diff --git a/SPECGAPS.md b/SPECGAPS.md
index 4cfe8ba..8b2bca5 100644
--- a/SPECGAPS.md
+++ b/SPECGAPS.md
@@ -118,7 +118,7 @@ All reconciled to follow code. Specs must be self-consistent and match code.
 | 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 | **Resolved:** added 4 tests in `template_lookup_priority_test.exs` — with no Template rows for the project, `load_template_source/3` resolves bundled single-post/post-list/not-found defaults (and still resolves when the project has no `templates/` directory at all) |
 | D1-4 | ~~UserTemplateDirectoryOverridesBundledDefaults~~ | template.allium:75 | **Resolved:** added 2 tests in `template_lookup_priority_test.exs` — a published project Template row with the bundled default slug (`single-post`) wins over the bundled default both when resolving `:post` with no explicit slug and when the slug is requested explicitly |
-| D1-5 | LiquidTagSubset (5 tags only) | template.allium:179 | Write test: unsupported tag raises error |
+| D1-5 | ~~LiquidTagSubset (5 tags only)~~ | template.allium:179 | **Resolved:** added `BDS.Rendering.LiquidParser`, a restricted Liquex parser recognizing only the subset (if/for/assign/render + `{{ }}` output); any other tag (`unless`, `case`, `capture`, `tablerow`, `cycle`, `increment`, …) leaves unmatched input and fails `eos/0`. Wired into `validate_liquid` (publish gate), `template_selection.render_template`, `filters.render_macro_source`, and MCP `validate_template` so validation and rendering share the same surface; 6 parametrized tests added asserting unsupported tags are rejected at publish |
 | D1-6 | LiquidFilterSubset (4 standard + 2 custom) | template.allium:191 | Write test: unsupported filter raises error |
 | D1-7 | LiquidOperatorSubset | template.allium:210 | Write test: unsupported operator raises error |
 | D1-8 | MacroTimeout guarantee | script.allium:94-95 | Write test: macro times out within budget |
diff --git a/lib/bds/mcp/tools.ex b/lib/bds/mcp/tools.ex
index 24e21ed..d641756 100644
--- a/lib/bds/mcp/tools.ex
+++ b/lib/bds/mcp/tools.ex
@@ -71,7 +71,7 @@ defmodule BDS.MCP.Tools do
 
   @spec validate_template(String.t()) :: {:ok, %{valid: boolean(), errors: [String.t()]}}
   def validate_template(source) when is_binary(source) do
-    case Liquex.parse(source) do
+    case Liquex.parse(source, BDS.Rendering.LiquidParser) do
       {:ok, _ast} ->
         {:ok, %{valid: true, errors: []}}
 
diff --git a/lib/bds/rendering/filters.ex b/lib/bds/rendering/filters.ex
index 01b9fe1..917f86e 100644
--- a/lib/bds/rendering/filters.ex
+++ b/lib/bds/rendering/filters.ex
@@ -155,7 +155,7 @@ defmodule BDS.Rendering.Filters do
   end
 
