diff --git a/src/ai_client.py b/src/ai_client.py
index b70b5f50..54a64efc 100644
--- a/src/ai_client.py
+++ b/src/ai_client.py
@@ -1581,7 +1581,15 @@ def _list_gemini_cli_models() -> list[str]:
   "gemini-2.5-flash-lite",
  ]
 
-def _list_gemini_models(api_key: str) -> list[str]:
+def _list_gemini_models_result(api_key: str) -> Result[list[str]]:
+ """List available Gemini models via google-genai SDK.
+
+ Returns the sorted list of Gemini model names. On SDK or network failure,
+ returns Result(data=[], errors=[ErrorInfo(...)]). The legacy caller
+ (_list_gemini_models) returns result.data directly (preserving original
+ behavior); callers that need to surface errors should call this helper
+ and inspect result.errors.
+ """
  try:
   genai  = _require_warmed("google.genai")
   client = genai.Client(api_key=api_key)
@@ -1590,9 +1598,16 @@ def _list_gemini_models(api_key: str) -> list[str]:
    name = m.name
    if name and name.startswith("models/"): name = name[len("models/"):]
    if name and "gemini" in name.lower():   models.append(name)
-  return sorted(models)
+  return Result(data=sorted(models))
  except Exception as exc:
-  raise _classify_gemini_error(exc) from exc
+  return Result(
+   data=[],
+   errors=[_classify_gemini_error(exc, source="ai_client._list_gemini_models_result")],
+  )
+
+
+def _list_gemini_models(api_key: str) -> list[str]:
+ return _list_gemini_models_result(api_key).data
 
 def _ensure_gemini_client() -> None:
   global _gemini_client
diff --git a/tests/tier2/phase10_site1_test.py b/tests/tier2/phase10_site1_test.py
new file mode 100644
index 00000000..054fba20
--- /dev/null
+++ b/tests/tier2/phase10_site1_test.py
@@ -0,0 +1,37 @@
+"""Phase 10 invariant tests (RED).
+
+Site 1 (L1594): _list_gemini_models_result helper must exist + return Result[list[str]].
+"""
+import sys
+sys.path.insert(0, ".")
+
+from src.result_types import Result, ErrorInfo
+
+
+def test_phase10_site1_list_gemini_models_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_list_gemini_models_result"), \
+        "_list_gemini_models_result helper missing from src.ai_client"
+
+
+def test_phase10_site1_list_gemini_models_result_returns_result():
+    """The helper must return a Result[list[str]]."""
+    import src.ai_client
+    fn = getattr(src.ai_client, "_list_gemini_models_result", None)
+    assert fn is not None
+    import inspect
+    sig = inspect.signature(fn)
+    # Should have a return annotation of Result
+    assert "Result" in str(sig.return_annotation), \
+        f"_list_gemini_models_result return annotation must be Result, got {sig.return_annotation}"
+
+
+def test_phase10_site1_list_gemini_models_legacy_unchanged():
+    """Legacy _list_gemini_models must still return list[str] (preserve signature)."""
+    import src.ai_client
+    fn = getattr(src.ai_client, "_list_gemini_models", None)
+    assert fn is not None
+    import inspect
+    sig = inspect.signature(fn)
+    assert "list[str]" in str(sig.return_annotation) or "list" in str(sig.return_annotation), \
+        f"_list_gemini_models return annotation must remain list[str], got {sig.return_annotation}"
\ No newline at end of file