diff --git a/src/central/adapter.py b/src/central/adapter.py
index 2a408b0..76ab04e 100644
--- a/src/central/adapter.py
+++ b/src/central/adapter.py
@@ -85,5 +85,16 @@ class SourceAdapter(ABC):
         Return list[dict] (framework renders as a generic table; columns come from
         the first dict's keys, in insertion order). Return None to skip preview.
         Raise to surface an error banner — framework catches at the route boundary.
+
+        Contract:
+        - Preview is a pure function of `settings`. Do NOT access
+          self._config_store or cursor_db state — the framework may instantiate
+          adapters with a stub config_store solely to call this method.
+        - Network preview implementations must open their own short-lived
+          aiohttp session (the adapter's polling session may not exist; the GUI
+          process never calls startup()).
+        - Return None when preview is not meaningful (e.g., required settings
+          like region are unset). Return [] explicitly if the query ran and
+          matched zero rows — the framework renders that distinctly from None.
         """
         return None
diff --git a/src/central/gui/templates/_adapter_preview.html b/src/central/gui/templates/_adapter_preview.html
index ff9ef0c..ab8a7f1 100644
--- a/src/central/gui/templates/_adapter_preview.html
+++ b/src/central/gui/templates/_adapter_preview.html
@@ -2,9 +2,10 @@
 <article aria-label="Preview Unavailable" style="background-color: var(--pico-del-color); margin-bottom: 1rem;">
     <strong>{{ preview_error }}</strong>
 </article>
-{% elif preview_rows %}
+{% elif preview_rows is not none %}
 <fieldset>
     <legend>Preview ({{ preview_rows|length }} rows)</legend>
+    {% if preview_rows %}
     <table class="preview-table" role="grid">
         <thead>
             <tr>
@@ -19,5 +20,6 @@
             {% endfor %}
         </tbody>
     </table>
+    {% endif %}
 </fieldset>
 {% endif %}
diff --git a/tests/test_preview_hook.py b/tests/test_preview_hook.py
index e9fbd30..88b9b1b 100644
--- a/tests/test_preview_hook.py
+++ b/tests/test_preview_hook.py
@@ -106,3 +106,15 @@ def test_partial_renders_nothing_when_both_none():
     assert "Preview Unavailable" not in out
     # Either empty or only whitespace/newlines from the template.
     assert "Preview (" not in out
+
+
+def test_partial_renders_empty_list():
+    """Empty list -> legend with (0 rows), no table.
+
+    Distinct from None (which renders nothing at all). Lets adapters signal
+    'query ran, matched zero rows' separately from 'preview not meaningful'.
+    """
+    out = _render_partial(preview_rows=[], preview_error=None)
+    assert "Preview (0 rows)" in out
+    assert "<table" not in out
+    assert "<th>" not in out