We have our Hydra (https://hydra.holo.host) set up to build PRs for https://github.com/Holo-Host/holo-nixpkgs in project holo-nixpkgs.
It is set up declaratively. Jobsets are set up in jobsets.nix, JSON spec is here.
When the project is created via Hydra web UI, .jobsets get evaluated, and all existing (open) PRs appear in jobsets list of holo-nixpkgs. When new PRs are opened against the repo, .jobsets gets re-evaluated (confirmed that github-pulls-sorted.json and jobsets.json are re-generated and contain the new jobsets), but these jobsets don’t appear in holo-nixpkgs jobsets list.
Hydra server info: