Estimate the distinct reach of a filter without creating an audience. Sizes the filter tree against the precomputed intent index and returns distinct person / email / mobile counts, so a builder can show a live reach estimate as the filter changes.

The estimate is distinct reach only and never includes a billable record count: the row-per-email fan-out that billing is based on only exists after a build. Treat the returned counts as approximate reach; the exact billed record count is known only after the audience is created.

Not every filter can be sized from the index. When supported is false, the index cannot stand behind a count (for example a filter shape it does not cover, or an intent window not yet fully indexed); fall back to creating the audience to get an exact size. A 200 is returned for any well-formed request, even when supported is false; only a malformed body returns 400.

Project-scoped: name the project with the X-Project-Id header or a ?project_id= query parameter, as with the other audience endpoints.