Content-Type: "text/plain"

The Context.Text(format, ...args) is the method which sends plain text responses to the client. It accepts the value (like fmt package works). It's identical to Context.WriteString and Context.Writef.

func handler(ctx iris.Context) {
    response := "Hello, %s!"
    ctx.Text(response, "World")
    // ctx.Writef(response, "World")
}

Result

Hello, World!

That's all.