fix(preview): harden sandbox and transformation

tests/test_tools.py

@@ -514,9 +514,25 @@ def test_asking_for_a_preview_language_or_pointer_interaction_offers_the_tool():
         assert tools.wants_render_preview(prompt), prompt
 
     # Still narrow: ordinary chat must not pay for a tool turn.
-    for prompt in ("what's the weather vibe today", "summarise this email thread"):
+    for prompt in (
+        "what's the weather vibe today",
+        "summarise this email thread",
+        "write a concise paragraph about caching",
+    ):
         assert not tools.wants_render_preview(prompt), prompt
 
+    assert tools.wants_render_preview("compare these graphs")
+
+
+def test_external_preview_resources_are_rejected_in_attributes_and_css():
+    for markup in (
+        '<img src=https://example.com/chart.png alt="chart">',
+        '<style>.chart { background: url("https://example.com/chart.png"); }</style>',
+        '<style>@import "https://example.com/chart.css";</style>',
+    ):
+        issues = tools._critique_shared(markup)
+        assert any("external http(s)" in issue for issue in issues), markup
+
 
 def test_every_preview_language_hints_for_itself():
     for lang in tools.PREVIEW_LANGS: