If an API client reports Unexpected token '<', first check the response—not the parser. The body may be an HTML login page, frontend fallback, or server or proxy error instead of JSON. Record the status, final URL, Content-Type, and a short raw-body preview; together, those clues help identify which layer answered the request.
What the response can tell you
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No single clue proves the cause in every stack. Compare the response status, redirect history and final URL, Content-Type, body signature, and the system responsible for the response: API handler, authentication layer, frontend router, or proxy. A body beginning with < is a useful warning that the response may be HTML, but inspect it rather than assuming the parser is at fault.
- Login form or authentication message: check whether credentials are missing, invalid, or formatted incorrectly.
- Frontend app shell: check the request URL and route. A wrong path or prefix can land on a website fallback rather than an API handler.
- Server or proxy error text: investigate the component that generated the error and whether it returned the representation your client expects.
Microsoft’s ASP.NET Core documentation describes an API call 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 errors, including missing endpoints. Microsoft’s API error-handling guidance is specific to ASP.NET Core; other frameworks have their own error-handling configuration.
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Trace the response from request to body
- Verify the request target. Record the method, host, path, environment, and complete URL. Compare them with the API documentation, including any required base path or version prefix. Seeing
/api/in a URL does not prove the intended API handler received the request. - Inspect status and headers. Check the HTTP status and
Content-Type, then read a short preview of the raw body before attempting to parse it. The header is informative, not conclusive: confirm the representation in the body too. - Check redirects and the final URL. A client may follow a redirect and show the destination page, obscuring the original response. Inspect redirect history and compare the final URL with the API route you intended to call.
- Verify authentication. Confirm that the credentials and authorization format match the service’s instructions. For example, Cloudinary’s Admin API documentation identifies missing credentials and incorrectly formatted credentials as causes of HTML responses; it specifically calls out incorrect Base64 encoding when manually constructing the
Authorizationheader. This is a Cloudinary-specific example, not a universal diagnosis. Cloudinary Admin API documentation - Check the proxy path. If the request passes through a proxy, inspect its routing and diagnostics. Postman recommends reviewing its Console for proxy-server debugging information. Postman proxy troubleshooting
- If you own the API, inspect its error paths. Make missing-route and exception responses conform to the API’s documented machine-readable contract. In ASP.NET Core, Microsoft documents configuring API errors to return JSON rather than HTML.
- Parse only the representation you expect. Check the response before decoding it as JSON. Avoid silently swallowing parse failures: that can hide both the HTML body and the upstream status that would help locate the problem.
Match the evidence to the likely source
| Observation | Likely area to investigate | Next check |
|---|---|---|
| HTML login or authentication error | Authentication layer | Verify credentials and authorization-header format against the service documentation. |
| Website shell or unrelated page | URL, route, or frontend fallback | Compare the actual request path and final URL with the documented API route. |
| HTML exception response | API error handler or server | Inspect server logs and configure API errors to use the documented response format. |
| Proxy-branded error or unexpected routing | Proxy or network path | Review proxy diagnostics and request details, including Postman Console output when applicable. |
These are diagnostic patterns, not guarantees. A status code or header alone does not identify the responsible layer; use the URL, redirects, headers, and body together.
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Make the API’s error contract predictable
If you maintain the API, define machine-readable responses for expected failures as well as successful requests. Error handling should cover missing routes and unhandled exceptions, not just the endpoint’s normal return path. Microsoft’s ASP.NET Core guidance describes configuring a web API to return JSON for missing endpoints and unhandled exceptions. Ensure the client can distinguish an API error from a response generated by a frontend or intermediary.
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On the client side, preserve enough information to diagnose a mismatch: status, response headers, final URL, and a safe preview of the body. Do not treat every non-JSON response as a JSON parsing problem; it may be evidence that a different layer handled the request.
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