CORS Error - Cross-Origin Request Blocked

1. API doesn't include CORS headers in response
2. API allow-origin header doesn't match request origin
3. Credentials (cookies) sent without proper CORS config
4. OPTIONS preflight request not handled
5. Missing wildcard or specific origin in CORS config

Add CORS headers to API: Response headers { 'Access-Control-Allow-Origin': '*' }

For credentials: 'Access-Control-Allow-Origin': 'https://your-domain.com' (not wildcard)

Include: 'Access-Control-Allow-Credentials': 'true'

Handle OPTIONS: if (request.method === 'OPTIONS') return new Response(null, { headers: corsHeaders })

Use middleware to apply CORS to all routes