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An “Unexpected token <” error usually means your client received an HTML document where it expected JSON. Before changing parser code, inspect the response itself: its status, Content-Type, final URL, redirect history, and a short preview of the raw body. Those clues help distinguish a wrong route, an authentication page, an API error handler, or a proxy response.
What to check first
Record what the client actually sent and received. A URL that contains /api/ is not proof that the intended API handler received the request; a wrong host, path prefix, route, or environment can lead to a frontend page or another unrelated response. Compare the full request URL and method with the API’s documented route. Cloudinary’s Admin API documentation is one service-specific example of why the exact endpoint and request requirements matter.
- Request: method, host, path, and environment.
- Response: status code and
Content-Type. - Navigation: redirects and the final URL after the client follows them.
- Body: a short raw preview, not just the parser’s error message.
An HTML preview can reveal whether the response is a login form, a frontend app shell, or a server or proxy error page. A redirect can obscure the original response: a client that follows it may show the destination page instead of making the original authentication or routing problem obvious. The status, content type, final URL, and body signature are useful clues, but none proves the cause in every stack. For the general symptom and inspection approach, see this HTML response troubleshooting explainer.
Which layer might be returning HTML?
Wrong URL or frontend fallback
If the host or route is wrong, the request may reach a website rather than an API endpoint. A frontend router can serve its standard HTML app shell for a path that it does not recognize. Check the actual URL sent by the client—not only a URL assembled in code—and compare its host and path with the API documentation.
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An API request that lacks credentials or formats them incorrectly may receive an HTML login or error page. Cloudinary specifically identifies missing or incorrectly formatted credentials as common reasons its Admin API returns HTML instead of the expected JSON. Its documentation also calls out incorrect Base64 encoding when manually constructing the Authorization header. This is a Cloudinary-specific example, not a universal explanation: verify the authentication requirements for the service you are calling. Cloudinary Admin API documentation
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API error handling
The API itself can return HTML when an endpoint fails. Microsoft’s ASP.NET Core documentation describes a client expecting JSON receiving an unhandled-exception response with Content-Type: text/html. It also explains that an API can be configured to return JSON for missing endpoints and unhandled exceptions. If you own the API, check its exception and not-found handling so its error responses follow the documented machine-readable contract. Microsoft’s ASP.NET Core API error-handling guidance
Proxy or intermediary
A proxy in the request path can affect routing or produce its own error response. If a proxy is involved, inspect its configuration and diagnostics as well as the API server’s logs. Postman recommends using its Console to review proxy-related request information during troubleshooting. Postman’s request troubleshooting documentation
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Follow a practical diagnostic sequence
- Verify the request target. Capture the method, host, path, and environment the client used. Compare them with the API’s documented route; do not assume a path that merely looks like an API path reached the API.
- Read the response metadata. Note the status and
Content-Type, then inspect a short raw-body preview. A login form, app shell, or error text can point toward a different layer. - Check redirects. Review the redirect history and final URL. If the client followed a redirect, determine whether the destination is a login page or another unexpected route.
- Verify authentication. Confirm that credentials are present and that the authorization format matches the service’s instructions. For Cloudinary’s Admin API, check the required credential formatting, including Base64 encoding if you construct the header manually.
- Inspect the proxy path. If a proxy is involved, review its routing and diagnostics. In Postman, the Console can help expose proxy-server request information.
- Correct API error responses if you control the server. Configure missing-route and exception paths to return the API’s documented machine-readable response rather than an HTML page.
- Parse only after checking the representation. Preserve the status and response details when parsing fails; do not silently catch the error and hide evidence that the body was HTML.
Use the response clues together
Compare the observations rather than treating one header or status as conclusive. An HTML body with a login form and a redirect to a sign-in route points toward authentication; an app shell with an unexpected final URL suggests a route or frontend fallback; an exception page may indicate server-side error handling; and proxy diagnostics can indicate an intermediary response. These are diagnostic patterns, not guarantees—confirm the responsible component using the request path and the relevant service, framework, or proxy documentation.
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