root-cause-tracing

<skill_overview>
Bugs manifest deep in the call stack; trace backward until you find the original trigger, then fix at source, not where error appears.
</skill_overview>

<rigidity_level>
MEDIUM FREEDOM - Follow the backward tracing process strictly, but adapt instrumentation and debugging techniques to your language and tools.
</rigidity_level>

<quick_reference>
| Step | Action | Question |
|------|--------|----------|
| 1 | Read error completely | What failed and where? |
| 2 | Find immediate cause | What code directly threw this? |
| 3 | Trace backward one level | What called this code? |
| 4 | Keep tracing up stack | What called that? |
| 5 | Find where bad data originated | Where was invalid value created? |
| 6 | Fix at source | Address root cause |
| 7 | Add defense at each layer | Validate assumptions as backup |

**Core rule:** Never fix just where error appears. Fix where problem originates.
</quick_reference>

<when_to_use>
- 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
- Error message points to utility/library code

**Example symptoms:**
- "Database rejects empty string" ← Where did empty string come from?
- "File not found: ''" ← Why is path empty?
- "Invalid argument to function" ← Who passed invalid argument?
- "Null pointer dereference" ← What should have been initialized?
</when_to_use>

<the_process>
## 1. Observe the Symptom

Read the complete error:

```
Error: Invalid email format: ""
  at validateEmail (validator.ts:42)
  at UserService.create (user-service.ts:18)
  at ApiHandler.createUser (api-handler.ts:67)
  at HttpServer.handleRequest (server.ts:123)
  at TestCase.test_create_user (user.test.ts:10)
```

**Symptom:** Email validation fails on empty string
**Location:** Deep in validator utility

**DON'T fix here yet.** This might be symptom, not source.

---

## 2. Find Immediate Cause

What code directly caus