Systematic Debugging

Systematic Debugging

Find root cause before attempting fixes. Symptom fixes mask underlying issues.

The Four Phases

Phase 1: Root Cause Investigation

Before attempting any fix:

1. Read error messages carefully — they often contain the solution. Read stack traces completely; note line numbers, file paths, error codes.
2. Reproduce consistently — exact steps, every time. If not reproducible, gather more data.
3. Check recent changes — git diff, new dependencies, config changes, environmental differences.
4. Trace data flow — where does the bad value originate? For deep call stack errors, use the root-cause-tracing skill.

For multi-component systems, add diagnostic logging at each component boundary before proposing fixes. Log what enters and exits each component.

Phase 2: Pattern Analysis

1. Find working examples — similar working code in the same codebase.
2. Compare against references — read reference implementations completely, not skimmed.
3. Identify differences — list every difference, however small.
4. Understand dependencies — settings, config, environment assumptions.

Phase 3: Hypothesis and Testing

1. Form single hypothesis — "I think X is the root cause because Y."
2. Test minimally — smallest possible change, one variable at a time.
3. Verify — didn't work? Form new hypothesis. Don't stack fixes.

Phase 4: Implementation

1. Create failing test — simplest reproduction. Use the test-driven-development skill.
2. Implement single fix — address the root cause. One change at a time. No "while I'm here" improvements.
3. Verify fix — test passes, no other tests broken, issue resolved.
4. If fix doesn't work — return to Phase 1 with new information. If 3+ fixes have failed, see below.

When 3+ Fixes Fail: Question Architecture

If each fix reveals a new problem in a different place, this is not a failed hypothesis — it's a wrong architecture.

Stop and question fundamentals:

• Is this pattern fundamentally sound?
• Are we sticking with it through inertia?
• Should we refactor architecture vs. continue fixing symptoms?

Discuss with the user before attempting more fixes. Present findings: "3 fix attempts, each revealed new problem."

Quick Reference

Red Flags — Stop and Return to Phase 1

• Proposing fixes before tracing data flow
• "Quick fix for now, investigate later"
• "I don't fully understand but this might work"
• Each fix reveals a new problem in a different place
• 3+ failed fix attempts without questioning architecture

Integration

• root-cause-tracing — for deep call stack errors (Phase 1)
• test-driven-development — for failing test case (Phase 4)
• defense-in-depth — add validation layers after fix
• verification-before-completion — verify fix before claiming success