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If a browser SDK request fails with a CORS error, do not start by changing SDK code. First use the browser’s Console and Network panels to determine whether the browser rejected a server response, a preflight check failed, credentials were not permitted, or the request never reached the server. CORS is enforced by browsers, and the API server controls the headers that allow a page to read a cross-origin response.
Why am I getting a CORS error with my SDK?
Browser APIs such as fetch and XMLHttpRequest apply the same-origin policy to cross-origin requests. Cross-Origin Resource Sharing (CORS) is the HTTP-header mechanism a server can use to permit a browser to expose a response to a page from another origin. The SDK can initiate the request, but it cannot grant itself permission to read the response.
A console message such as “Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at [some site]” is the browser’s report, not proof that the SDK is defective. Browser security restrictions also mean page JavaScript generally cannot inspect the detailed reason for a CORS rejection; use developer tools for that evidence. MDN’s CORS error guide notes that most CORS errors can only be resolved on the server because the server controls whether cross-origin access is allowed.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsHow to diagnose the failure in browser developer tools
- Reproduce the failure with developer tools open. In the Console, record the exact browser message. In the Network panel, locate the failing URL and inspect its request and response details.
- Look for an OPTIONS preflight. If one appears immediately before the SDK request, inspect its response. A preflight is the browser asking whether the server permits the planned origin, method, and headers. If the check fails, the browser does not send the actual request.
- Check the CORS response headers. Confirm that the server permits the page’s origin and, when applicable, the intended method and request headers. A client-side SDK setting cannot substitute for the server’s permission.
- Check credential rules separately. If the request is meant to send cookies or other credentials, verify the client’s credential setting and the server’s response headers, as well as whether the browser sent the credentials.
- Rule out a transport failure. If no usable response appears, check the Network panel for DNS errors, timeouts, refused connections, TLS errors, mixed content, or a missing endpoint response.
What the Network panel can tell you
| Evidence | What it suggests | What to check next |
|---|---|---|
| An OPTIONS request followed by no SDK request | The browser likely stopped after a failed preflight. | Compare the preflight response’s allowed origin, method, and headers with what the browser requested. MDN’s preflight documentation explains this browser check. |
| An actual request and a response, but the browser blocks access | The server response may not authorize the requesting origin, or may not satisfy other CORS requirements. | Inspect the response’s CORS headers and compare them with the page origin and request. |
| A credentialed request with wildcard origin | The response does not meet the origin requirement for credentialed CORS. | Use an explicit allowed origin and the credential permission described below. |
| No endpoint response, or a visible DNS, TLS, timeout, mixed-content, or connection error | The request may have failed at the network or protocol layer rather than because a CORS response header is missing. | Resolve connectivity, HTTPS, or endpoint availability first. |
How preflight failures differ from ordinary CORS failures
A preflight is an HTTP OPTIONS request that the browser sends before certain cross-origin requests. It asks whether the server permits the planned request, including its method and headers. The browser proceeds with the actual request only if the preflight succeeds.
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In the Network panel, compare what the browser requested with what the preflight response allows: the requesting origin, the intended method, and the requested headers. If an OPTIONS request fails and the SDK request never follows, focus on the API’s preflight response rather than rewriting the SDK call.
What changes when the SDK sends credentials?
For a credentialed cross-origin request, the server must return Access-Control-Allow-Credentials: true and explicitly allow the requesting origin. Access-Control-Allow-Origin: * is not accepted for a credentialed response. Check that the client intends to send credentials and that the response meets both server requirements. Browser third-party-cookie policies can still affect cookie behavior independently.
When “CORS request did not succeed” is a network problem
A browser’s “CORS request did not succeed” message does not necessarily mean the server omitted a CORS header. It can also accompany a failure to obtain an endpoint response, such as DNS resolution trouble, a timeout, a refused connection, a TLS error, or mixed content. Inspect the Network panel, confirm the API is running and responding, and verify the page and API use an appropriate HTTPS setup before changing CORS configuration. See MDN’s explanation of this error.
Choose a fix based on who controls the API
If you control the API
Correct the API’s CORS response behavior for the requesting origin and, where required, the method, headers, and credentials. Because the server controls these response headers, changing the browser SDK alone cannot authorize access.
If the API belongs to another provider
If the remote server does not allow your page’s origin, ask the provider to permit it or use another supported integration path. A server-side proxy may be appropriate when you control and can secure it, but it adds an intermediary dependency; it does not make a browser SDK setting equivalent to permission from the API.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why no-cors is not a general fix
Setting a request to no-cors does not make a blocked API response readable. The browser returns an opaque response whose body and headers are unavailable to JavaScript. MDN describes limited cases where that mode can be useful if the caller does not need to read the response body or headers, but it is not a workaround for an SDK that needs API data. See MDN’s documentation on request modes.
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