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.opencode/skills/ck-debug/references/root-cause-tracing.md

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+# Root Cause Tracing
+
+Systematically trace bugs backward through call stack to find original trigger.
+
+## Core Principle
+
+**Trace backward through call chain until finding original trigger, then fix at source.**
+
+Bugs often manifest deep in call stack (git init in wrong directory, file created in wrong location). Instinct is to fix where error appears, but that's treating symptom.
+
+## 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 problem
+
+## The Tracing Process
+
+### 1. Observe the Symptom
+```
+Error: git init failed in /Users/jesse/project/packages/core
+```
+
+### 2. Find Immediate Cause
+What code directly causes this?
+```typescript
+await execFileAsync('git', ['init'], { cwd: projectDir });
+```
+
+### 3. Ask: What Called This?
+```typescript
+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?
+```typescript
+const context = setupCoreTest(); // Returns { tempDir: '' }
+Project.create('name', context.tempDir); // Accessed before beforeEach!
+```
+
+## Adding Stack Traces
+
+When can't trace manually, add instrumentation:
+
+```typescript
+async function gitInit(directory: string) {
+  const stack = new Error().stack;
+  console.error('DEBUG git init:', {
+    directory,
+    cwd: process.cwd(),
+    stack,
+  });
+
+  await execFileAsync('git', ['init'], { cwd: directory });
+}
+```
+
+**Critical:** Use `console.error()` in tests (not logger - may not show)
+
+**Run and capture:**
+```bash
+npm test 2>&1 | grep 'DEBUG git init'
+```
+
+**Analyze stack traces:**
+- Look for test file names
+- Find line number triggering call
+- Identify pattern (same test? same parameter?)
+
+## Finding Which Test Causes Pollution
+
+If something appears during tests but don't know which test:
+
+Use bisection script: `scripts/find-polluter.sh`
+
+```bash
+./scripts/find-polluter.sh '.git' 'src/**/*.test.ts'
+```
+
+Runs tests one-by-one, stops at first polluter.
+
+## Key Principle
+
+**NEVER fix just where error appears.** Trace back to find original trigger.
+
+When found immediate cause:
+- Can trace one level up? → Trace backwards
+- Is this the source? → Fix at source
+- Then add validation at each layer (see defense-in-depth.md)
+
+## Real Example
+
+**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