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SKILL.md
npx skills add neolabhq/context-engineering-kit --skill root-cause-tracing
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
npx skills add neolabhq/context-engineering-kit --skill root-cause-tracing
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.
Use when:
What code directly causes this?
What value was passed?
projectDir = '' (empty string!)cwd resolves to process.cwd()Where did empty string come from?
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:
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.
Symptom: .git created in packages/core/ (source code)
Trace chain:
git init runs in process.cwd() ← empty cwd parametercontext.tempDir before beforeEach{ tempDir: '' } initiallyRoot cause: Top-level variable initialization accessing empty value
Fix: Made tempDir a getter that throws if accessed before beforeEach
Also added defense-in-depth:
NEVER fix just where the error appears. Trace back to find the original trigger.
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
From debugging session (2025-10-03):
Error: git init failed in /Users/jesse/project/packages/coreawait execFileAsync('git', ['init'], { cwd: projectDir });WorktreeManager.createSessionWorktree(projectDir, sessionId)
→ called by Session.initializeWorkspace()
→ called by Session.create()
→ called by test at Project.create()const context = setupCoreTest(); // Returns { tempDir: '' }
Project.create('name', context.tempDir); // Accessed before beforeEach!// 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 });
}npm test 2>&1 | grep 'DEBUG git init'./find-polluter.sh '.git' 'src/**/*.test.ts'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";
}