https://github.com/sendgrid/csharp-http-client/blob/master/CSharpHTTPClient/Client.cs#L311 Here, the client replaces single quotes with double quotes. That is unexpected behavior. Single quotes are perfectly valid inside JSON string tokens. # https://github.com/sendgrid/sendgrid-csharp/issues/256