kensaurus/cursor-kenji/backend-error-handling

Error Handling Skill

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

This skill vs neighbors

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.

Self-critique before reporting

• Reuse shape — searched ActionResult / ApiError / error.tsx before adding a type
• User-safe — INTERNAL_ERROR is generic; PII is not in the client message
• Layered — boundary + action result + toast, not toast-only
• Right owner — plan-only observability audit → plan-error-handling; live Sentry triage → debug-sentry-monitor

Layered error handling for full-stack apps: one action-result shape, boundaries, and toasts.

When to Use

• Adding error handling to new features
• Improving error user experience
• Standardizing error responses
• Debugging error propagation
• Adding error monitoring

Check existing first [LOW freedom — run exactly]

Before adding ANY error handling, verify:

1. Check for existing error types:

rg "type.*Error|interface.*Error" --type ts
rg "ActionResult|ApiError" --type ts

1. Check for existing error boundaries:

ls -la app/error.tsx app/global-error.tsx
rg "ErrorBoundary" --type tsx

1. Check for existing error utilities:

rg "formatError|handleError|reportError" --type ts
ls -la src/lib/errors* src/lib/error* 2>/dev/null # @/lib/errors

1. Check established error response patterns:

rg "success: false|error:" src/features/*/server/ --type ts | head -10

Why: Inconsistent error handling confuses users and complicates debugging. Always follow established patterns.

Error Handling Layers [HIGH freedom]

• UI layer — error boundaries, form validation errors, toast notifications.
• Application layer — Server Action errors, API route errors, business-logic errors.
• Data layer — database errors, Zod validation errors, external API errors.
• Every error is caught at the lowest layer that can name it and surfaces upward as ActionResult or a typed API error, never as a raw throw to the UI.

Layer diagram: references/server-patterns.md §Error handling layers.

Standard Error Types [HIGH freedom]

• One AppError { code, message, details? }; code is machine-readable, message is user-safe.
• ActionResult<T> is a discriminated union on success; never return { data, error } both-optional.
• Error codes: VALIDATION_ERROR, NOT_FOUND, UNAUTHORIZED, FORBIDDEN, CONFLICT, RATE_LIMITED, INTERNAL_ERROR.

// types/errors.ts
interface AppError { code: string; message: string; details?: unknown }
type ActionResult<T> =
  | { success: true; data: T }
  | { success: false; error: AppError }

Full ErrorCode constant: references/server-patterns.md §Standard error types.

Server Action Error Handling [HIGH freedom]

1. safeParse the FormData; return VALIDATION_ERROR with flatten().fieldErrors as details.
2. Check the session; return UNAUTHORIZED with a "please sign in" message.
3. Run the mutation, revalidatePath, return { success: true, data }.
4. Map known errors (Prisma P2002 → CONFLICT with a specific message).
5. Log unknown errors server-side; return a generic INTERNAL_ERROR message, never the stack.

} catch (error) {
  if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === 'P2002') {
    return { success: false, error: { code: 'CONFLICT', message: 'A user with this email already exists' } }
  }
  console.error('createUser error:', error)
  return { success: false, error: { code: 'INTERNAL_ERROR', message: 'Something went wrong. Please try again.' } }
}

Full createUser action: references/server-patterns.md §Server Action error handling.

Form Error Display (React 19+) [HIGH freedom]

• useActionState(createUser, null) for state; useFormStatus in a child SubmitButton for pending.
• Field errors come from state.error.details; render inline with aria-invalid and aria-describedby.
• Non-validation errors render once in a role="alert" banner above the fields.
• useOptimistic for optimistic UI updates.

Full UserForm component: references/ui-patterns.md §Form error display.

React Error Boundaries [HIGH freedom]

• app/error.tsx per page segment: 'use client', receives { error, reset }, logs in useEffect, shows a "Try again" button.
• app/global-error.tsx for the root: must render its own <html> and <body>.
• Boundaries catch render errors only; async and event errors go through ActionResult or TanStack Query.

// app/error.tsx
'use client'
export default function Error({ error, reset }: { error: Error & { digest?: string }; reset: () => void }) {
  useEffect(() => { console.error('Page error:', error) }, [error])
  return (
    <div className="flex flex-col items-center justify-center min-h-[400px]">
      <h2 className="text-xl font-semibold mb-4">Something went wrong</h2>
      <button onClick={reset} className="px-4 py-2 bg-primary text-primary-foreground rounded-lg">Try again</button>
    </div>
  )
}

Both boundaries in full: references/ui-patterns.md §React error boundaries.

API Route Error Handling [HIGH freedom]

• Same { error: { code, message, details? } } body shape for every non-2xx response.
• 400 for VALIDATION_ERROR and INVALID_JSON (SyntaxError from request.json()), 201 on create, 500 generic.
• Log the route name with the error; never return stack traces.

Full POST /api/products handler: references/server-patterns.md §API route error handling.

TanStack Query Error Handling [HIGH freedom]

• In queryFn/mutationFn, throw new Error(body.error?.message ?? fallback) when !res.ok so the UI gets the server message.
• retry: no retry on 401/403; up to 3 attempts otherwise.
• Mutations: onSuccess invalidates the query key and toast.success; onError → toast.error(error.message).

useProducts and useCreateProduct hooks: references/ui-patterns.md §TanStack Query error handling.

Error State UI Components [HIGH freedom]

• One reusable ErrorState({ title, message, onRetry? }): icon, heading, message, optional retry button.
• Use it for query errors (error + refetch) instead of ad-hoc red text per list.

const { data, error, isLoading, refetch } = useProducts()
if (error) return <ErrorState title="Failed to load products" message={error.message} onRetry={() => refetch()} />

Full ErrorState component: references/ui-patterns.md §Error state UI components.

Further reading

• Error Logging & Monitoring and the full checklist
• Server-side error patterns
• Client-side error patterns