Debugging & Diagnosis
Debugging is the skill of finding the actual cause of a failure rather than the first plausible one. It is assessed on method: forming hypotheses, testing the cheapest one first, and reaching a verified root cause rather than a symptom that stopped appearing.
Also called: troubleshooting, root cause analysis, incident diagnosis
What is debugging & diagnosis?
Debugging covers reproducing a failure reliably, forming and eliminating hypotheses in cost order, reading evidence rather than guessing, and confirming the fix addressed the cause rather than masking the symptom.
Debugging speed varies more between engineers than almost any other skill, and it is invisible on a CV. It is also the most transferable engineering skill across stacks and domains.
What do the levels of debugging & diagnosis mean?
Four levels, each defined by observable behaviour rather than by years. This is the definition employers set their bar against, and the one every proof is scored to.
- L1Aware
Can fix errors where the message points to the cause.
Evidence: Resolved bugs with clear error paths.
- L2Working
Reproduces reliably and narrows the search systematically instead of guessing.
Evidence: A written diagnosis showing the narrowing steps.
- L3Independent
Diagnoses failures where the error is misleading, tests the cheapest hypothesis first, and verifies the root cause before fixing.
Evidence: A root-cause analysis distinguishing the cause from the symptom, with verification.
- L4Leading
Diagnoses systemic and intermittent failures across service boundaries and improves the observability that makes future diagnosis faster.
Evidence: A resolved class of intermittent failures plus the instrumentation added.
| Level | Label | People proven here |
|---|---|---|
| L1 | Aware | 0 |
| L2 | Working | 0 |
| L3 | Independent | 3 |
| L4 | Leading | 0 |
How is debugging & diagnosis assessed?
Candidates diagnose a failure in an unfamiliar codebase with a misleading error message. Scoring rewards the elimination method and the verification step, not just arriving at the answer.
Which roles require debugging & diagnosis?
| Role | Level required | Status |
|---|---|---|
| AI Engineer | L3 Independent | Required |
| Founding Engineer | L3 Independent | Required |
How do you prove debugging & diagnosis?
A support bot is giving wrong answers with total confidence. Find out why — the answer is not in the prompt.
An intermittent production failure with an error message that points at the wrong thing.
Debugging & Diagnosis: questions people ask
- Is this about knowing a specific stack?
- No. The codebase is deliberately unfamiliar to everyone. That is the point — it isolates method from prior exposure, which is why this skill transfers across stacks.
- What if I don't find the bug in time?
- You can still score at Level 2 or 3. The rubric scores the elimination method, so a well-documented narrowing that ran out of time beats a lucky guess with no reasoning.
- Why is this assessed separately from coding?
- Because they diverge. Strong writers of new code are sometimes weak diagnosers of existing code, and most engineering time is spent on the second.