diff --git a/SPECGAPS.md b/SPECGAPS.md
index 1e7ae8a..759ff2f 100644
--- a/SPECGAPS.md
+++ b/SPECGAPS.md
@@ -16,7 +16,7 @@ Gap categories: **SC** = spec correct, fix code | **CS** = code correct, update
 | A1-4 | ~~`doNotTranslate: false` written to frontmatter despite "only when true"~~ | frontmatter.allium:398 | `file_sync.ex:78` now converts false→nil so serializer omits the key | **Resolved:** doNotTranslate omitted from frontmatter when false, test added |
 | A1-5 | ~~Auto-save after 3000ms idle~~ | editor_post.allium:183-188 | PostEditor schedules auto-save via parent timer on dirty change | **Resolved:** 3000ms idle auto-save timer in Bridges, tab-switch save in ShellLive, cancel on manual save, 3 tests added |
 | A1-6 | ~~On-demand rendering in preview server~~ | preview.allium:53-93 | `Preview.Router` matches post/archive/home/language routes and renders on-demand via `Rendering` | **Resolved:** `Preview.Router` implements on-demand template rendering for post, archive, home, date, tag, category, page, and language-prefixed routes; static file fallback retained for non-HTML assets (pagefind, feeds); 6 tests added |
-| A1-7 | Template lookup must use all 4 levels (post→tag→category→default) | template_context.allium:267-277 | Only levels 1 and 4 implemented; tag/category fallback unused | Fix code: implement levels 2-3 in template_selection.ex |
+| A1-7 | ~~Template lookup must use all 4 levels (post→tag→category→default)~~ | template_context.allium:267-277 | `resolve_post_template_slug/3` implements tag→category cascade; all callers (preview, generation) updated | **Resolved:** `resolve_post_template_slug/3` in template_selection.ex, callers in preview.ex, router.ex, outputs.ex updated, 8 tests added |
 | A1-8 | `ValidateLiquid`/`ValidateScript` before publish | template.allium:110, script.allium:165 | No validation gate before publish | Fix code: add validation step before publish |
 | A1-9 | 17 preset colors + custom hex in tag picker | editor_tags.allium | Native `<input type="color">`, no preset palette | Fix code: implement preset color palette popover |
 | A1-10 | Template file written on create | engine_side_effects.allium:151-153 | Draft templates have `file_path=""` | Fix code: write template file on create |
diff --git a/lib/bds/generation/outputs.ex b/lib/bds/generation/outputs.ex
index 220af39..8b8a5c1 100644
--- a/lib/bds/generation/outputs.ex
+++ b/lib/bds/generation/outputs.ex
@@ -5,6 +5,8 @@ defmodule BDS.Generation.Outputs do
   import BDS.Generation.Renderers
   import BDS.Generation.Sitemap, only: [render_feed: 3, render_atom: 3, render_calendar: 1]
 
+  alias BDS.Rendering.TemplateSelection
+
   @spec additional_languages(map()) :: [String.t()]
   def additional_languages(plan) do
     Enum.reject(plan.blog_languages, &(&1 == plan.language))
@@ -391,10 +393,12 @@ defmodule BDS.Generation.Outputs do
         canonical_variant = Map.get(translations_by_post_language, {post.id, main_language}, post)
         body = load_body(project_id, canonical_variant.file_path, canonical_variant.content)
 
+        effective_slug = effective_template_slug(project_id, post)
+
         {page_output_path(post.slug, nil),
          render_post_output(
            project_id,
-           post.template_slug,
+           effective_slug,
            %{
              id: canonical_variant.id,
              title: canonical_variant.title,
@@ -423,10 +427,12 @@ defmodule BDS.Generation.Outputs do
         |> Enum.map(fn post ->
           body = load_body(project_id, post.file_path, post.content)
 
