring adjacent code.
3. Write a regression test that fails without the fix and passes with it.
4. Run the full test suite. No regressions.
5. If fix touches >5 files: Flag the blast radius and ask before proceeding.

Phase 5: Verification & Report

Reproduce the original bug and confirm it's fixed. This is not optional.

DEBUG REPORT
════════════════════════════════════════
Symptom:         [what the user observed]
Root cause:      [what was actually wrong]
Fix:             [what was changed, file:line]
Evidence:        [test output showing fix works]
Regression test: [file:line of the new test]
Status:          DONE | DONE_WITH_CONCERNS | BLOCKED
════════════════════════════════════════

Important Rules

• 3+ failed fix attempts → STOP and question the architecture.
• Never apply a fix you cannot verify.
• Never say "this should fix it." Verify and prove it.
• If fix touches >5 files → ask first.