Systematic Debugging

Systematic Debugging

Overview

Random fixes waste time and create new bugs. Quick patches mask underlying issues.

Core principle: ALWAYS find root cause before attempting fixes. Symptom fixes are failure.

Announce at start: "I'm using the systematic-debugging skill to investigate this issue."

The Iron Laws

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

If you haven't completed Phase 1, you cannot propose fixes.

NO SHORTCUTS. NO WORKAROUNDS. FIX THE ROOT CAUSE.

Violating the letter of this rule is violating the spirit of debugging.

Forbidden Shortcuts

These are NEVER acceptable fixes:

TypeScript/JavaScript:

.NET/C#:

Both:

If you find yourself reaching for any of these, STOP.

Return to Phase 1. Find the root cause. Fix it properly.

When to Use

Use for ANY technical issue:

• Test failures
• Bugs in production
• Unexpected behavior
• Performance problems
• Build failures
• Integration issues

Use ESPECIALLY when:

• Under time pressure (emergencies make guessing tempting)
• "Just one quick fix" seems obvious
• You've already tried multiple fixes
• Previous fix didn't work

The Four Phases

You MUST complete each phase before proceeding to the next.

Phase 1: Root Cause Investigation

BEFORE attempting ANY fix:

1. Read Error Messages Carefully

   • Don't skip past errors or warnings
   • Read stack traces completely
   • Note line numbers, file paths, error codes

2. Reproduce Consistently

   • Can you trigger it reliably?
   • What are the exact steps?
   • If not reproducible → gather more data, don't guess

3. Check Recent Changes

   • What changed that could cause this?
   • Git diff, recent commits
   • New dependencies, config changes

4. Gather Evidence in Multi-Component Systems

   For .NET/React stack, check each layer:

   # Frontend
   echo "=== Browser Console ==="
   # Check DevTools console for errors
   
   # API Layer
   echo "=== API Response ==="
   curl -v http://localhost:5000/api/endpoint
   
   # Backend Logs
   echo "=== .NET Logs ==="
   # Check Serilog output
   
   # Database
   echo "=== Database State ==="
   # Check PostgreSQL for data issues
   

5. Trace Data Flow

   • Where does bad value originate?
   • Trace backward through call stack
   • Fix at source, not at symptom

Phase 2: Pattern Analysis

1. Find Working Examples

   • Locate similar working code in same codebase
   • What works that's similar to what's broken?

2. Compare Against References

   • Check instruction files in .github/instructions/ for common mistakes
   • Read the relevant instruction file completely

3. Identify Differences

   • What's different between working and broken?
   • List every difference, however small

Phase 3: Hypothesis and Testing

1. Form Single Hypothesis

   • State clearly: "I think X is the root cause because Y"
   • Be specific, not vague

2. Test Minimally

   • Make the SMALLEST possible change to test hypothesis
   • One variable at a time
   • Don't fix multiple things at once

3. Verify Before Continuing

   • Did it work? Yes → Phase 4
   • Didn't work? Form NEW hypothesis
   • DON'T add more fixes on top

4. When You Don't Know

   • Say "I don't understand X"
   • Don't pretend to know
   • Don't guess and hope
   • Ask for help or research more

Phase 4: Implementation

1. Create Failing Test Case

   • Follow TDD: write test BEFORE fix
   • Test must fail without the fix
   • Commit: test(<scope>): add test for <issue>

2. Implement Single Fix

   • Address the root cause identified
   • ONE change at a time
   • Commit: fix(<scope>): <description>

Scopes: backend, frontend, api, db, auth, tests, docs

3. Verify Fix

   • Test passes now?
   • No other tests broken?
   • Issue actually resolved?

4. If 3+ Fixes Failed: Question Architecture

   Pattern indicating architectural problem:

   • Each fix reveals new shared state/coupling
   • Fixes require "massive refactoring"
   • Each fix creates new symptoms elsewhere

   STOP and discuss with user before attempting more fixes

Red Flags - STOP and Follow Process

If you catch yourself thinking:

• "Quick fix for now, investigate later"
• "Just try changing X and see if it works"
• "It's probably X, let me fix that"
• "I don't fully understand but this might work"
• "Let me just comment this out for now"
• "I'll add @ts-ignore to make it compile"
• "I'll disable this lint rule"
• "I'll use any type here"
• Proposing solutions before tracing data flow
• Reaching for suppressions instead of fixes

ALL of these mean: STOP. Return to Phase 1.

User Signals You're Doing It Wrong

Watch for these redirections from the user:

• "Is that not happening?" → You assumed without verifying
• "Will it show us...?" → You should have added evidence gathering
• "Stop guessing" → You're proposing fixes without understanding
• "Go deeper" → You're treating symptoms, not root cause
• "Why did you comment that out?" → You took a shortcut
• "That's a workaround, not a fix" → Return to Phase 1

When you see these: STOP. Return to Phase 1.

Common Rationalizations

Quick Reference

Supporting Techniques

These techniques are part of systematic debugging and available in this directory:

Root Cause Tracing

When error is deep in call stack:

1. Find immediate cause
2. Ask: "What called this?"
3. Keep tracing up
4. Fix at the SOURCE, not the symptom

See root-cause-tracing.md for detailed process with .NET/React examples.

Defense-in-Depth

After finding root cause, add validation at EVERY layer:

1. Entry point (Controller/Form)
2. Business logic (Service)
3. Environment guards
4. Debug instrumentation

See defense-in-depth.md for the 4-layer pattern.

Condition-Based Waiting

For flaky tests, replace arbitrary delays with condition polling:

// ❌ await Task.Delay(500);
// ✅ await WaitForAsync(() => GetResult(), r => r != null, "result");

See condition-based-waiting.md for implementation patterns.

Integration with Other Skills

After debugging:

• verification — Verify fix before claiming success
• layered-review — Review changes before finalizing

Related skills:

• test-driven-development — Write failing test before fix (Phase 4)
• executing-plans — TDD enforcement during implementation
• pressure-test-scenarios — Scenarios 1, 10, 11, 12 test debugging discipline

Instruction files to consult:

• dotnet.instructions.md — Common .NET debugging patterns
• react.instructions.md — Common React debugging patterns
• testing-dotnet.instructions.md — xUnit/FluentAssertions patterns
• testing-playwright.instructions.md — E2E test debugging