CORS on Railway usually breaks for the same boring reasons it breaks everywhere else: wrong origin, missing preflight handling, credentials mixed with *, or a proxy layer eating headers. Railway itself is not the hard part. Your app is. This guide is the version I wish I could paste into every “CORS error on Railway” thread. What Railway changes Railway gives you deployed services on Railway-owned domains and often custom domains on top. That means your frontend and backend commonly end up on different origins: ...

Stripe webhooks and CORS get mixed together constantly, and that usually means someone is solving the wrong problem. I’ve seen this play out the same way more than once: payments work in Stripe Checkout, the webhook endpoint is live, then somebody opens DevTools, sees a failed browser request, and starts adding Access-Control-Allow-Origin: * to the webhook route. A few commits later, webhook signature verification breaks, preflight requests start showing up where they never mattered, and the team is less sure than before what CORS even applies to. ...

Caddy makes the easy path easy, but CORS is still CORS. The browser enforces it, the server has to answer correctly, and one wrong header can turn a simple API call into a weird frontend bug that eats an afternoon. This guide is the version I wish I had the first few times I configured CORS behind a reverse proxy. What CORS is actually doing CORS is the browser asking: ...

I’ve seen the same Netlify Edge Functions CORS bug more times than I can count: the function works perfectly in curl, looks fine in local testing, then the browser blows up with a vague CORS error and the frontend team starts blaming fetch. Usually the problem is simple. The Edge Function returns JSON, but forgets the preflight request, forgets Vary: Origin, or hardcodes * while also trying to send cookies. That combo is enough to turn a clean deployment into an afternoon of browser-tab archaeology. ...

CORS on Vultr is usually not a Vultr problem. It’s an app server, reverse proxy, or object storage config problem that just happens to show up on a Vultr VM, Kubernetes cluster, or load balancer. I’ve seen teams lose hours blaming firewalls, DNS, even TLS, when the real bug was one missing OPTIONS response or a wildcard used with credentials. So here’s the practical version: what to set, where to set it, and what not to do. ...

CORS with Cloudflare Zero Trust tends to fail in ways that look random until you understand who is actually answering the browser. That’s the first mistake: treating CORS like an app-only problem when Cloudflare is sitting in front of your app, enforcing Access policies, redirecting unauthenticated users, and sometimes answering OPTIONS before your origin ever sees it. If you’ve ever said “but my API sends Access-Control-Allow-Origin just fine” while the browser still throws a CORS error, this is probably why. ...

CORS and Linkerd live at different layers, and that mismatch is where most confusion starts. Linkerd is a service mesh. CORS is a browser enforcement model for cross-origin HTTP requests. Linkerd is great at mTLS, traffic policy, retries, and observability between services. It is not, by itself, a CORS engine. If you expect Linkerd to “handle CORS” the way an API gateway or app framework does, you’ll hit a wall pretty quickly. ...

A lot of teams assume Azure Front Door will “handle CORS” because it sits in front of everything. That assumption burns time. I’ve seen this play out the same way more than once: the API works in Postman, works from curl, even works when you hit the backend directly — but the browser says no. Then someone starts adding random Access-Control-* headers at Front Door, somebody else enables caching, and suddenly the failures become intermittent. That’s when the real fun starts. ...

If you build with SST long enough, you’ll eventually hit the classic wall: “Blocked by CORS policy” And the annoying part is that SST usually makes the happy path feel simple. Then one custom header, one cookie-based auth flow, or one frontend deployed to a different domain later, and you’re deep in browser errors that barely explain what’s actually wrong. Here are the CORS mistakes I see most often in SST projects, plus the fixes that actually work. ...

CORS performance usually gets treated like background noise until your frontend starts making hundreds of API calls and every second request triggers an OPTIONS preflight. Then it becomes very obvious: bad CORS config can waste latency, server CPU, and CDN cache efficiency. I’ve seen teams obsess over query performance while every browser quietly burns extra round trips on preflights they could have avoided. This guide is the practical version: what gets cached, what does not, and what headers to set when you want cross-origin requests to be fast without turning your policy into mush. ...