Root Cause Tracing (Investigating bugs skill)

Part of the investigating-bugs skill. The skill links here when your agent needs the detail.

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 ~/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:

npm 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: Bisect: run the test files one at a time (or in halves) and check for the artifact after each run. The first run that produces it is the polluter. A five-line shell loop over the test files is enough; stop at the first hit.

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

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() 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

Source: library/skills/investigating-bugs/root-cause-tracing.md, rendered from summer-engine@4.3.0 on npm.

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Root Cause Tracing (Investigating bugs skill)

Overview

When to Use

The Tracing Process

1. Observe the Symptom

2. Find Immediate Cause

3. Ask: What Called This?

4. Keep Tracing Up

5. Find Original Trigger

Adding Stack Traces

Finding Which Test Causes Pollution

Real Example: Empty projectDir

Key Principle

Stack Trace Tips

​Overview

​When to Use

​The Tracing Process

​1. Observe the Symptom

​2. Find Immediate Cause

​3. Ask: What Called This?

​4. Keep Tracing Up

​5. Find Original Trigger

​Adding Stack Traces

​Finding Which Test Causes Pollution

​Real Example: Empty projectDir

​Key Principle

​Stack Trace Tips

Overview

When to Use

The Tracing Process

1. Observe the Symptom

2. Find Immediate Cause

3. Ask: What Called This?

4. Keep Tracing Up

5. Find Original Trigger

Adding Stack Traces

Finding Which Test Causes Pollution

Real Example: Empty projectDir

Key Principle

Stack Trace Tips