Error Handling Skill

Degree of freedom: MIXED. Which layer and message [HIGH freedom]; existing-type/boundary probes [LOW freedom — run exactly].

How to reason

1. Observe — existing error types, boundaries, ActionResult / ApiError
2. Interpret — missing layer vs inconsistent shape vs swallowed catch
3. Classify — reuse-existing / extend-codes / add-boundary / toast-only
4. Severity — unhandled 500 on a mutation outranks a missing toast

Worked example

Observe: createUser throws Prisma P2002; UI shows a blank catch; ActionResult already lives in types/errors.ts. Interpret: known conflict is unmapped; no field/toast path. Classify: reuse ActionResult + CONFLICT; toast the message; do not add a second error type. Verify: duplicate email returns { success: false, error: { code: 'CONFLICT' } }; form alert shown.