Next.js API Route Returns 404 on POST Request

1. Using page.ts instead of route.ts in API route directory
2. Default export instead of named exports for HTTP methods (GET, POST, etc)
3. Route file in pages directory (Pages Router) instead of app directory (App Router)
4. Incorrect URL path matching route.ts file structure
5. Missing HttpResponse wrapper in Vercel's Request/Response types

Use route.ts: API routes must be named route.ts (not page.ts or api.ts) in app/api/your-route/ directory.

Export handlers: Export named async functions for each method:
export async function GET(request: Request) { ... }
export async function POST(request: Request) { ... }

App Router pattern: For /api/users endpoint, create app/api/users/route.ts file.

Test with correct method: Verify fetch call uses correct method: fetch(url, { method: 'POST' })