CORS Preflight Request Blocked on Replit
API calls fail with CORS error: 'Access to XMLHttpRequest blocked by CORS policy'. Browsers block requests from your frontend to your API.
Same-origin policy requires explicit CORS configuration.
Error Messages You Might See
Common Causes
- No CORS headers in API response
- Access-Control-Allow-Origin doesn't include frontend origin
- Preflight OPTIONS method not handled
- Custom headers not in Access-Control-Allow-Headers
- Credentials mode mismatch (include vs omit)
How to Fix It
For Spring Boot, use @CrossOrigin annotation or WebMvcConfigurer. Set allowedOrigins=" to allow Replit domain. Handle OPTIONS method: @RequestMapping(method={RequestMethod.OPTIONS, RequestMethod.POST}). Client must use credentials: 'include' if sending cookies. Allow custom headers needed by client.
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Get HelpFrequently Asked Questions
What's a preflight request?
Browser sends OPTIONS request before POST/PUT/DELETE. Server must respond with CORS headers
How do I enable CORS in Spring Boot?
Use @CrossOrigin(origins="https://your-domain.replit.dev") on controller class or method