What is root-cause-tracing?

The Root Cause Tracing skill is a sophisticated debugging methodology designed to identify the original triggers of complex software defects. Instead of merely addressing surface-level symptoms, it provides a systematic framework for tracing errors backward through the call stack, implementing strategic instrumentation, and establishing defense-in-depth layers to ensure permanent bug resolution and architectural stability.

When should I use root-cause-tracing?

root-cause-tracing is useful in the following scenarios: • Tracing the origin of invalid data or null pointers that cause crashes deep within complex execution chains. • Identifying the specific test case or code block responsible for environment pollution, such as unauthorized file system modifications or database state changes. • Debugging silent failures where an operation succeeds but produces incorrect side effects in a different part of the system. • Implementing multi-layered validation and logging strategies to prevent the recurrence of elusive, non-deterministic bugs. • Analyzing long call stacks in legacy codebases to understand how a specific parameter value was derived and passed through multiple layers.

name	root-cause-tracing
description	Use when errors occur deep in execution and you need to trace back to find the original trigger - systematically traces bugs backward through call stack, adding instrumentation when needed, to identify source of invalid data or incorrect behavior

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

digraph when_to_use {
    "Bug appears deep in stack?" [shape=diamond];
    "Can trace backwards?" [shape=diamond];
    "Fix at symptom point" [shape=box];
    "Trace to original trigger" [shape=box];
    "BETTER: Also add defense-in-depth" [shape=box];

    "Bug appears deep in stack?" -> "Can trace backwards?" [label="yes"];
    "Can trace backwards?" -> "Trace to original trigger" [label="yes"];
    "Can trace backwards?" -> "Fix at symptom point" [label="no - dead end"];
    "Trace to original trigger" -> "BETTER: Also add defense-in-depth";
}

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 C:\Users\developer\project\src\MyProject.Core

2. Find Immediate Cause

What code directly causes this?

await ProcessHelper.RunAsync("git", "init", workingDirectory: projectDir);

3. Ask: What Called This?

WorktreeManager.CreateSessionWorktreeAsync(projectDir, sessionId)
  → called by Session.InitializeWorkspaceAsync()
  → called by Session.CreateAsync()
  → called by test at Project.CreateAsync()

4. Keep Tracing Up

What value was passed?

projectDir = "" (empty string!)
Empty string as working directory resolves to current directory
That's the source code directory!

5. Find Original Trigger

Where did empty string come from?

var context = SetupCoreTest(); // Returns { TempDir = "" }
await Project.CreateAsync("name", context.TempDir); // Accessed before setup!

Adding Stack Traces

When you can't trace manually, add instrumentation:

// Before the problematic operation
public static async Task GitInitAsync(string directory)
{
    var stackTrace = Environment.StackTrace;
    Console.Error.WriteLine($"DEBUG git init: {new {
        Directory = directory,
        CurrentDirectory = Environment.CurrentDirectory,
        Environment = Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT"),
        StackTrace = stackTrace
    }}");

    await ProcessHelper.RunAsync("git", "init", workingDirectory: directory);
}

Critical: Use Console.Error.WriteLine() in tests (not ILogger - may not show)

Run and capture:

dotnet test --logger "console;verbosity=detailed" 2>&1 | findstr "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: @find-polluter.sh

./find-polluter.sh '.git' 'src/**/*.test.ts'

Runs tests one-by-one, stops at first polluter. See script for usage.

Real Example: Empty projectDir

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

Trace chain:

git init runs in current directory ← empty working directory parameter
WorktreeManager called with empty projectDir
Session.CreateAsync() passed empty string
Test accessed context.TempDir before setup
SetupCoreTest() returns { TempDir = "" } initially

Root cause: Top-level variable initialization accessing empty value

Fix: Made TempDir a property that throws if accessed before setup

Also added defense-in-depth:

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

Key Principle

digraph principle {
    "Found immediate cause" [shape=ellipse];
    "Can trace one level up?" [shape=diamond];
    "Trace backwards" [shape=box];
    "Is this the source?" [shape=diamond];
    "Fix at source" [shape=box];
    "Add validation at each layer" [shape=box];
    "Bug impossible" [shape=doublecircle];
    "NEVER fix just the symptom" [shape=octagon, style=filled, fillcolor=red, fontcolor=white];

    "Found immediate cause" -> "Can trace one level up?";
    "Can trace one level up?" -> "Trace backwards" [label="yes"];
    "Can trace one level up?" -> "NEVER fix just the symptom" [label="no"];
    "Trace backwards" -> "Is this the source?";
    "Is this the source?" -> "Trace backwards" [label="no - keeps going"];
    "Is this the source?" -> "Fix at source" [label="yes"];
    "Fix at source" -> "Add validation at each layer";
    "Add validation at each layer" -> "Bug impossible";
}

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

Stack Trace Tips

In tests: Use Console.Error.WriteLine() not ILogger - logger may be suppressed Before operation: Log before the dangerous operation, not after it fails Include context: Directory, current directory, environment variables, timestamps Capture stack: Environment.StackTrace shows complete call chain

Real-World Impact

From debugging session (2025-10-03):

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