Systematic Debugging

---
name: systematic-debugging
description: Use when encountering any bug, test failure, or unexpected behavior. 4-phase root cause investigation — NO fixes without understanding the problem first.
version: 1.1.0
author: Hermes Agent (adapted from obra/superpowers)
license: MIT
metadata:
  hermes:
    tags: [debugging, troubleshooting, problem-solving, root-cause, investigation]
    related_skills: [test-driven-development, writing-plans, subagent-driven-development]
---

# Systematic Debugging

## Overview

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

**Core principle:** Find the root cause before attempting a fix; a symptom fix leaves the defect in place and usually creates a new one. Propose fixes only after Phase 1 has identified the cause.

## 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

Work through the phases in order; each one supplies the evidence the next depends on.

---

## 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

**Action:** Use `search_files` to trace references:

```python
# Find where the function is called
search_files("function_name(", path="src/", file_glob="*.py")

# Find where the variable is set
search_files("variable_name\\s*=", path="src/", file_glob="*.py")
```

### Phase 1 Completion Checklist

- [ ] Error messages fully read and understood
- [ ] Issue reproduced consistently
- [ ] Recent changes identified and reviewed
- [ ] Evidence gathered (logs, state, data flow)
- [ ] Problem isolated to specific component/code
- [ ] Root cause hypothesis formed

**STOP:** Do not proceed to Phase 2 until you understand WHY it's happening.

---

## Phase 2: Pattern Analysis

**Find the pattern before fixing:**

### 1. Find Working Examples

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

**Action:** Use `search_files` to find comparable patterns:

```python
search_files("similar_pattern", path="src/", file_glob="*.py")
```

### 2. Compare Against References

- If implementing a pattern, read the reference implementation COMPLETELY
- Don't skim — read every line
- Understand the pattern fully before applying

### 3. Identify Differences

- What's different between working and broken?
- List every difference, however small
- Don't assume "that can't matter"

### 4. Understand Dependencies

- What other components does this need?
- What settings, config, environment?
- What assumptions does it make?

---

## Phase 3: Hypothesis and Testing

**Scientific method:**

### 1. Form a Single Hypothesis

- State clearly: "I think X is the root cause because Y"
- Write it down
- Be specific, not vague

### 2. Test Minimally

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

### 3. Verify Before Continuing

- Did it work? → 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
- Ask the user for help
- Research more

---

## Phase 4: Implementation

**Fix the root cause, not the symptom:**

### 1. Create Failing Test Case

- Simplest possible reproduction
- Automated test if possible
- MUST have before fixing
- Use the `test-driven-development` skill

### 2. Implement Single Fix

- Address the root cause identified
- ONE change at a time
- No "while I'm here" improvements
- No bundled refactoring

### 3. Verify Fix

```bash
# Run the specific regression test
pytest tests/test_module.py::test_regression -v

# Run full suite — no regressions
pytest tests/ -q
```

### 4. If Fix Doesn't Work — The Rule of Three

- **STOP.**
- Count: How many fixes have you tried?
- If < 3: Return to Phase 1, re-analyze with new information
- **If ≥ 3: STOP and question the architecture (step 5 below)**
- DON'T attempt Fix #4 without architectural discussion

### 5. If 3+ Fixes Failed: Question Architecture

**Pattern indicating an architectural problem:**
- Each fix reveals new shared state/coupling in a different place
- Fixes require "massive refactoring" to implement
- Each fix creates new symptoms elsewhere

**STOP and question fundamentals:**
- Is this pattern fundamentally sound?
- Are we "sticking with it through sheer inertia"?
- Should we refactor the architecture vs. continue fixing symptoms?

**Discuss with the user before attempting more fixes.**

This is NOT a failed hypothesis — this is a wrong architecture.

---

## When to return to Phase 1

Return to Phase 1 when a fix is being proposed before the data flow has been traced, or when several changes are being made at once so the effective one cannot be isolated. If three or more fixes have failed, or each fix surfaces a new problem in a different place, treat it as an architectural problem and question the design (Phase 4 step 5) rather than attempting another fix.

## Quick Reference

| Phase | Key Activities | Success Criteria |
|-------|---------------|------------------|
| **1. Root Cause** | Read errors, reproduce, check changes, gather evidence, trace data flow | Understand WHAT and WHY |
| **2. Pattern** | Find working examples, compare, identify differences | Know what's different |
| **3. Hypothesis** | Form theory, test minimally, one variable at a time | Confirmed or new hypothesis |
| **4. Implementation** | Create regression test, fix root cause, verify | Bug resolved, all tests pass |

## Hermes Agent Integration

### Investigation Tools

Use these Hermes tools during Phase 1:

- **`search_files`** — Find error strings, trace function calls, locate patterns
- **`read_file`** — Read source code with line numbers for precise analysis
- **`terminal`** — Run tests, check git history, reproduce bugs
- **`web_search`/`web_extract`** — Research error messages, library docs

### With delegate_task

For complex multi-component debugging, dispatch investigation subagents:

```python
delegate_task(
    goal="Investigate why [specific test/behavior] fails",
    context="""
    Follow systematic-debugging skill:
    1. Read the error message carefully
    2. Reproduce the issue
    3. Trace the data flow to find root cause
    4. Report findings — do NOT fix yet

    Error: [paste full error]
    File: [path to failing code]
    Test command: [exact command]
    """,
    toolsets=['terminal', 'file']
)
```

### With test-driven-development

When fixing bugs:
1. Write a test that reproduces the bug (RED)
2. Debug systematically to find root cause
3. Fix the root cause (GREEN)
4. The test proves the fix and prevents regression