diff --git a/tests/tier2/phase12_site4_test.py b/tests/tier2/phase12_site4_test.py
new file mode 100644
index 00000000..6b47a96a
--- /dev/null
+++ b/tests/tier2/phase12_site4_test.py
@@ -0,0 +1,41 @@
+"""Phase 12 site 4: _list_anthropic_models Result migration.
+
+Site 4 (L1337):
+  try: anthropic = _require_warmed('anthropic'); ... client.models.list() ...
+  except Exception as exc:
+      raise _classify_anthropic_error(exc) from exc
+
+BUG: _classify_anthropic_error(exc) returns ErrorInfo (not an Exception).
+'raise ErrorInfo from exc' would fail at runtime. Migrate to Result.
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase12_site4_list_anthropic_models_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_list_anthropic_models_result"), \
+        "_list_anthropic_models_result helper missing"
+
+
+def test_phase12_site4_helper_returns_result():
+    import src.ai_client
+    import inspect
+    fn = src.ai_client._list_anthropic_models_result
+    sig = inspect.signature(fn)
+    assert "Result" in str(sig.return_annotation), \
+        f"_list_anthropic_models_result return must be Result, got {sig.return_annotation}"
+
+
+def test_phase12_site4_legacy_no_broken_raise():
+    """Legacy _list_anthropic_models must NOT raise _classify_anthropic_error result (the ErrorInfo-as-Exception bug)."""
+    import inspect
+    import src.ai_client
+    src_text = inspect.getsource(src.ai_client._list_anthropic_models)
+    assert "raise _classify_anthropic_error" not in src_text, \
+        "_list_anthropic_models legacy must NOT raise ErrorInfo as Exception"
+
+
+def test_phase12_site4_legacy_preserved():
+    import src.ai_client
+    assert callable(getattr(src.ai_client, "_list_anthropic_models", None))
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