diff --git a/lib/travis/web/app.rb b/lib/travis/web/app.rb
index f6420ac0..31466a28 100644
--- a/lib/travis/web/app.rb
+++ b/lib/travis/web/app.rb
@@ -11,12 +11,18 @@ class Travis::Web::App
   # Simple Rack router that behaves like a hash.
   # Key is the path, value the response.
   class Router < DelegateClass(Hash)
-    def initialize
+    attr_reader :main_app
+    def initialize(main_app)
+      @main_app = main_app
       super({})
     end
 
     def call(env)
-      self[env['PATH_INFO']]
+      if main_app.custom_branch?(env)
+        main_app.response_for_custom_branch(env)
+      else
+        self[env['PATH_INFO']]
+      end
     end
   end
 
@@ -31,7 +37,7 @@ class Travis::Web::App
     @options       = options
     @environment   = options.fetch(:environment)
     @root          = options.fetch(:root)
-    @router        = Router.new
+    @router        = Router.new(self)
     @app           = builder.to_app
     @version       = File.read File.expand_path('version', root)
     @last_modified = Time.now
@@ -43,16 +49,53 @@ class Travis::Web::App
     app.call(env)
   end
 
+  def response_for_custom_branch(env)
+    status, headers, body = response_for File.join(root, 'index.html'), custom_branch: custom_branch(env)
+    response = Rack::Response.new body, status, headers
+
+    if disable_custom_branch?(env)
+      response.delete_cookie 'custom_branch'
+    elsif custom_branch_from_params(env)
+      response.set_cookie 'custom_branch', value: custom_branch_from_params(env), expires: Time.now + 31536000
+    end
+
+    response.finish
+  end
+
+  def custom_branch?(env)
+    custom_branch(env) || disable_custom_branch?(env)
+  end
+
   private
 
+    def disable_custom_branch?(env)
+      env['QUERY_STRING'] =~ /disable[_-]custom[_-]branch/
+    end
+
+    def custom_branch_from_params(env)
+      branch = custom_branch_from_string env['QUERY_STRING']
+    end
+
+    def custom_branch_from_cookie(env)
+      custom_branch_from_string env['HTTP_COOKIE']
+    end
+
+    def custom_branch_from_string(string)
+      $1 if string =~ /(?<!disable.)custom[_-]branch=([^&]+)/
+    end
+
+    def custom_branch(env)
+      custom_branch_from_params(env) || custom_branch_from_cookie(env)
+    end
+
     def load_routes
       each_file { |f| router[route_for(f)] = response_for(f) }
       router.default = router['/']
     end
 
-    def response_for(file)
+    def response_for(file, options = {})
       content = File.read(file)
-      set_config(content) if config_needed? file
+      set_config(content, options) if config_needed? file
 
       headers = {
         'Content-Length'   => content.bytesize.to_s,
@@ -113,13 +156,18 @@ class Travis::Web::App
       Rack::Mime.mime_type File.extname(file)
     end
 
-    def set_config(string)
+    def set_config(string, options = {})
       string.gsub! %r(<meta (rel|name)="travis\.([^"]*)" (href|value)="([^"]*)"[^>]*>) do
         %(<meta #{$1}="travis.#{$2}" #{$3}="#{options[$2.to_sym] || $4}">)
       end
 
-      string.gsub! %r{(src|href)="(\/?)((styles|scripts)\/[^"]*)"} do
-        %(#$1="#$2#{version}/#$3")
+      string.gsub! %r{(src|href)="(?:\/?)((styles|scripts)\/[^"]*)"} do
+        if options[:custom_branch]
+          url = "https://s3.amazonaws.com/travis-web-production/assets/#{options[:custom_branch]}/#{$2}"
+          %(#$1="#{url}")
+        else
+          %(#$1="/#{version}/#$2")
+        end
       end
     end
 
diff --git a/spec/app_spec.rb b/spec/app_spec.rb
index 3b4f4698..584616ba 100644
--- a/spec/app_spec.rb
+++ b/spec/app_spec.rb
@@ -30,4 +30,24 @@ describe Travis::Web::App do
     example { headers['Cache-Control'].should be == 'no-cache' }
     example { headers['Vary'].split(',').should_not include('Accept') }
   end
+
+  describe 'custom branch' do
+    context 'when passing custom branch as a param' do
+      before { get('/?custom-branch=foo') }
+      example { last_response.should be_ok }
+      example { last_response.body.should include('/assets/foo/styles/app.css') }
+      example { last_response.body.should include('/assets/foo/scripts/app.js') }
+      example { headers['Set-Cookie'].should include('custom_branch=foo') }
+    end
+
+    context 'disabling custom branch' do
+      before { get('/?disable-custom-branch=true') }
+      example { last_response.should be_ok }
+      example { last_response.body.should =~ %r{src="/[^\/]+/scripts/app.js} }
+      example { last_response.body.should_not include('/assets/true/styles/app.css') }
+      example { last_response.body.should_not include('/assets/foo/styles/app.css') }
+      example { last_response.body.should_not include('/assets/foo/scripts/app.js') }
+      example { headers['Set-Cookie'].should include('custom_branch=;') }
+    end
+  end
 end