I had to dig a little for this. The answer is simply:

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headers["Status"] = "301 Moved Permanently"
redirect_to "/"

Here is the relevant snippet from response.rb in ActionPack:

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def redirect(to_url, permanently = false)
  @headers["Status"]   = "302 Found" unless @headers["Status"] == "301 Moved Permanently"
  @headers["location"] = to_url
  ...

So as you can see, it actively checks for that particular header. Kinda weird that it doesn’t have a :status flag like so many other methods, but that’s the way it is.