How to use Newtonsoft.Json as default in Core Web Api?

In .NET Core 3.0+ include the NuGet package Microsoft.AspNetCore.Mvc.NewtonsoftJson and then replace


in ConfigureServices with


This is a pre-release NuGet package in .NET Core 3.0 but a full release package in .NET Core 3.1.

I came across this myself, but I’ve found that the same answer with some additional info is in this SO question and answer.

Edit: As a useful update: code with the call to AddNewtonsoftJson() will compile and run even without installing the Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package. If you do that, it runs with both converters installed, but defaulting to the System.Text.Json converter which you presumably don’t want since you’re reading this answer. So you must remember to install the NuGet package for this to work properly (and remember to re-install it if you ever clear down and redo your NuGet dependencies).

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