The border marking technique is something I've been using for a long time, much earlier than when I first made Tapa Borders. Though for the purpose of making that image I did reuse the same drawing code.

Of course it's not quite the same as the Borders wall, since that one specifies exactly one black cell and this one just says not both.

Hm, I usually use an X on the border to indicate exactly one of the two cells (same as I do for Star Battle), but it's not terribly visible. I'll have to try bolding the border.

Thanks for the guides -- getting the push at R1C6 was just what I needed.

The classic sudoku wasn't indeed just another classic sudoku. It had a very pretty move to see - the one Thomas missed and also, certainly, the one that explains the troubles encountered by the test-solvers since it is really "the one step" that gives the puzzle its difficulty and value.

You should reach this point without too much trouble - give or take the 6 in R7C4, that is not that easy to spot but it isn't needed in what follows anyway.



Here, you just have to focus on the 9s. Thomas, you were on the right track by targeting them - just, you did not need coloring.

First, note that in region 4, they have to be in row 5 or 6 ; and same in region 6. Therefore, in region 5, they are in either column 4 or 5.
In region 2, 9 also has to be in one of those two columns. And here we are (already) : the only valid cell for a 9 in region 8 is R9C6.



It really wasn't that hard, but as is always the case on such puzzles with an extremely narrow path, you cannot afford to miss a single step. Anyway, the puzzle was a pretty little one, so much more pleasant to solve than the average computer-generated one.

That's a really nice demonstration on the sudoku's sticking point. I caught the same simple step when I solved it after the test. Often my eye catches the right digit to search, but not the simplest way to find use it (fastest, but not simplest). Here I failed to have the existing note in column 9 about a 9 in box 6, but reconstructed what I needed from just considering the effect of a 9 in R9C7 (which is bad!).

My approach on the Gapped Kakuro was actually center before left, which meant I didn't have the benefit of knowing the 6 in the middle row.

What I noticed is that the columns of 40 and 43 mean that you need two 9s and two 7s in those columns. The 16 accounts for one of each, and the 15 accounts for at most one of the other. So where's the fourth? It turns out you can eliminate it from all the rows except for the 2-cell 9 clue -- and putting the 7 there leads to a contradiction.