+          effective_slug = effective_template_slug(project_id, post)
+
           {page_output_path(post.slug, language),
            render_post_output(
              project_id,
-             post.template_slug,
+             effective_slug,
              %{
                id: post.id,
                title: post.title,
@@ -521,10 +527,12 @@ defmodule BDS.Generation.Outputs do
         canonical_variant = Map.get(translations_by_post_language, {post.id, main_language}, post)
         body = load_body(project_id, canonical_variant.file_path, canonical_variant.content)
 
+        effective_slug = effective_template_slug(project_id, post)
+
         {post_output_path(post),
          render_post_output(
            project_id,
-           post.template_slug,
+           effective_slug,
            %{
              id: canonical_variant.id,
              title: canonical_variant.title,
@@ -551,10 +559,12 @@ defmodule BDS.Generation.Outputs do
         Enum.map(posts, fn post ->
           body = load_body(project_id, post.file_path, post.content)
 
+          effective_slug = effective_template_slug(project_id, post)
+
           {post_output_path(post, language),
            render_post_output(
              project_id,
-             post.template_slug,
+             effective_slug,
              %{
                id: post.id,
                title: post.title,
@@ -571,4 +581,18 @@ defmodule BDS.Generation.Outputs do
 
     post_outputs ++ translation_outputs
   end
+
+  defp effective_template_slug(project_id, post) do
+    slug = Map.get(post, :template_slug)
+
+    if is_binary(slug) and slug != "" do
+      slug
+    else
+      TemplateSelection.resolve_post_template_slug(
+        project_id,
+        Map.get(post, :tags) || [],
+        Map.get(post, :categories) || []
+      )
+    end
+  end
 end
diff --git a/lib/bds/preview.ex b/lib/bds/preview.ex
index a024125..ab00398 100644
--- a/lib/bds/preview.ex
+++ b/lib/bds/preview.ex
@@ -9,6 +9,7 @@ defmodule BDS.Preview do
   alias BDS.Projects
   alias BDS.Repo
   alias BDS.Rendering
+  alias BDS.Rendering.TemplateSelection
 
   @host "127.0.0.1"
   @port 4123
@@ -219,6 +220,7 @@ defmodule BDS.Preview do
 
   defp draft_preview_payload(post, query_params) do
     requested_language = query_params |> Map.get("lang") |> normalize_requested_language()
+    effective_slug = post.template_slug || TemplateSelection.resolve_post_template_slug(post.project_id, post.tags, post.categories)
 
     case draft_preview_translation(post.id, requested_language, post.language) do
       %Translation{} = translation ->
@@ -230,7 +232,7 @@ defmodule BDS.Preview do
           slug: post.slug,
           language: translation.language,
           excerpt: translation.excerpt,
-          template_slug: post.template_slug
+          template_slug: effective_slug
         }
 
       nil ->
@@ -242,7 +244,7 @@ defmodule BDS.Preview do
           slug: post.slug,
           language: post.language,
           excerpt: post.excerpt,
-          template_slug: post.template_slug
+          template_slug: effective_slug
         }
     end
   end
diff --git a/lib/bds/preview/router.ex b/lib/bds/preview/router.ex
index de7450b..aadb385 100644
--- a/lib/bds/preview/router.ex
+++ b/lib/bds/preview/router.ex
@@ -10,6 +10,7 @@ defmodule BDS.Preview.Router do
   alias BDS.Posts.Post
   alias BDS.Posts.Translation
   alias BDS.Rendering
+  alias BDS.Rendering.TemplateSelection
   alias BDS.Repo
 
   @type route ::
@@ -219,7 +220,9 @@ defmodule BDS.Preview.Router do
       _post_record: effective_record
     }
 
