systematic-debugging

name	systematic-debugging
description	Use when encountering any bug, test failure, or unexpected behavior. Enforces root cause investigation before proposing fixes -- no guessing, no random patches. Language-aware.

Systematic Debugging

Follow a rigorous 4-phase process to find and fix bugs. Random fixes waste time and create new bugs.

Detect the project language from file extensions and project files. Apply the matching language-specific guidance below. If the language is not listed, infer conventions from the project's existing code and community standards.

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

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

When to Use

Use for ANY technical issue:

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

Use this 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
• You don't fully understand the issue

The Four Phases

Phase 1: Root Cause Investigation

BEFORE attempting ANY fix:

1. Read Error Messages Carefully

   • Don't skip past errors or warnings
   • They often contain the exact solution
   • Read stack traces completely
   • Note line numbers, file paths, error codes

2. Reproduce Consistently

   • Can you trigger it reliably?
   • What are the exact steps?
   • Does it happen every time?
   • 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
   • Environmental differences

4. Trace Data Flow

   • Where does the bad value originate?
   • What called this with the bad value?
   • Keep tracing up until you find the source
   • Fix at source, not at symptom

5. Gather Evidence in Multi-Component Systems

   • For EACH component boundary: log what enters and exits
   • Run once to gather evidence showing WHERE it breaks
   • THEN analyze evidence to identify the failing component
   • THEN investigate that specific component

Phase 2: Pattern Analysis

1. Find Working Examples -- locate similar working code in the same codebase
2. Compare Against References -- read the reference implementation COMPLETELY
3. Identify Differences -- list every difference, however small
4. Understand Dependencies -- what other components, settings, or environment does this need?

Phase 3: Hypothesis and Testing

1. Form Single Hypothesis -- "I think X is the root cause because Y"
2. Test Minimally -- make the SMALLEST possible change to test the hypothesis
3. Verify Before Continuing -- did it work? Yes -> Phase 4. No -> form NEW hypothesis.
4. When You Don't Know -- say "I don't understand X". Don't pretend.

Phase 4: Implementation

1. Create Failing Test Case -- simplest possible reproduction
2. Implement Single Fix -- ONE change at a time
3. Verify Fix -- test passes? No other tests broken? Run full suite.
4. If Fix Doesn't Work -- STOP. Count: how many fixes have you tried?
   • If < 3: Return to Phase 1
   • If 3 fixes failed: STOP and question the architecture (step 5)
5. If 3 Fixes Failed -- escalate to architectural discussion with the user. Do NOT attempt Fix #4. Document findings in the GitHub issue and pause.

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Language-Specific Debugging -- C# / .NET

Technique	Command
Build errors	dotnet build --no-restore -- read full output
Test failures	dotnet test --no-build --verbosity normal
Verbose test	dotnet test --no-build --verbosity detailed --filter "FullyQualifiedName~<TestName>"
Format check	dotnet format --verify-no-changes

Language-Specific Debugging -- PowerShell

Technique	Command
Verbose output	Run with -Verbose
Error details	Inspect $Error[0], $Error[0].Exception, $Error[0].ScriptStackTrace
Module reload	Always Import-Module ... -Force after changes
Pester output	Invoke-Pester -Output Diagnostic

Language-Specific Debugging -- TypeScript

Technique	Command
Type errors	npx tsc --noEmit
Vitest debug	npx vitest run --reporter=verbose <file>
Playwright trace	npx playwright test --trace on

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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"
• "Add multiple changes, run tests"
• "It's probably X, let me fix that"
• "One more fix attempt" (when already tried 3)

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

Quick Reference

Phase	Key Activities	Success Criteria
1. Root Cause	Read errors, reproduce, check changes, gather evidence	Understand WHAT and WHY
2. Pattern	Find working examples, compare	Identify differences
3. Hypothesis	Form theory, test minimally	Confirmed or new hypothesis
4. Implementation	Create test, fix, verify	Bug resolved, tests pass