Q:

auto_discover_link_tag ignores url_options

According to the docs,
auto_discovery_link_tag(:atom, {controller: "home", action: "public_archives"})`

Should return 
<link rel="alternate" type="application/atom+xml" title="ATOM" href="http://www.currenthost.com/home/public_archives" />

but I'm getting just the host without the controller or action.
<link href="http://currenhost.com/" rel="alternate" title="ATOM" type="application/atom+xml" />

A:

Since your root_path is home#public_archives, auto_discovery_link_tag is behaving as expected.
For example, if you put:
auto_discovery_link_tag(:atom, {controller: "articles", action: "index"})

The output would be:
<link rel="alternate" type="application/atom+xml" title="ATOM" href="http://www.currenthost.com/articles" />

Not
<link rel="alternate" type="application/atom+xml" title="ATOM" href="http://www.currenthost.com/articles/index" />

because the path for articles#index is /articles. The href it is returning is the first path in your routes file that matches the listed controller and action. In your case, contoller home and action public_archives first matches /.

