In my experience is usually caused by client side timeout. In.net 8, when long polling timeouts, the client throws a timeout exception, while on the server side, it logs this as an exception and returns status code 499. The application is deployed on an azure web app.
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So sounds like you would just need to increase the timeout. Its likely the client has timeout out the operation. I have a number of functions in a.net 9 function app, running under the isolated model, that occassionally return a 499 error after the actual work of the function.
For 499 errors make some adjustments in timeout settings, modifying middleware configurations, optimizing request handling logic, or making changes to the deployment setup.
