What is root-cause-tracing?

This Claude skill provides a systematic methodology for root cause analysis by tracing bugs backward through the call stack. It empowers developers to move beyond fixing symptoms, focusing instead on identifying and resolving the original triggers of complex errors in deep execution chains or polluted test environments.

When should I use root-cause-tracing?

root-cause-tracing is useful in the following scenarios: • Tracing the origin of invalid parameters or null values passed through multiple nested function calls to find the initial source of corruption. • Identifying which specific test case is causing side effects or environment pollution (e.g., creating files in the wrong directory) in a large test suite. • Debugging silent failures or unexpected behavior in complex asynchronous workflows by adding strategic instrumentation and stack trace logging. • Resolving file system or database errors where the operation fails deep in a library but the root cause is an upstream configuration or initialization error. • Implementing multi-layered defensive validation and 'defense-in-depth' strategies to catch and log the source of corrupted state before it causes a crash.

name	root-cause-tracing
description	Systematically trace bugs backward through call stack to find original trigger. Use when errors occur deep in execution and you need to trace back to find the original trigger.
version	1.1.0

Root Cause Tracing

Overview

Bugs often manifest deep in the call stack (git init in wrong directory, file created in wrong location, database opened with wrong path). Your instinct is to fix where the error appears, but that's treating a symptom.

Core principle: Trace backward through the call chain until you find the original trigger, then fix at the source.

When to Use

Use when:

Error happens deep in execution (not at entry point)
Stack trace shows long call chain
Unclear where invalid data originated
Need to find which test/code triggers the problem

The Tracing Process

1. Observe the Symptom

Error: git init failed in ~/project/packages/core

2. Find Immediate Cause

What code directly causes this?

await execFileAsync('git', ['init'], { cwd: projectDir });

3. Ask: What Called This?

WorktreeManager.createSessionWorktree(projectDir, sessionId)
  → called by Session.initializeWorkspace()
  → called by Session.create()
  → called by test at Project.create()

4. Keep Tracing Up

What value was passed?

projectDir = '' (empty string!)
Empty string as cwd resolves to process.cwd()
That's the source code directory!

5. Find Original Trigger

Where did empty string come from?

const context = setupCoreTest(); // Returns { tempDir: '' }
Project.create('name', context.tempDir); // Accessed before beforeEach!

Adding Stack Traces

When you can't trace manually, add instrumentation:

// Before the problematic operation
async function gitInit(directory: string) {
  const stack = new Error().stack;
  console.error('DEBUG git init:', {
    directory,
    cwd: process.cwd(),
    nodeEnv: process.env.NODE_ENV,
    stack,
  });

  await execFileAsync('git', ['init'], { cwd: directory });
}

Critical: Use console.error() in tests (not logger - may not show)

Run and capture:

bun test 2>&1 | grep 'DEBUG git init'

Analyze stack traces:

Look for test file names
Find the line number triggering the call
Identify the pattern (same test? same parameter?)

Finding Which Test Causes Pollution

If something appears during tests but you don't know which test:

Use the bisection script to run tests one-by-one:

# Example: find which test creates .git in wrong place
bun test --run --bail 2>&1 | tee test-output.log

Runs tests one-by-one, stops at first polluter.

Real Example: Empty projectDir

Symptom: .git created in packages/core/ (source code)

Trace chain:

git init runs in process.cwd() ← empty cwd parameter
WorktreeManager called with empty projectDir
Session.create() passed empty string
Test accessed context.tempDir before beforeEach
setupCoreTest() returns { tempDir: '' } initially

Root cause: Top-level variable initialization accessing empty value

Fix: Made tempDir a getter that throws if accessed before beforeEach

Also added defense-in-depth:

Layer 1: Project.create() validates directory
Layer 2: WorkspaceManager validates not empty
Layer 3: NODE_ENV guard refuses git init outside tmpdir
Layer 4: Stack trace logging before git init

Key Principle

NEVER fix just where the error appears. Trace back to find the original trigger.

Stack Trace Tips

In tests: Use console.error() not logger - logger may be suppressed Before operation: Log before the dangerous operation, not after it fails Include context: Directory, cwd, environment variables, timestamps Capture stack: new Error().stack shows complete call chain

Real-World Impact

From debugging session:

Found root cause through 5-level trace
Fixed at source (getter validation)
Added 4 layers of defense
1847 tests passed, zero pollution

root-cause-tracing

When & Why to Use This Skill

Use Cases