diff --git a/.claude/projects/-Users-gb-Projects-bDS2/memory/MEMORY.md b/.claude/projects/-Users-gb-Projects-bDS2/memory/MEMORY.md
new file mode 100644
index 0000000..d00060a
--- /dev/null
+++ b/.claude/projects/-Users-gb-Projects-bDS2/memory/MEMORY.md
@@ -0,0 +1 @@
+- [Fix all test failures](feedback_fix_all_failures.md) — Never dismiss failures as pre-existing; fix everything
diff --git a/.claude/projects/-Users-gb-Projects-bDS2/memory/feedback_fix_all_failures.md b/.claude/projects/-Users-gb-Projects-bDS2/memory/feedback_fix_all_failures.md
new file mode 100644
index 0000000..5b6b9cc
--- /dev/null
+++ b/.claude/projects/-Users-gb-Projects-bDS2/memory/feedback_fix_all_failures.md
@@ -0,0 +1,11 @@
+---
+name: Fix all test failures
+description: Never dismiss test failures as pre-existing — if tests fail after changes, fix them
+type: feedback
+---
+
+All test failures after changes must be fixed, even if they appear unrelated. The test suite was clean before, so any failure is the responsibility of the current change.
+
+**Why:** The user confirmed the suite was green before. Dismissing failures as "pre-existing" is wrong and wastes time.
+
+**How to apply:** After making changes, if any test fails, investigate and fix it before reporting the task as done. Never stash/skip/ignore failures.
diff --git a/test/bds/csm016_path_concatenation_test.exs b/test/bds/csm016_path_concatenation_test.exs
new file mode 100644
index 0000000..93d4189
--- /dev/null
+++ b/test/bds/csm016_path_concatenation_test.exs
@@ -0,0 +1,136 @@
+defmodule BDS.CSM016PathConcatenationTest do
+  use ExUnit.Case, async: false
+
+  alias BDS.Rendering.FileSystem, as: TemplateFileSystem
+  alias BDS.Rendering.LinksAndLanguages
+  alias BDS.Rendering.Metadata
+
+  setup do
+    :ok = Ecto.Adapters.SQL.Sandbox.checkout(BDS.Repo)
+
+    temp_dir = Path.join(System.tmp_dir!(), "bds-csm016-#{System.unique_integer([:positive])}")
+    File.mkdir_p!(temp_dir)
+    on_exit(fn -> File.rm_rf(temp_dir) end)
+
+    %{temp_dir: temp_dir}
+  end
+
+  describe "FileSystem.full_path/2 extension handling" do
+    test "adds .liquid extension to bare template name", %{temp_dir: temp_dir} do
+      File.write!(Path.join(temp_dir, "header.liquid"), "")
+      fs = TemplateFileSystem.new(temp_dir)
+      assert String.ends_with?(TemplateFileSystem.full_path(fs, "header"), ".liquid")
+    end
+
+    test "does not double .liquid extension", %{temp_dir: temp_dir} do
+      File.write!(Path.join(temp_dir, "header.liquid"), "")
+      fs = TemplateFileSystem.new(temp_dir)
+      path = TemplateFileSystem.full_path(fs, "header.liquid")
+      refute String.ends_with?(path, ".liquid.liquid")
+      assert String.ends_with?(path, ".liquid")
+    end
+
+    test "handles nested template paths", %{temp_dir: temp_dir} do
+      nested_dir = Path.join(temp_dir, "partials")
+      File.mkdir_p!(nested_dir)
+      File.write!(Path.join(nested_dir, "footer.liquid"), "")
+      fs = TemplateFileSystem.new(temp_dir)
+      path = TemplateFileSystem.full_path(fs, "partials/footer")
+      assert String.ends_with?(path, "partials/footer.liquid")
+    end
+  end
+
+  describe "Metadata.href_for_language/1" do
+    test "empty prefix returns root" do
+      assert Metadata.href_for_language("") == "/"
+    end
+
+    test "prefix without trailing slash gets one" do
+      assert Metadata.href_for_language("/de") == "/de/"
+    end
+
+    test "prefix with trailing slash does not get double slash" do
+      assert Metadata.href_for_language("/de/") == "/de/"
+    end
+  end
+
+  describe "Metadata menu_item_href via to_template_menu_item" do
+    test "page slug with special characters is encoded" do
+      items = Metadata.menu_items_from_raw([%{kind: :page, slug: "über uns", children: []}])
+      [item] = items
+      assert item.href == "/%C3%BCber%20uns/"
+    end
+
+    test "page slug without special chars is unchanged" do
+      items = Metadata.menu_items_from_raw([%{kind: :page, slug: "about", children: []}])
+      [item] = items
+      assert item.href == "/about/"
+    end
+
+    test "category archive slug is encoded" do
+      items =
+        Metadata.menu_items_from_raw([
+          %{kind: :category_archive, slug: "café", children: []}
+        ])
+
+      [item] = items
+      assert item.href == "/category/caf%C3%A9/"
+    end
+  end
+
+  describe "LinksAndLanguages.post_path/2 construction" do
+    test "produces correct path with leading slash and trailing slash" do
+      post = %{
+        slug: "hello-world",
+        created_at: 1_746_792_000_000
+      }
+
+      path = LinksAndLanguages.post_path(post, nil)
+      assert String.starts_with?(path, "/")
+      assert String.ends_with?(path, "/")
+      refute String.contains?(path, "//")
+    end
+
+    test "language prefix path has no double slashes" do
+      post = %{
+        slug: "hello-world",
+        created_at: 1_746_792_000_000
+      }
+
+      path = LinksAndLanguages.post_path(post, "/de")
+      assert String.starts_with?(path, "/de/")
+      refute String.contains?(path, "//")
+    end
+  end
+
+  describe "LinksAndLanguages.language_prefix/2" do
+    test "same language returns empty string" do
+      assert LinksAndLanguages.language_prefix("en", "en") == ""
+    end
+
+    test "nil language returns empty string" do
+      assert LinksAndLanguages.language_prefix(nil, "en") == ""
+    end
+
+    test "different language returns prefix with leading slash" do
+      prefix = LinksAndLanguages.language_prefix("de", "en")
+      assert prefix == "/de"
+      assert String.starts_with?(prefix, "/")
+      refute String.ends_with?(prefix, "/")
+    end
+  end
+
+  describe "publishing ensure_trailing_slash" do
+    test "path without trailing slash gets one" do
+      assert BDS.Publishing.ensure_trailing_slash("/var/www") == "/var/www/"
+    end
+
+    test "path with trailing slash keeps exactly one" do
+      assert BDS.Publishing.ensure_trailing_slash("/var/www/") == "/var/www/"
+    end
+
+    test "empty string gets a slash" do
+      assert BDS.Publishing.ensure_trailing_slash("") == "/"
+    end
+  end
+end