diff --git a/README.md b/README.md
index d1338d6..586a263 100644
--- a/README.md
+++ b/README.md
@@ -58,7 +58,7 @@ defmodule TwitterClientTest do
       Plug.Conn.resp(conn, 429, ~s<{"errors": [{"code": 88, "message": "Rate limit exceeded"}]}>)
     end)
 
-    {:ok, client} = TwitterClient.start_link(url: endpoint_url(bypass.port))
+    {:ok, client} = TwitterClient.start_link(url: bypass.endpoint_url)
     assert {:error, :rate_limited} == TwitterClient.post_tweet(client, "Elixir is awesome!")
   end
 
@@ -68,7 +68,7 @@ defmodule TwitterClientTest do
       Plug.Conn.resp(conn, 200, "")
     end)
 
-    {:ok, client} = TwitterClient.start_link(url: endpoint_url(bypass.port))
+    {:ok, client} = TwitterClient.start_link(url: bypass.endpoint_url)
 
     assert :ok == TwitterClient.post_tweet(client, "Elixir is awesome!")
 
@@ -85,8 +85,6 @@ defmodule TwitterClientTest do
 
     assert :ok == TwitterClient.post_tweet(client, "Elixir is awesome!")
   end
-
-  defp endpoint_url(port), do: "http://localhost:#{port}/"
 end
 ```
 
@@ -134,11 +132,9 @@ test configuration is basically the same, there are only two differences:
          Plug.Conn.resp(conn, 429, ~s<{"errors": [{"code": 88, "message": "Rate limit exceeded"}]}>)
        end)
 
-       {:ok, client} = TwitterClient.start_link(url: endpoint_url(shared.bypass.port))
+       {:ok, client} = TwitterClient.start_link(url: shared.bypass.endpoint_url))
        assert {:error, :rate_limited} == TwitterClient.post_tweet(client, "Elixir is awesome!")
      end
-
-     defp endpoint_url(port), do: "http://localhost:#{port}/"
    end
    ```
 
diff --git a/lib/bypass.ex b/lib/bypass.ex
index 4d7ae77..08d90cc 100644
--- a/lib/bypass.ex
+++ b/lib/bypass.ex
@@ -5,12 +5,12 @@ defmodule Bypass do
              |> String.split("<!-- MDOC !-->")
              |> Enum.fetch!(1)
 
-  defstruct pid: nil, port: nil
+  defstruct pid: nil, port: nil, endpoint_url: nil
 
   @typedoc """
   Represents a Bypass server process.
   """
-  @type t :: %__MODULE__{pid: pid, port: non_neg_integer}
+  @type t :: %__MODULE__{pid: pid, port: non_neg_integer, endpoint_url: String.t()}
 
   import Bypass.Utils
   require Logger
@@ -44,7 +44,7 @@ defmodule Bypass do
     pid = start_instance(opts)
     port = Bypass.Instance.call(pid, :port)
     debug_log("Did open connection #{inspect(pid)} on port #{inspect(port)}")
-    bypass = %Bypass{pid: pid, port: port}
+    bypass = %Bypass{pid: pid, port: port, endpoint_url: "http://localhost:#{port}/"}
     setup_framework_integration(test_framework(), bypass)
     bypass
   end
diff --git a/test/bypass_test.exs b/test/bypass_test.exs
index 6f41000..791a8de 100644
--- a/test/bypass_test.exs
+++ b/test/bypass_test.exs
@@ -587,4 +587,9 @@ defmodule BypassTest do
       Bypass.open(:error)
     end
   end
+
+  test "Bypass.open/1 open fills endpoint_url" do
+    bypass = Bypass.open(port: 8000)
+    assert bypass.endpoint_url == "http://localhost:8000/"
+  end
 end