fix(mcp): reject YAML merge keys in OpenAPI specs

litellm/proxy/_experimental/mcp_server/openapi_to_mcp_generator.py

@@ -182,7 +182,13 @@ def _parse_openapi_spec(text: str) -> Mapping[str, Any]:
     except json.JSONDecodeError:
         import yaml
 
-        parsed = yaml.safe_load(text)
+        class _NoMergeSafeLoader(yaml.SafeLoader):
+            def flatten_mapping(self, node: Any) -> None:
+                if any(key_node.tag == "tag:yaml.org,2002:merge" for key_node, _ in node.value):
+                    raise yaml.YAMLError("YAML merge keys are not supported")
+                super().flatten_mapping(node)
+
+        parsed = yaml.load(text, Loader=_NoMergeSafeLoader)
 
     if not isinstance(parsed, dict):
         raise TypeError("OpenAPI spec must be a JSON or YAML object")

tests/unit/proxy/_experimental/mcp_server/test_openapi_to_mcp_generator.py

@@ -312,6 +312,24 @@ def test_parse_openapi_spec_rejects_non_object_document() -> None:
         gen._parse_openapi_spec("- not an OpenAPI object\n")
 
 
+def test_parse_openapi_spec_rejects_yaml_merge_aliases() -> None:
+    import yaml
+
+    with pytest.raises(yaml.YAMLError, match="YAML merge keys are not supported"):
+        gen._parse_openapi_spec(
+            """
+base: &base
+  openapi: 3.0.0
+  info:
+    title: Base
+    version: '1.0.0'
+  paths: {}
+merged:
+  <<: *base
+"""
+        )
+
+
 def _create_mock_client(method: str, response_text: str, status_code: int = 200) -> AsyncMock:
     """Utility to create a mocked async httpx client for the given method.