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.
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: git init failed in /Users/jesse/project/packages/core
What code directly causes this?
await execFileAsync('git', ['init'], { cwd: projectDir });
WorktreeManager.createSessionWorktree(projectDir, sessionId)
→ called by Session.initializeWorkspace()
→ called by Session.create()
→ called by test at Project.create()
What value was passed?
projectDir = '' (empty string!)cwd resolves to process.cwd()Where did empty string come from?
const context = setupCoreTest(); // Returns { tempDir: '' }
Project.create('name', context.tempDir); // Accessed before beforeEach!
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:
If something appears during tests but you don't know which test:
Use the bisection script: @find-polluter.sh
./find-polluter.sh '.git' 'src/**/*.test.ts'
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:
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.
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):
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).