systematic-debugging

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# 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.

**Violating the letter of this process is violating the spirit of debugging.**

## 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

**Don't skip when:**
- Issue seems simple (simple bugs have root causes too)
- You're in a hurry (rushing guarantees rework)
- Someone wants it fixed NOW (systematic is faster than thrashing)

## 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
- They often contain the exact solution
- Read stack traces completely
- Note line numbers, file paths, error codes

**Action:** Use `read_file` on the relevant source files. Use `search_files` to find the error string in the codebase.

### 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

**Action:** Use the `terminal` tool to run the failing test or trigger the bug:

```bash
# Run specific failing test
pytest tests/test_module.py::test_name -v

# Run with verbose output
pytest tests/test_module.py -v --tb=long
```

### 3. Check Recent Changes

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

**Action:**

```bash
# Recent commits
git log --oneline -10

# Uncommitted changes
git diff

# Changes in specific file
git log -p --follow src/problematic_file.py | head -100
```

### 4. Gather Evidence in Multi-Component Systems

**WHEN system has multiple components (API → service → database, CI → build → deploy):**

**BEFORE proposing fixes, add diagnostic instrumentation:**

For EACH component boundary:
- Log what data enters the component
- Log what data exits the component
- Verify environment/config propagation
- Check state at each layer

Run once to gather evidence showing WHERE it breaks.
THEN analyze evidence to identify the failing component.
THEN investigate that specific component.

### 5. Trace Data Flow

**WHEN error is deep in the call stack:**

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

From Tommy-yw/RunbookHermes, MIT.

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