validated-handler

Next.js Validated Handler Pattern

Type-safe API route handler with automatic Zod validation for Next.js App Router.

When to Use This Skill

Use this skill when:

• Building Next.js API routes (App Router)
• Want automatic input validation with Zod
• Need consistent error handling across API routes
• Want to eliminate boilerplate validation code
• Building type-safe APIs with TypeScript

The Problem

Without a validated handler, every API route has repetitive validation code:

// ❌ REPETITIVE - Every route looks like this
export async function GET(request: NextRequest) {
  try {
    // 1. Parse query params
    const { searchParams } = new URL(request.url);
    const rawPage = searchParams.get('page');
    const rawLimit = searchParams.get('limit');

    // 2. Validate each param manually
    if (!rawPage || isNaN(Number(rawPage))) {
      return NextResponse.json(
        { error: 'Invalid page parameter' },
        { status: 400 }
      );
    }

    if (!rawLimit || isNaN(Number(rawLimit))) {
      return NextResponse.json(
        { error: 'Invalid limit parameter' },
        { status: 400 }
      );
    }

    const page = Number(rawPage);
    const limit = Number(rawLimit);

    // 3. Validate ranges
    if (page < 1) {
      return NextResponse.json(
        { error: 'Page must be >= 1' },
        { status: 400 }
      );
    }

    if (limit < 1 || limit > 100) {
      return NextResponse.json(
        { error: 'Limit must be between 1 and 100' },
        { status: 400 }
      );
    }

    // 4. Finally, business logic
    const data = await fetchData(page, limit);
    return NextResponse.json(data);

  } catch (error) {
    console.error(error);
    return NextResponse.json(
      { error: 'Internal server error' },
      { status: 500 }
    );
  }
}

Problems:

• 30+ lines of boilerplate per route
• Error-prone manual validation
• Inconsistent error messages
• No type safety
• Hard to maintain across 100+ routes

The Solution: validatedHandler