Test whether an API accepts cross-origin calls, your browser makes the real request and the tool explains the result.
CORS (Cross-Origin Resource Sharing) is the mechanism browsers use to decide whether a site's JavaScript (the "origin", e.g. https://jkit.tools) can read the response from a request made to another domain (e.g. https://api.thirdparty.com). By default this is blocked by the same-origin policy, unless the target server responds with the Access-Control-Allow-Origin header (and, depending on the case, Access-Control-Allow-Methods, Access-Control-Allow-Headers and Access-Control-Allow-Credentials) explicitly authorizing that origin. This tool makes your browser run the real call against the URL you provide, with the method and headers you choose, and interprets the result, success means your origin was authorized; a block means it wasn't, which is just as valid a diagnostic result as the other.
Paste the code into your HTML and the tool shows up on your page, without J-Kit's navigation and ads. It still runs in the browser of whoever visits your site.
<iframe
src="https://jkit.tools/embed/en-US/cors-tester"
width="100%"
height="600"
style="border:0"
loading="lazy"
title="CORS Tester"
></iframe>These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
That's exactly the symptom of a CORS block: the browser rejects the fetch() promise before handing any data to JavaScript, without detailing why. The catch is that this same generic message also shows up for DNS, TLS or connectivity failures, so the browser does not distinguish the two for the script that made the call.
Your browser makes the real request to the URL you enter, exactly like any page would. If the call completes, the current origin was authorized. If it's blocked, the browser never shows the page any data, that alone is the result.
This request only uses a method and headers CORS considers "safe" (GET/HEAD/POST with a simple Content-Type and standard headers), so the browser sends it directly, with no OPTIONS check first.
The test request leaves your browser directly for the URL you entered, the J-Kit server never sees, resolves or intermediates that address. It's the same kind of call any script on your own page would make.