diff --git a/test/bds/csm011_url_state_test.exs b/test/bds/csm011_url_state_test.exs
new file mode 100644
index 0000000..12a318c
--- /dev/null
+++ b/test/bds/csm011_url_state_test.exs
@@ -0,0 +1,127 @@
+defmodule BDS.CSM011UrlStateTest do
+  use ExUnit.Case, async: false
+
+  import Phoenix.ConnTest
+  import Phoenix.LiveViewTest
+
+  @endpoint BDS.Desktop.Endpoint
+
+  setup do
+    :ok = Ecto.Adapters.SQL.Sandbox.checkout(BDS.Repo)
+
+    prev = System.get_env("BDS_DESKTOP_AUTOMATION")
+    System.put_env("BDS_DESKTOP_AUTOMATION", "1")
+
+    on_exit(fn ->
+      if prev,
+        do: System.put_env("BDS_DESKTOP_AUTOMATION", prev),
+        else: System.delete_env("BDS_DESKTOP_AUTOMATION")
+    end)
+
+    temp_dir =
+      Path.join(System.tmp_dir!(), "bds-csm011-#{System.unique_integer([:positive])}")
+
+    File.mkdir_p!(temp_dir)
+    on_exit(fn -> File.rm_rf(temp_dir) end)
+
+    {:ok, project} = BDS.Projects.create_project(%{name: "CSM011", data_path: temp_dir})
+    {:ok, post} = BDS.Posts.create_post(%{project_id: project.id, title: "Test Post"})
+    %{project: project, post: post}
+  end
+
+  describe "mount reads URL params" do
+    test "/?view=media activates media view" do
+      {:ok, view, _html} = live(build_conn(), "/?view=media")
+
+      assert get_workbench(view).active_view == :media
+    end
+
+    test "/ defaults to posts view" do
+      {:ok, view, _html} = live(build_conn(), "/")
+
+      assert get_workbench(view).active_view == :posts
+    end
+
+    test "invalid view param falls back to posts" do
+      {:ok, view, _html} = live(build_conn(), "/?view=nonexistent")
+
+      assert get_workbench(view).active_view == :posts
+    end
+
+    test "/?tab=post:<id> opens the tab", %{post: post} do
+      {:ok, view, _html} = live(build_conn(), "/?tab=post:#{post.id}")
+
+      assert get_workbench(view).active_tab == {:post, post.id}
+    end
+
+    test "/?view=media&tab=post:<id> applies both", %{post: post} do
+      {:ok, view, _html} = live(build_conn(), "/?view=media&tab=post:#{post.id}")
+
+      workbench = get_workbench(view)
+      assert workbench.active_view == :media
+      assert workbench.active_tab == {:post, post.id}
+    end
+  end
+
+  describe "select_view pushes url-state event" do
+    test "selecting media view pushes url with view=media" do
+      {:ok, view, _html} = live_isolated(build_conn(), BDS.Desktop.ShellLive)
+
+      render_click(view, "select_view", %{"view" => "media"})
+
+      assert_push_event(view, "url-state", %{path: "/?view=media"})
+    end
+
+    test "selecting posts view pushes clean URL" do
+      {:ok, view, _html} = live_isolated(build_conn(), BDS.Desktop.ShellLive)
+
+      render_click(view, "select_view", %{"view" => "posts"})
+
+      assert_push_event(view, "url-state", %{path: "/"})
+    end
+  end
+
+  describe "select_tab pushes url-state event" do
+    test "selecting a tab pushes URL with tab param", %{post: post} do
+      {:ok, view, _html} = live_isolated(build_conn(), BDS.Desktop.ShellLive)
+
+      render_click(view, "select_tab", %{"type" => "post", "id" => post.id})
+
+      assert_push_event(view, "url-state", %{path: path})
+      assert path =~ "tab=post"
+      assert path =~ post.id
+    end
+  end
+
+  describe "close_tab pushes url-state event" do
+    test "closing the active tab removes tab from URL", %{post: post} do
+      {:ok, view, _html} = live_isolated(build_conn(), BDS.Desktop.ShellLive)
+
+      render_click(view, "select_tab", %{"type" => "post", "id" => post.id})
+      render_click(view, "close_tab", %{"type" => "post", "id" => post.id})
+
+      assert_push_event(view, "url-state", %{path: "/"})
+    end
+  end
+
+  describe "open_sidebar_item pushes url-state event" do
+    test "opening a sidebar item pushes URL with tab param", %{post: post} do
+      {:ok, view, _html} = live_isolated(build_conn(), BDS.Desktop.ShellLive)
+
+      render_click(view, "open_sidebar_item", %{
+        "route" => "post",
+        "id" => post.id,
+        "title" => "Test Post"
+      })
+
+      assert_push_event(view, "url-state", %{path: path})
+      assert path =~ "tab=post"
+      assert path =~ post.id
+    end
+  end
+
+  defp get_workbench(view) do
+    state = :sys.get_state(view.pid)
+    state.socket.assigns.workbench
+  end
+end