Counts visible talent only by default (isHidden is not true and archivedAt is null). Pass onlyReturnHiddenTalent=true to count only hidden talent instead, or includeHidden=true to count BOTH visible and hidden talent (archived still excluded). For Match Score counts, vectors[matchUuid] takes the Pool entity's own uuid (for matchType Opening, a Hire funnel id is also resolved via the opening's hireExternalId); an identifier found in neither space is rejected as a 400 naming the unresolvable id. averageScore is DEPRECATED here and returns null unless the query carries vectors — use GET /talents/averageScore instead. It stays in this response for one release so existing consumers keep parsing. Removing it is also what moves the Average Match tile in the talent table onto GET /talents/averageScore; until then the tile reads it here, because the count computes the number inside a vector search it is already running. The filter lists common fields; any talent field works.