ASP.NET Core 3.0 [FromBody] string content returns "The JSON value could not be converted to System.String."

Not sure this help but I think they made some change in .net core 3.0 Newtonsoft.JSON package so you can try this

Install Microsoft.AspNetCore.Mvc.NewtonsoftJson package.

In your startup.cs add

services.AddControllers().AddNewtonsoftJson();

If you are using asp.net core 3.0 then this has built-in JSON support. I have use the following and it works without setting the custom input handler.

[HttpPost]
public async Task<IActionResult> Index([FromBody] JsonElement body)
{

    string json = System.Text.Json.JsonSerializer.Serialize(body);
    return Ok();

}

Change [FromBody] string content to [FromBody] object content and then if you want/need to read as string use content.ToString()