diff --git a/SPECGAPS.md b/SPECGAPS.md
index 52568bd..0d1e2f9 100644
--- a/SPECGAPS.md
+++ b/SPECGAPS.md
@@ -114,7 +114,7 @@ All reconciled to follow code. Specs must be self-consistent and match code.
 | ID | Claim | Spec | Path |
 |---|---|---|---|
 | D1-1 | ~~UniqueMediaTranslation invariant~~ | media.allium:108 | **Resolved:** test added (re-upsert updates not duplicates; direct duplicate insert rejected). Test exposed a real bug — `Media.Translation` declared the migration's index name but ecto_sqlite3 derives the violated-constraint name from columns, so violations crashed instead of returning a changeset error; fixed `unique_constraint` name to `:media_translations_translation_for_language_index` |
-| D1-2 | UniqueTranslationPerLanguage invariant | translation.allium:94 | Write test: create duplicate post translation, expect rejection |
+| D1-2 | ~~UniqueTranslationPerLanguage invariant~~ | translation.allium:94 | **Resolved:** test added (re-upsert updates not duplicates; direct duplicate insert rejected). Same bug as D1-1 — `Posts.Translation` declared the migration's index name but ecto_sqlite3 derives the violated-constraint name from columns, so duplicates crashed instead of returning a changeset error; fixed `unique_constraint` name to `:post_translations_translation_for_language_index` |
 | D1-3 | BundledDefaultTemplatesExistOutsideProjectData | template.allium:65 | Write test: render with no Template rows, bundled template found |
 | D1-4 | UserTemplateDirectoryOverridesBundledDefaults | template.allium:75 | Write test: project template overrides bundled same-slug |
 | D1-5 | LiquidTagSubset (5 tags only) | template.allium:179 | Write test: unsupported tag raises error |
diff --git a/lib/bds/posts/translation.ex b/lib/bds/posts/translation.ex
index bc58894..7e335c5 100644
--- a/lib/bds/posts/translation.ex
+++ b/lib/bds/posts/translation.ex
@@ -79,6 +79,6 @@ defmodule BDS.Posts.Translation do
       :updated_at
     ])
     |> foreign_key_constraint(:translation_for)
-    |> unique_constraint(:language, name: :post_translations_translation_language_idx)
+    |> unique_constraint(:language, name: :post_translations_translation_for_language_index)
   end
 end
diff --git a/test/bds/post_translations_test.exs b/test/bds/post_translations_test.exs
index 5e70de9..9183b50 100644
--- a/test/bds/post_translations_test.exs
+++ b/test/bds/post_translations_test.exs
@@ -310,6 +310,61 @@ defmodule BDS.PostTranslationsTest do
     assert report.do_not_translate_posts == [ignored_post.id]
   end
 
+  test "UniqueTranslationPerLanguage: a post has at most one translation per language", %{
+    project: project
+  } do
+    assert {:ok, post} =
+             Posts.create_post(%{
+               project_id: project.id,
+               title: "Canonical Post",
+               content: "Hello world",
+               language: "en"
+             })
+
+    assert {:ok, first} =
+             Posts.upsert_post_translation(post.id, "de", %{title: "Titel"})
+
+    # Re-upserting the same language updates the existing row instead of adding a second.
+    assert {:ok, second} =
+             Posts.upsert_post_translation(post.id, "de", %{title: "Geändert"})
+
+    assert second.id == first.id
+
+    assert [persisted] =
+             BDS.Posts.Translation
+             |> where([t], t.translation_for == ^post.id and t.language == "de")
+             |> Repo.all()
+
+    assert persisted.title == "Geändert"
+
+    # A direct insert of a distinct row with the same (post, language) is rejected by the
+    # unique constraint backing the invariant, returning a changeset error rather than crashing.
+    now = BDS.Persistence.now_ms()
+
+    duplicate =
+      BDS.Posts.Translation.changeset(%BDS.Posts.Translation{}, %{
+        id: Ecto.UUID.generate(),
+        project_id: post.project_id,
+        translation_for: post.id,
+        language: "de",
+        title: "Duplicate",
+        status: :draft,
+        created_at: now,
+        updated_at: now
+      })
+
+    assert {:error, changeset} = Repo.insert(duplicate)
+    assert %{language: ["has already been taken"]} = errors_on(changeset)
+  end
+
+  defp errors_on(changeset) do
+    Ecto.Changeset.traverse_errors(changeset, fn {message, opts} ->
+      Regex.replace(~r"%{(\w+)}", message, fn _, key ->
+        opts |> Keyword.get(String.to_existing_atom(key), key) |> to_string()
+      end)
+    end)
+  end
+
   defp configure_auto_translation_test_runtime do
     assert {:ok, _endpoint} =
              AI.put_endpoint(