Cursor
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CORS Headers Missing After API Refactoring
After Cursor refactored your API server, requests from the frontend are blocked by CORS errors. The browser blocks responses due to missing Access-Control headers.
CORS middleware was removed or misconfigured.
Error Messages You Might See
Access to XMLHttpRequest blocked by CORS policy
No 'Access-Control-Allow-Origin' header
PreflightError
Cross-origin request blocked
Common Causes
- CORS middleware removed or commented out
- Origin whitelist too restrictive or changed
- Allow-Methods not including required methods (PUT, DELETE)
- Allow-Headers not including custom headers (Authorization)
- Credentials handling broken (withCredentials vs CORS)
How to Fix It
Enable CORS: app.use(cors({origin: 'http://localhost:3000', credentials: true})). Or manually set headers: res.setHeader('Access-Control-Allow-Origin', '*'). Ensure preflight handled: OPTIONS method. Allow methods: GET, POST, PUT, DELETE.
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Get HelpFrequently Asked Questions
What's a preflight request?
Browser sends OPTIONS request first to check permissions for complex requests (PUT, DELETE, custom headers). Server must respond with CORS headers.
When is CORS needed?
When frontend (different origin) calls API. Not needed for same-origin. Localhost:3000 != localhost:3001.