-    case Rendering.render_post_page(project_id, post.template_slug, assigns) do
+    effective_slug = post.template_slug || TemplateSelection.resolve_post_template_slug(project_id, post.tags, post.categories)
+
+    case Rendering.render_post_page(project_id, effective_slug, assigns) do
       {:ok, rendered} -> {:ok, rendered}
       {:error, _reason} -> {:error, :not_found}
     end
diff --git a/lib/bds/rendering/template_selection.ex b/lib/bds/rendering/template_selection.ex
index 575d2f7..8c9413a 100644
--- a/lib/bds/rendering/template_selection.ex
+++ b/lib/bds/rendering/template_selection.ex
@@ -4,13 +4,52 @@ defmodule BDS.Rendering.TemplateSelection do
   import Ecto.Query
 
   alias BDS.Frontmatter
+  alias BDS.Metadata
   alias BDS.Projects
   alias BDS.Rendering.FileSystem
   alias BDS.Rendering.Filters
   alias BDS.Repo
   alias BDS.StarterTemplates
+  alias BDS.Tags.Tag
   alias BDS.Templates.Template
 
+  @spec resolve_post_template_slug(String.t(), [String.t()], [String.t()]) ::
+          String.t() | nil
+  def resolve_post_template_slug(project_id, tag_names, category_names) do
+    resolve_from_tags(project_id, tag_names) ||
+      resolve_from_categories(project_id, category_names)
+  end
+
+  defp resolve_from_tags(_project_id, []), do: nil
+
+  defp resolve_from_tags(project_id, tag_names) do
+    Repo.all(
+      from tag in Tag,
+        where:
+          tag.project_id == ^project_id and
+            tag.name in ^tag_names and
+            not is_nil(tag.post_template_slug) and
+            tag.post_template_slug != "",
+        select: tag.post_template_slug,
+        limit: 1
+    )
+    |> List.first()
+  end
+
+  defp resolve_from_categories(_project_id, []), do: nil
+
+  defp resolve_from_categories(project_id, category_names) do
+    {:ok, state} = Metadata.get_project_metadata(project_id)
+    settings = state.category_settings || %{}
+
+    Enum.find_value(category_names, fn cat_name ->
+      case Map.get(settings, cat_name) do
+        %{"post_template_slug" => slug} when is_binary(slug) and slug != "" -> slug
+        _ -> nil
+      end
+    end)
+  end
+
   @spec load_template_source(String.t(), atom(), String.t() | nil) ::
           {:ok, String.t()} | {:error, term()}
   def load_template_source(project_id, kind, slug) do
diff --git a/test/bds/template_lookup_priority_test.exs b/test/bds/template_lookup_priority_test.exs
new file mode 100644
index 0000000..883b1f0
--- /dev/null
+++ b/test/bds/template_lookup_priority_test.exs
@@ -0,0 +1,215 @@
+defmodule BDS.TemplateLookupPriorityTest do
+  use ExUnit.Case, async: false
+
+  alias BDS.Rendering.TemplateSelection
+
+  setup do
+    :ok = Ecto.Adapters.SQL.Sandbox.checkout(BDS.Repo)
+    temp_dir = Path.join(System.tmp_dir!(), "bds-tlp-#{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: "TLP Test", data_path: temp_dir})
+
+    {:ok, _metadata} =
+      BDS.Metadata.update_project_metadata(project.id, %{
+        main_language: "en",
+        blog_languages: ["en"]
+      })
+
+    %{project: project}
+  end
+
+  defp create_published_template(project_id, slug, content) do
+    {:ok, template} =
+      BDS.Templates.create_template(%{
+        project_id: project_id,
+        title: "Template #{slug}",
+        kind: :post,
+        content: content
+      })
+
+    if template.slug != slug do
+      BDS.Repo.update!(Ecto.Changeset.change(template, slug: slug))
+    end
+
+    {:ok, published} = BDS.Templates.publish_template(template.id)
+    published
+  end
+
+  defp create_tag(project_id, name, post_template_slug) do
+    {:ok, tag} =
+      BDS.Tags.create_tag(%{
+        project_id: project_id,
+        name: name,
+        post_template_slug: post_template_slug
+      })
+
+    tag
+  end
+
+  describe "resolve_post_template_slug/3" do
+    test "level 1: returns post's own template_slug when set", %{project: project} do
+      result =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          ["some-tag"],
+          ["article"]
+        )
+
+      assert result == nil
+    end
+
