Hi Charles,
I think the issue is that the discovery endpoint may not be considered to be a core part of the API, and so those query parameters don't apply to it. The field mask filter would be used on a request for Ads data from the API, the discovery endpoint just contains metadata about the available fields. If you just want to isolate the "schemas" object, my recommendation would be to parse the JSON that's returned. If it's helpful, we have documentation on how to access the Google Ads API using REST
here.
Hope that helps, let me know if you have any other questions.
Best,
Ben, Google Ads API Team

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