   defp render_macro_source(template_path, template_source, assigns, context) do
-    with {:ok, template_ast} <- Liquex.parse(template_source),
+    with {:ok, template_ast} <- Liquex.parse(template_source, BDS.Rendering.LiquidParser),
          {:ok, rendered} <- safe_liquex_render(template_ast, context, assigns) do
       rendered
     else
diff --git a/lib/bds/rendering/liquid_parser.ex b/lib/bds/rendering/liquid_parser.ex
new file mode 100644
index 0000000..e4aeef0
--- /dev/null
+++ b/lib/bds/rendering/liquid_parser.ex
@@ -0,0 +1,66 @@
+defmodule BDS.Rendering.LiquidParser do
+  @moduledoc """
+  Restricted Liquid parser enforcing the `LiquidTagSubset` invariant
+  (`specs/template.allium`).
+
+  Only the tags used by the bundled starter templates are recognized:
+
+    * `{% if %}` / `{% elsif %}` / `{% else %}` / `{% endif %}`
+    * `{% for %}` / `{% endfor %}`
+    * `{% assign %}`
+    * `{% render 'partial', name: value %}`
+    * `{{ object }}` output (and whitespace-stripped variants `{%- -%}` / `{{- -}}`)
+
+  Any tag outside this subset (`unless`, `case`, `capture`, `raw`, `comment`,
+  `cycle`, `tablerow`, `increment`, `decrement`, `liquid`, `echo`, `include`)
+  leaves unmatched input and fails the `eos/0` check, producing a parse error.
+
+  Pass this module as the second argument to `Liquex.parse/2` (and
+  `Liquex.parse!/2`) so validation and rendering share the same surface.
+  """
+
+  import NimbleParsec
+
+  @tags [
+    Liquex.Tag.AssignTag,
+    Liquex.Tag.ForTag,
+    Liquex.Tag.IfTag,
+    Liquex.Tag.RenderTag,
+    Liquex.Tag.ObjectTag
+  ]
+
+  tags_parser = Enum.map(@tags, &tag(&1.parse(), {:tag, &1}))
+
+  # Ensure the tags are loaded into scope, otherwise function_exported? will
+  # return false.
+  Enum.each(@tags, &Code.ensure_loaded!/1)
+
+  liquid_tags_parser =
+    @tags
+    |> Enum.filter(&function_exported?(&1, :parse_liquid_tag, 0))
+    |> Enum.map(&tag(&1.parse_liquid_tag(), {:tag, &1}))
+    |> choice()
+
+  # Special case for leading spaces before `{%-` and `{{-`.
+  leading_whitespace =
+    empty()
+    # credo:disable-for-lines:1
+    |> Liquex.Parser.Literal.whitespace(1)
+    |> lookahead(choice([string("{%-"), string("{{-")]))
+    |> ignore()
+
+  base =
+    choice(
+      tags_parser ++
+        [
+          # credo:disable-for-lines:2
+          Liquex.Parser.Literal.text(),
+          leading_whitespace
+        ]
+    )
+
+  defcombinatorp(:document, repeat(base))
+  defcombinatorp(:liquid_tag_contents, repeat(liquid_tags_parser))
+
+  defparsec(:parse, parsec(:document) |> eos())
+end
diff --git a/lib/bds/rendering/template_selection.ex b/lib/bds/rendering/template_selection.ex
index 8c9413a..506b6ea 100644
--- a/lib/bds/rendering/template_selection.ex
+++ b/lib/bds/rendering/template_selection.ex
@@ -132,7 +132,7 @@ defmodule BDS.Rendering.TemplateSelection do
   @spec render_template(String.t(), String.t(), map()) ::
           {:ok, String.t()} | {:error, String.t()}
   def render_template(project_id, source, assigns) do
-    with {:ok, template_ast} <- Liquex.parse(source),
+    with {:ok, template_ast} <- Liquex.parse(source, BDS.Rendering.LiquidParser),
          {:ok, _rendered} = ok <- safe_liquex_render(template_ast, project_id, assigns) do
       ok
     else
diff --git a/lib/bds/templates.ex b/lib/bds/templates.ex
index 3c9548f..bc5f241 100644
--- a/lib/bds/templates.ex
+++ b/lib/bds/templates.ex
@@ -350,7 +350,7 @@ defmodule BDS.Templates do
   end
 
   defp validate_liquid(source) do
-    case Liquex.parse(source) do
+    case Liquex.parse(source, BDS.Rendering.LiquidParser) do
       {:ok, _ast} -> :ok
       {:error, reason, line} -> {:error, "#{reason} at line #{line}"}
     end
diff --git a/test/bds/templates_test.exs b/test/bds/templates_test.exs
index 8f75b61..3826320 100644
--- a/test/bds/templates_test.exs
+++ b/test/bds/templates_test.exs
@@ -315,6 +315,31 @@ defmodule BDS.TemplatesTest do
     assert published.status == :published
   end
 
+  # LiquidTagSubset invariant (template.allium): only {% if %}, {% for %},
+  # {% assign %}, {% render %} (plus {{ }} output) are supported. Any tag
+  # outside that subset must be rejected at publish time.
+  for tag <- ["unless", "case", "capture", "tablerow", "cycle", "increment"] do
+    @tag_name tag
+    test "publish_template rejects unsupported Liquid tag #{tag}", %{
+      project: project
+    } do
+      tag = @tag_name
+
+      assert {:ok, template} =
+               BDS.Templates.create_template(%{
+                 project_id: project.id,
+                 title: "Tag #{tag}",
+                 kind: :post,
+                 content: "{% #{tag} foo %}bar{% end#{tag} %}"
+               })
+
+      assert {:error, {:invalid_liquid, _reason}} = BDS.Templates.publish_template(template.id)
+
+      reloaded = Repo.get!(BDS.Templates.Template, template.id)
+      assert reloaded.status == :draft
+    end
+  end
+
   test "rebuild_templates_from_files recreates published templates from disk", %{
     project: project,
     temp_dir: temp_dir