4-cell killer cage summing to 13

3 digit sets sum to 13 across 4 cells. Without other constraints this cage is loose; the value comes from pairing it with the cage's row, column, or box context — every digit ruled out elsewhere narrows which combinations remain.

Locked digits: 1. Whichever combination is in play, this digit sit somewhere inside the cage — meaning the cage's shared row, column, and box can't have it anywhere outside the cage.

Forbidden digits: 8, 9. These digits never appear in this cage no matter which combination resolves; you can eliminate all of them from every cell of the cage as you pencil in candidates.