Harden alpha8 publication safety

test/opencode/message_artifacts_test.rb

@@ -7,6 +7,23 @@ require "test_helper"
 # coverage — including ActiveStorage attachment, Transform application,
 # error reporting via Opencode::ErrorReporter — lives in the host app.
 class Opencode::MessageArtifactsTest < Minitest::Test
+  Message = Struct.new(:id, keyword_init: true)
+  Sandbox = Struct.new(:entries, keyword_init: true) do
+    def exists? = true
+    def files(after: nil) = entries
+  end
+  SandboxEntry = Struct.new(:basename, :attached, keyword_init: true) do
+    def as_artifact
+      self.attached = true
+      raise "destination must not use the identity attachment path"
+    end
+  end
+
+  class Transform < Opencode::Transform
+    def source_filename = "source.json"
+    def destination_filename = "rendered.html"
+  end
+
   def test_initialize_takes_message_feature_and_optional_transforms
     params = Opencode::MessageArtifacts.instance_method(:initialize).parameters
     by_kind = params.group_by(&:first).transform_values { |list| list.map(&:last) }
@@ -23,4 +40,17 @@ class Opencode::MessageArtifactsTest < Minitest::Test
     assert_equal [ :attach_from ], Opencode::MessageArtifacts.instance_methods(false),
       "MessageArtifacts's only public verb is #attach_from"
   end
+
+  def test_agent_authored_transform_destination_never_uses_default_attachment
+    entry = SandboxEntry.new(basename: "rendered.html", attached: false)
+    artifacts = Opencode::MessageArtifacts.new(
+      message: Message.new(id: 1),
+      feature: "test",
+      transforms: [ Transform.new ]
+    )
+
+    artifacts.attach_from(sandbox: Sandbox.new(entries: [ entry ]))
+
+    refute entry.attached
+  end
 end