SignalR is a websocket library for .NET. During setup and local development there are a number of issues I have encountered. In this post I’ll explain how to fix these. I will assume you are using ASP.NET core, version 2.2 or later. And your client is just plain old vanilla JavaScript . Obviously your first step should be reproducing your issue with new empty projects, where you can be sure nothing else is interfering. Beyond that, here are a few tips: Client Scripts Updated To use SignalR in JavaScript you should be using the official client-side library from Microsoft. This is usually a single file called signalr.js or signalr.min.js (the minified version). You can find basic HTML/JS example use here to check you are using it correctly. If you have got the client scripts from a non-official source, or there is a chance they might be out-of-date, make sure you replace them with the latest version. You can get hold of the latest version of the scripts using Vi...