diff --git a/tests/unit/proxy/_experimental/mcp_server/test_openapi_spec_path_url.py b/tests/unit/proxy/_experimental/mcp_server/test_openapi_spec_path_url.py
index 5b231aac09d..81ad3cd2bec 100644
--- a/tests/unit/proxy/_experimental/mcp_server/test_openapi_spec_path_url.py
+++ b/tests/unit/proxy/_experimental/mcp_server/test_openapi_spec_path_url.py
@@ -197,3 +197,52 @@ def test_load_openapi_spec_url_plain_text_body_raises(
 
     with pytest.raises(ValueError, match="Expecting value"):
         gen.load_openapi_spec(url)
+
+
+def test_load_openapi_spec_invalid_yaml_raises(tmp_path) -> None:
+    """Invalid YAML in a .yaml file must raise ValueError, not crash."""
+    p = tmp_path / "openapi.yaml"
+    p.write_text("openapi: [unclosed\n  bad: indent", encoding="utf-8")
+    with pytest.raises(ValueError, match="Invalid YAML OpenAPI spec"):
+        gen.load_openapi_spec(str(p))
+
+
+def test_load_openapi_spec_non_mapping_yaml_raises(tmp_path) -> None:
+    """A YAML document that is not a mapping must raise TypeError."""
+    p = tmp_path / "openapi.yaml"
+    p.write_text("just a plain string", encoding="utf-8")
+    with pytest.raises(TypeError, match="expected a JSON/YAML mapping"):
+        gen.load_openapi_spec(str(p))
+
+
+def test_load_openapi_spec_missing_file_raises(tmp_path) -> None:
+    """A missing local spec file must raise FileNotFoundError."""
+    with pytest.raises(FileNotFoundError):
+        gen.load_openapi_spec(str(tmp_path / "does-not-exist.yaml"))
+
+
+def test_load_openapi_spec_json_file_with_yaml_content(tmp_path) -> None:
+    """A .json file containing YAML falls back to the YAML parser."""
+    p = tmp_path / "openapi.json"
+    p.write_text(
+        "openapi: '3.0.0'\ninfo:\n  title: YAML in JSON clothing\n  version: '1.0.0'\npaths: {}",
+        encoding="utf-8",
+    )
+    spec = gen.load_openapi_spec(str(p))
+    assert spec["info"]["title"] == "YAML in JSON clothing"
+
+
+def test_import_yaml_missing_raises_clear_error(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Without PyYAML installed the error must say how to fix it."""
+    import builtins
+
+    real_import = builtins.__import__
+
+    def fake_import(name, *args, **kwargs):
+        if name == "yaml":
+            raise ImportError("No module named 'yaml'")
+        return real_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(builtins, "__import__", fake_import)
+    with pytest.raises(ImportError, match="pip install pyyaml"):
+        gen._import_yaml()