diff --git a/config/initializers/tenanting.rb b/config/initializers/tenanting.rb
index 85ef97ba7..a63b37caa 100644
--- a/config/initializers/tenanting.rb
+++ b/config/initializers/tenanting.rb
@@ -1,19 +1,37 @@
-Rails.application.config.after_initialize do
-  # in production and staging, we're using a two-level subdomain like "tenant.fizzy.37signals.com".
-  # in development and beta it's only a single-level subdomain.
-  Rails.application.config.active_record_tenanted.tenant_resolver = ->(request) do
-    tld_length = [ "37signals.com", "37signals-staging.com" ].include?(request.domain) ? 2 : 1
+module AccountSlug
+  PATTERN = /(\d{7,})/
+  FORMAT = "%07d"
+  PATH_INFO_MATCH = /\A(\/#{AccountSlug::PATTERN})/
 
-    if request.path == "/up"
-      nil
-    elsif subdomain = request.subdomain(tld_length).presence
-      subdomain
-    elsif request.path =~ %r{^/(queenbee|signup)\b}
-      nil
-    else
-      "return-404" # this is a quick fix for now
+  # We're using account id prefixes in the URL path. Rather than namespace
+  # all our routes, we're "mounting" the Rails app at this URL prefix.
+  def self.extract(request)
+    # $1, $2, $' == script_name, slug, path_info
+    if request.script_name && request.script_name =~ PATH_INFO_MATCH
+      # Likely due to restarting the action cable connection after upgrade
+      AccountSlug.decode($2)
+    elsif request.path_info =~ PATH_INFO_MATCH
+      # Yanks the prefix off PATH_INFO and move it to SCRIPT_NAME
+      request.engine_script_name = request.script_name = $1
+      request.path_info   = $'.empty? ? "/" : $'
+
+      # Limit session cookies to the slug path.
+      # TODO TEST ME
+      request.env["rack.session.options"][:path] = $1
+
+      # Return the account id for tenanting.
+      AccountSlug.decode($2)
     end
   end
+
+  def self.decode(slug) slug.to_i end
+  def self.encode(id) FORMAT % id end
+end
+
+Rails.application.config.after_initialize do
+  Rails.application.config.active_record_tenanted.tenant_resolver = ->(request) do
+    AccountSlug.extract(request)
+  end
 end
 
 ActiveSupport.on_load(:action_controller_base) do