+    test "level 2: falls back to tag's post_template_slug", %{project: project} do
+      _tag = create_tag(project.id, "photo", "photo-layout")
+
+      result =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          ["photo"],
+          []
+        )
+
+      assert result == "photo-layout"
+    end
+
+    test "level 2: uses first matching tag with a template slug", %{project: project} do
+      _tag1 = create_tag(project.id, "news", nil)
+      _tag2 = create_tag(project.id, "gallery", "gallery-layout")
+
+      result =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          ["news", "gallery"],
+          []
+        )
+
+      assert result == "gallery-layout"
+    end
+
+    test "level 3: falls back to category's post_template_slug", %{project: project} do
+      {:ok, _} = BDS.Metadata.add_category(project.id, "review")
+
+      {:ok, _} =
+        BDS.Metadata.update_category_settings(project.id, "review", %{
+          "post_template_slug" => "review-layout"
+        })
+
+      result =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          [],
+          ["review"]
+        )
+
+      assert result == "review-layout"
+    end
+
+    test "level 2 takes priority over level 3", %{project: project} do
+      _tag = create_tag(project.id, "featured", "featured-layout")
+      {:ok, _} = BDS.Metadata.add_category(project.id, "review")
+
+      {:ok, _} =
+        BDS.Metadata.update_category_settings(project.id, "review", %{
+          "post_template_slug" => "review-layout"
+        })
+
+      result =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          ["featured"],
+          ["review"]
+        )
+
+      assert result == "featured-layout"
+    end
+
+    test "level 4: returns nil when no tag or category has a template", %{project: project} do
+      _tag = create_tag(project.id, "plain", nil)
+
+      result =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          ["plain"],
+          ["article"]
+        )
+
+      assert result == nil
+    end
+  end
+
+  describe "end-to-end template lookup with rendering" do
+    test "post renders with tag-specific template when no post template set", %{
+      project: project
+    } do
+      template =
+        create_published_template(project.id, "photo-layout", "<h1>PHOTO: {{ page.title }}</h1>")
+
+      _tag = create_tag(project.id, "photo", template.slug)
+
+      {:ok, post} =
+        BDS.Posts.create_post(%{
+          project_id: project.id,
+          title: "My Photo Post",
+          content: "photo content",
+          language: "en",
+          tags: ["photo"]
+        })
+
+      resolved_slug =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          post.tags,
+          post.categories
+        )
+
+      assert resolved_slug == "photo-layout"
+
+      {:ok, rendered} =
+        TemplateSelection.load_template_source(project.id, :post, resolved_slug)
+
+      assert rendered =~ "PHOTO:"
+    end
+
+    test "post renders with category-specific template when no post or tag template", %{
+      project: project
+    } do
+      template =
+        create_published_template(
+          project.id,
+          "review-layout",
+          "<h1>REVIEW: {{ page.title }}</h1>"
+        )
+
+      {:ok, _} = BDS.Metadata.add_category(project.id, "review")
+
+      {:ok, _} =
+        BDS.Metadata.update_category_settings(project.id, "review", %{
+          "post_template_slug" => template.slug
+        })
+
+      {:ok, post} =
+        BDS.Posts.create_post(%{
+          project_id: project.id,
+          title: "My Review",
+          content: "review content",
+          language: "en",
+          categories: ["review"]
+        })
+
+      resolved_slug =
+        TemplateSelection.resolve_post_template_slug(
+          project.id,
+          post.tags,
+          post.categories
+        )
+
+      assert resolved_slug == "review-layout"
+
+      {:ok, rendered} =
+        TemplateSelection.load_template_source(project.id, :post, resolved_slug)
+
+      assert rendered =~ "REVIEW:"
+    end
+  end
+end