Did you mean /sharing/rest/...?
You are using an unsupported version of this API located at https://gis.cityofsanmateo.org/portal/sharing. Avoid using and/or developing with this unsupported version. There are pitfalls in doing so and no advantages over using the released and supported version of the API, which is logically equivalent. The supported version is located at https://gis.cityofsanmateo.org/portal/sharing/rest/.