Swagger returns an unexpected HTML code response when endpoint is called

When Swagger returns an unexpected HTML code response instead of the expected JSON response for an API endpoint, it's typically an indication that there might be an issue with the configuration or that the server is returning an error page instead of the actual JSON data.

Here are some common reasons and steps to troubleshoot the issue:

1. Incorrect API Endpoint URL: Double-check that you are calling the correct API endpoint URL in Swagger. Make sure the URL matches the exact endpoint you want to test, including any query parameters if applicable.

2. CORS (Cross-Origin Resource Sharing) Issue: If the API endpoint is on a different domain than the Swagger UI, ensure that CORS is properly configured on the server to allow cross-origin requests. Check the browser's developer console for any CORS-related error messages.

3. Authentication Issues: If the API endpoint requires authentication, ensure that you have provided the correct authentication credentials or tokens in Swagger to access the endpoint.

4. Server Error: Check if the server is encountering any errors while processing the request. A server error could cause it to return an HTML error page instead of the expected JSON data. Review the server logs for any error messages.

5. Invalid Request Format: Ensure that you are sending the correct request format to the API endpoint (e.g., using the correct HTTP method, including required headers, and providing valid request parameters).

6. Content-Type Header: Verify that the Content-Type header of your request is correctly set to application/json to indicate that you are expecting JSON data as the response.

7. Swagger Configuration Issue: Check if there is any misconfiguration in the Swagger specification that is causing it to generate incorrect requests or responses.

8. Swagger UI Cache: If you have made changes to the API or Swagger specification, the Swagger UI might be caching the previous version. Try clearing the browser cache or restarting the Swagger UI.

9. API Documentation Issue: If Swagger is auto-generating the API documentation, there might be an issue with how the server code is documented, leading to incorrect responses being displayed in Swagger.

10. Server Response Manipulation: Verify if any server-side middleware or scripts are modifying the response content type or payload.

By investigating these potential reasons and following the steps to troubleshoot the issue, you should be able to determine the root cause of the unexpected HTML code response in Swagger and resolve the problem. If the issue persists, review the server-side code and configuration to ensure it is correctly handling API requests and returning the expected JSON responses.