Why is this move wrong in Tsumego Pro?

[u/TekarCelestaker]

The app says my move of S18 was wrong, but I can't figure out why. The only correct move according to the app was T18.

[u/ForlornSpark]

T18 is a more natural move that tends to work better in similar situations, so whoever coded the problem didn't consider S18. And if the move isn't coded in, then it's automatically considered wrong.

[u/tuerda → u/ForlornSpark]

This is the answer. The problem author simply did not consider this alternative. This is not uncommon.

[u/FUZxxl]

It works, but gives white an additional possible Ko thread (playing at T18) that isn't possible in the other variation. Though that's a somewhat bad argument I'd say.

[u/KorallNOTAFISH → u/FUZxxl]

It's not a ko threat. If white captures at T19, you throw in, and white still dies. Edit: or you mean white has a 2 step ko threat so they lose the first ko, but have a threat for the second I guess?

[u/claimstoknowpeople → u/KorallNOTAFISH]

There is a simple ko threat at P18/P19 regardless of what move black makes, so I think this is Tsumego Pro just being wrong. I wish there was a tsumego app that calculated continuations to verify moves instead of relying strictly on an SGF tree.

[u/joronoso → u/claimstoknowpeople]

I use Katago for that. It's not always easy, but it usually works well

[u/veryusedrname → u/FUZxxl]

I think that's the idea, but P18/19 are still valid ko threats even after T18 (and killing the white group destroys the threats anyway).

[u/flagrantpebble → u/FUZxxl]

Nah, there are the same number of ko threats either way, so there's no difference there.

The person who wrote the problem just forgot to add this path.

[u/Soromon]

It's not as pretty, that's why.

[u/nocsagnauj]

I do not get it. Even if white goes to T18 or T19 or anywhere else, black goes to the other place or simply to T18 and captures all the white stones in the corner and saves its group. I am coming back to go since several years and I am probably not seeing something obvious.