6-cell killer cage summing to 21

A 6-cell killer cage of 21 is fully locked: the only way to make 21 from 6 distinct digits is {1, 2, 3, 4, 5, 6}. Whatever order they sit in, those digits are the cage's contents — every one of them eliminates from every cell of the cage's row, column, and box that lies outside the cage.

Locked digits: 1, 2, 3, 4, 5, 6. Whichever combination is in play, these digits sit somewhere inside the cage — meaning the cage's shared row, column, and box can't have any of them anywhere outside the cage.

Forbidden digits: 7, 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.