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If a browser SDK request fails with a CORS error, the SDK may not be the problem. The browser enforces cross-origin access, and the server decides whether to permit it through its response headers. Check the browser’s Console and Network panels first: they can distinguish a CORS policy rejection from a failed preflight, credential mismatch, or lower-level network problem.
Why am I getting a CORS error with my SDK?
Browsers apply Cross-Origin Resource Sharing (CORS) rules to cross-origin requests made through APIs such as fetch and XMLHttpRequest. When the browser blocks access to a response, JavaScript generally does not receive the detailed reason; the browser reports it in developer tools instead. A CORS message therefore does not, by itself, show that the SDK is defective. MDN’s CORS error guide describes the browser’s reported reason, while its CORS overview explains the server-controlled HTTP-header mechanism.
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One documented console message is: “Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at [some site]”. The wording is a clue to investigate the request, not a diagnosis of which component needs changing.
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Capture the browser’s report
Open developer tools before reproducing the problem. In the Console, note the exact error text. In the Network panel, identify the failing request and its URL. This evidence matters because the JavaScript error usually does not expose the browser’s detailed CORS rejection reason. MDN explains why these details appear in developer tools.
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Check whether an OPTIONS preflight occurred
Look for an
OPTIONSrequest immediately before the SDK’s request. A browser sends this preflight when the request’s characteristics require permission for its planned method and headers. Check whether the response permits the requesting origin, intended method, and requested headers. If the preflight fails, the browser does not send the actual request. MDN’s preflight explanation describes this exchange. -
Inspect the API’s CORS response
For a CORS request to succeed, the server must return headers allowing the requesting origin and, when applicable, the planned method and headers. In the Network panel, inspect the response headers and status for the preflight and any actual request that was sent. A client-side SDK cannot grant itself access that the resource server has not allowed.
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Check credentials as a separate requirement
If the request is meant to include cookies or other credentials, verify both the client’s credential setting and the server’s response. The server must return
Access-Control-Allow-Credentials: trueand explicitly name an allowed origin.Access-Control-Allow-Origin: *is not accepted for credentialed responses. Browser third-party-cookie policies can also affect whether cookies are sent. MDN documents the credential requirements.Recommended: PC Feels Slow? A Free Scan Shows What's Dragging Windows Down →Recommended: Update Every Outdated Driver on Your PC in One Scan - Free →Recommended: Fix Windows Errors and Clear Junk Files in Minutes - Free Scan →Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy. -
Rule out a transport or protocol failure
If the browser reports “CORS request did not succeed,” do not assume a missing CORS header is the cause. Check the Network panel for DNS resolution problems, timeouts, refused connections, TLS errors, mixed content, or an endpoint that returned no response. Confirm the request uses the appropriate HTTPS URL and that the API is running and responding. MDN lists the kinds of failures associated with this message.
Match the fix to what the evidence shows
| What you see | What it indicates | Next step |
|---|---|---|
An OPTIONS request fails, and the actual request is absent |
The browser’s preflight check did not permit the planned request. | On the API, allow the requesting origin, method, and requested headers as needed. |
| The request uses credentials, but the response has a wildcard origin or omits credential permission | The server response does not meet the requirements for credentialed cross-origin access. | Configure an explicit allowed origin and Access-Control-Allow-Credentials: true; also check browser cookie policy. |
| The browser says the CORS request did not succeed, and no usable endpoint response appears | A network or protocol failure may have interrupted the request before a CORS response could be evaluated. | Investigate DNS, connection, TLS, mixed-content, timeout, and API availability issues. |
| The remote API does not allow your origin, and you do not control its configuration | The browser cannot be instructed by the SDK to bypass the server’s access policy. | Ask the provider to allow the origin or, where appropriate, use a server-side proxy you control. |
What if you do not control the API?
When you control the API, correct its CORS response behavior. When the API belongs to an external provider, request that the provider allow your origin. A server-side proxy may be appropriate if you control it and can accept the added intermediary and its operational dependency; it is not a browser-side switch that makes the remote server authorize your page. MDN notes that most CORS errors can only be resolved on the server because the server controls whether cross-origin access is allowed. MDN Web Docs, “CORS errors.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why no-cors is not an API fix
Setting a request to no-cors does not make a cross-origin API response readable. The browser returns an opaque response: JavaScript cannot access its body or headers. MDN describes limited cases where this mode can be useful only when the caller does not need to read either. See MDN’s documentation of request modes.
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