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 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 2. Find Immediate Cause What code directly causes this? 3. Ask: What Called This? 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? Adding Stack Traces When you can't trace manually, add instrumentation: Critical: Use console.error() in tests (not logger may not show) Run and capture: 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 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: 1. git init runs in process.cwd() ← empty cwd parameter 2. WorktreeManager called with empty projectDir 3. Session.create() passed empty string 4. Test accessed context.tempDir before beforeEach 5. 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 (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