What if the application itself is the problem, not Nginx?

If you've increased `client_max_body_size` and still get 413 errors, the problem likely lies within the application. The application might be sending the request headers incorrectly, or it might be attempting to process a larger request than it's designed to handle. Debug the application code and ensure it correctly parses the request and handles the data. Consider increasing the application's internal buffer size as well.