Simpler Cage/Unit overlaps are handled by Captured Candidates. This applies to all puzzles with cages.

This is an important Killer Sudoku strategy and very useful and gives you a bit of a break from looking for Innies and Outies. But you'll need to be aware of the cage combinations. It is related to Intersection Removal. Whereas IR is the overlap of rows/columns with boxes, this is the overlap of 'cages' with rows, columns and boxes.

Each 'cage' is made up of one or more 'combinations' - sets of numbers that total the cage clue. If you can find a candidate number inside that cage that is not found elsewhere on the row, columns or box the cage is aligned on, then you know that number must appear in the cage. That part is self-evident since, in the solver at least, these numbers are displayed. Given that number is true, we can remove all combinations which omit that number. Often that means we can remove a bunch of numbers.

Example 1 (Link)
In the first example below the 3-cell cage with in the middle has a clue of 10, which means the combinations (visible if you hover over the cage on the solver) are {1,2,7}, {1,3,6}, {1,4,5} and {2,3,5}. But try as you might only {1,3,6} can fit in the remaining cells. Hence we can remove all candidates that can see all the same candidate in the cage. notice that 3 can only go in D6 so it acts like a single and knocks out all those other 3s.

Example 2 (Link) (Untick Killer Combs Easy)

In this second example Captured Candidates are detected in the cage starting A7. We are fortunate B7 has been solved as this drastically reduces the possible combinations from seven to one. To make 13 with a 9 we must have 1 and 3 as well. That makes 1 and 3 like a Hidden Pair - it is unique to the cage, box 3 and column 7. All other 3s that can see AB7 must be eliminated.

Overall, most Killer Sudoku puzzles will have at least one example of this strategy so they are well worth looking out for, and often you can reduce the puzzle with this method while looking into the cages with multiple combinations. Keep an eye out on the rows, columns and boxes you are studying.

This is a strange situation and at first glance it is not obvious why the solver comes up with this solution for the first cage. Example 3 (Link)

Cage vs Unit: the cage starting on A1 has combinations 
which require 3 AND they align in a unit, so
 - 3 can be removed from A8
 - 3 can be removed from A9

The 3s are highlighted because they allegedly align with row A. But what about B3? The remaining combinations are 13589, 13679, 23579 and 23678. Each one has a 3 in it but it turns out you can't fit 3 in B3 and then the other numbers elsewhere. 3 can in fact be removed from B3 but that is another step.

If Type 1 above is related to Box Line Reduction, then you'd recognize that Type 2 will look like Pointing Pairs, it's opposite. Credit to Caleb Andrews for finding this example and stating the logic so elegantly.



In this puzzle (untick Innies/Outies 2+) (also used in the Innies/Outies example) we consider the 4-cage in a square CD67. Because the 4 is fixed in C7 we only need to consider the combinations that have 4, but
1489 <- no 1 left in the cage
2479 <- no 2 left in the cage
3469
3478
4567 <- no 5 left in the cage
so we are left with two combinations. Both contain 3 and 4. 4 is not helpful as we've already placed it, but look where 3 fits in the cage. It can only go in D6 and D7. That makes 3 a certainty in those cells. So it acts like a Pointing Pair aligned on the row. We can remove 3 from D1235.

There is another large example of this strategy in the same puzzle, on the big L cage starting on D3.

The third theme of Cage Unit Overlap considers cages of size N with N remaining numbers. All numbers are necessary to the cage (locked set). Then, if any one or more of those candidates are exclusive to any row, box or column they point along that unit to candidates which can be eliminated. On small straight cages this will often be a Naked Pair, Triple etc and dealt with earlier. But for larger cages that 'dog-leg' it is possible some remaining candidates will be on one part of the 'leg' and aligned with a unit.



In this example the 5-cell with 25+ starting on D2 has {1,3,6,7,8} to fill all cells. But 8 is exclusive to column 2 and 1 is exclusive to both column 2 and box 4. So eliminations are made on those units.
Load this example or From the start [Added Jan 2025]

Here is an interesting situation with the yellow cage with seven cells at the bottom of the board. 'Captured Candidates' is going to find something first and call it a Cage Hidden Single. This board A to the right. This cage has seven remaining candidates spread over seven cells making it a locked set. But 4 exists only once in the cage in J4. As a 4 is necessary there being 7 for 7 in the cage, we must assume 4 is the solution for J4.

If this was a cage with 4 cells it would be a Naked Quad but we don't go that high for cages of size 7. Unticking 'Captured Candidates' gives us another view on the cage (B). This is turned up by Cage/Unit Overlap. It identifies it is a locked set and starts to remove all other candidates that can see all of each N in the cage. The single 4 is "pointing" to all 4s it can see just as you'd expect from a single. Trace the other eliminations.

Another type of elimination I've chosen, a bit arbitrarily, to insert into the search at this strategy point is to compare a cage with a bi-value cell. This was a tip from Ivar Ling.



In this puzzle the 2-cage with a sum of 12 can only be {4/8} or {5/7} (with {3/9} already removed). Now a bi-value cell B1 can see all the cells in this cage and it contains a pair of candidates which matches one of those combinations. Since B1 must be either 4 or 8 it removes {4/8} combination from the cage. This will be reported as

Cage/Unit Overlap
CAGE vs BIVALUE: the cage starting on B6 has combinations which are ruled out by bi-value cell B1 which can see all cells in the cage, so
- we can remove 4/8 from B6
- we can remove 4/8 from B7

Update 26 August 2025: Cages longer than two cells can now be detected with this pattern. This is an example.