Root Cause First

Use this skill to prevent premature patches. The goal is to make the smallest correct change only after the failure mechanism is understood.

Red Flags — Stop If You Catch Yourself Thinking

These thoughts mean you are about to ship a band-aid. Stop and find the mechanism first.

Relationship To SSOT Enforcer

If the root cause is duplicated constants, scattered policy, parallel calculations, copied parsing, or multiple modules answering the same question, switch to ssot-enforcer for the consolidation step. After consolidation, return here to verify the fix against the original failure.

When Not To Use

Do not use this skill for:

• New features or design work where there is no failure to explain.
• Mechanical edits such as renames, formatting, dependency bumps, or documentation cleanup.
• Broad architecture cleanup unless it is tied to a concrete symptom, failing test, bad runtime behavior, or user-reported regression.

Required Workflow

1. State the observed failure in concrete terms.
   • Quote the exact error, log line, failed assertion, bad behavior, or user-visible symptom.
   • Identify where in the execution path the failure happens.
2. Reconstruct the intended contract.
   • Determine what the component is supposed to guarantee.
   • Identify upstream inputs, downstream consumers, and invariants that must hold.
3. Trace the real path before editing.
   • Read the code that produces the failure and the code that consumes its output.
   • Prefer logs, tests, traces, raw API responses, local reproduction, and call sites over assumptions.
   • Distinguish provider/tool behavior from adapter/parser/orchestrator behavior.
4. Name the root cause.
   • Explain the mechanism that makes the observed failure inevitable or likely.
   • If there are multiple plausible causes, list what evidence confirms or eliminates each one.
   • Do not implement until there is one primary cause or a clearly bounded uncertainty.
5. Reject band-aids explicitly.
   • Do not add fallback, retry, catch-and-continue, synthetic success, timeout inflation, broad parsing, or error suppression unless the root cause proves that behavior is the correct product contract.
   • If such handling is truly needed, state why it is not masking the failure and what invariant it preserves.
6. Implement the root fix.
   • Change the source of the bad state or the incorrect contract boundary.
   • Keep the patch scoped to the root cause.
   • If the source of the bad state is duplicated or scattered truth, consolidate it through ssot-enforcer instead of adding a local workaround.
   • Add diagnostics when future failures would otherwise be ambiguous.
7. Verify against the original failure.
   • Add or update a regression test that fails on the old behavior.
   • Run the narrow test first, then broader typecheck/test coverage as risk requires.
   • If the fix cannot be fully verified in-environment (visual/UI/device-only behavior, or unavailable services), say so explicitly and name exactly what a human must check. Do not claim success you could not observe.
   • In the final answer, report root cause, fix, and verification.

Worked Example

Symptom. TypeError: cannot read 'email' of undefined thrown in renderProfile() for ~2% of users.

Band-aid (what to avoid).

// Crash disappears, but those users now silently render a blank email.
const email = user.profile?.email ?? "";

The optional chain suppresses the only signal that something upstream is wrong.

Root-cause trace. Follow where user is built. loadUser() returns the record before profile is hydrated for accounts created before the profile backfill. The invariant "every user has a profile" is violated at the data layer, not in the view.

Root fix. Enforce the invariant at the source so every consumer can trust it:

// loadUser() now owns the invariant: it always returns a non-null profile.
function loadUser(id) {
  const row = db.fetchUser(id);
  return { ...row, profile: row.profile ?? defaultProfile(row.id) };
}

Now renderProfile() can trust its input and the ?. is unnecessary. (The invariant has exactly one owner — see ssot-enforcer.)

Verification. Add a regression test: loadUser() for a pre-backfill account returns a non-null profile; renderProfile() renders without optional chaining and the old input reproduces the original crash on the unpatched code.

Stop Conditions

Stop and ask or report a blocker when:

• The failure cannot be reproduced or located after reading the relevant path.
• Required credentials, services, logs, or artifacts are unavailable.
• Fixing the real cause requires a product decision rather than an engineering correction.

Final Answer Shape

Keep the close-out short:

• Root cause: one concrete mechanism.
• Fix: what changed and where.
• Verification: commands/tests run and any remaining risk.