CORS Errors: Blocked by Browser? How to Let Your App Talk to Your API

You click a button that should load data and nothing happens. The console says “blocked by CORS policy” or your tool mentions a cross‑origin error. Your browser is protecting people by stopping your app from talking to a server it doesn’t fully trust yet. This is common in AI‑first development and no‑code builds because frontends and backends often sit on different addresses when you move fast.

What’s going on behind the scenes

Your app lives at one address (for example, yoursite.com) and the API lives at another (for example, api.yoursite.com or a third‑party service). Browsers block requests between them unless the server says, “this app is allowed.” That permission is what people mean by “CORS configuration.”

How users experience CORS errors

• A loading spinner that never finishes after clicking a button
• A form that saves locally but doesn’t update the account online
• A sign‑in that works once and fails on refresh

These are frustrating because they feel random. In reality, the browser is being strict on purpose.

A straightforward fix

• Confirm the exact address where your app runs (the full URL)
• In your backend or service dashboard, add that address to the “allowed” list
• If you have staging and production, add both to avoid surprises
• Try the action again and watch the request succeed

Most popular services provide a simple place to add allowed origins. If you use platforms like Supabase, Firebase, Vercel, or Netlify, look for settings that mention “CORS,” “allowed origins,” or “domain restrictions.”

Keep it stable as you move fast

Vibe‑coding encourages quick iteration, which sometimes means changing addresses (preview links, staging, production). Every time the address changes, the server needs to know it. Keep a tiny list of the URLs you use and make sure the backend recognizes them. Cursor, Claude Code, Lovable, and Bolt.new help you spin up frontends fast; a minute spent allowing the right origins keeps those frontends talking to the right backends.

When to ask for help

If you’ve added the correct URL and still see a blocked request, a rescue can be quicker than guessing. Spin by fryga can jump in to identify the missing setting and connect your app to its API so features work reliably again.

Real‑world examples

Local works, live fails. Your app calls https on production, but the backend only allows http or the wrong domain. Update the backend to allow your public https domain and try again.

Staging passes, production blocks. You allowed the staging URL in the service but forgot to add the production URL. Add both so preview links and the public site work consistently.

Third‑party widget blocked. The widget fetches from its own domain, which your Content Security Policy blocks. Add the widget’s domain to the allowed list and confirm requests succeed.

Small habits that prevent CORS surprises

• Keep a list of all URLs your app uses (preview, staging, production)
• Whenever the URL changes, update allowed origins on your backend and services
• Test the most important requests in production after each release

Founder FAQs

Do we need wildcards (allow all)? Avoid them. Be specific about which origins can talk to your API. Specific rules protect users while still letting your app work across preview, staging, and production.

Do mobile apps need CORS? Native mobile apps don’t, but mobile web apps do. If a mobile browser uses your web app, it follows the same rules.

Can AI fix CORS for me? AI can point to the right settings if you paste the exact error and the URLs involved. You still have to add those URLs to the allowed list in your backend or provider.