Summary
withCount(...relations: string[]) exists but only accepts relation names — no per-relation constraint callback (Eloquent's withCount(['comments' => fn])) and no aggregate siblings. The subquery builders (buildHasCountSubquery, buildBelongsToManyCountSubquery) are already factored, so adding withSum(relation, col) / withAvg / withMax / withMin and an optional callback arg is incremental.
Effort: M. Location: src/client.ts:~3519. High Eloquent-parity value. Found via internal code audit.