This document contains current known issues for Google Cloud Search.
API Explorer doesn't work
The API Explorer was disabled for Cloud Search APIs due to some UX issues with the Explorer and how the Cloud Search API and scopes work. One of the scopes present on most methods is currently only allowed with service accounts, but is enabled by default in the explorer. Users would experience OAuth errors unless they were careful to unselect the scope causing problems. Hopefully this is just a temporary fix until we can find a better solution.
As a workaround, use the standalone explorer.