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