One of the features of Rails 1.1 was the ability to automatically detect clients that could understand XML and send XML responses to them using the respond_to method. This also works for AJAX requests and HTML, of course. Unfortunately, though, this relies on XML and API clients sending the correct Accept headers in most cases, and, as we all know, users aren't necessarily so smart (even API users!).

In response to this problem, Edge Rails now accepts the format parameter on the URL, like so: http://www.yourapp.com/post/show/1?format=xml So while the Accept header method still works, you can also pass in the format directly via the URL. I suggested that also allowing a subdomain to specify the content type could also work (e.g: all requests to api.yourapp.com could require an XML response), and Ryan Daigle came up with an elegant solution here. For reference:

map.connect '/post/show', :controller => 'post', :action => 'show', :format => 'xml', :requirements => { :subdomain => 'api' }