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I played the following game:
https://online-go.com/game/25722260

The final board state, in case the link breaks

In the final position, it seems to me that the stones in the bottom left are in seki, but the score estimator says that the white stones are dead. Counting up my points, it seems to me that I should have won by 12.5 points, but it says I'm winning by over 30. What's happening here? Is there a tactic I'm not seeing or a rule I am unaware of? The scoring system being used is Chinese.

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It is possible depending on previous turns that some variants of the Superko rule would allow black to win that fight. For example, if black played A3, white might not be allowed to capture at A5 if that position had already appeared earlier in your game. However, even if that rule is in effect and relevant, I doubt it would be incorporated into the scoring system, so my guess is this is just a bug in the digital adaptation.

EDIT: PJTraill comments that we can see from your picture that H8 was the most recent move; since that space was empty before and is now filled forever, no earlier positions should be relevant for Superko. Thus there is no way to break that seki.

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  • Chinese rules so Positional Superko, I think. Would have been relevant for winning it earlier but not at this late juncture ^^;
    – Please stop being evil
    Jul 27, 2020 at 21:24
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    I believe that it is known that OGS bot games have this sort of problem, as in them there is effectively no stone removal agreement phase, and it is left to the OGS software, which does not always get it right. I think you can find this in the forum.
    – PJTraill
    Jul 28, 2020 at 19:07
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    I asked a similar question in the OGS forum: forums.online-go.com/t/… and was told that one should make group status as clear as possible before the game ends. Since you won anyway, it makes no difference, but in your case the only thing that might otherwise have helped was connecting at A5, which does not affect the seki or the (Chinese or Japanese) score.
    – PJTraill
    Jul 28, 2020 at 23:56
  • No form of superko lets ⬤(Black) capture. There are no superko constraints in the above position, as H8 was empty until the last move. It is now ◯(White)’s turn, but they can only usefully pass or resign. If they pass and ⬤ plays A3, ◯ can capture two with A5, which is atari, and if ⬤ is not allowed to recapture at A4, ◯ can remove the black group and thereby win the game. As far as I can see, A4 is forbidden by all 4 sorts of superko at Sensei’s: Positional (same position), Chinese (send 2 return 1), Situational (same situation as after pass) and Natural Situational (⬤ created this position).
    – PJTraill
    Jul 29, 2020 at 0:31
  • Computer scoring algorithms notoriously have trouble with some seki and some ko positions. In this case, if Black were to have a stone at A5, which would preserve the seki, then the OGS computer probably could score the position correctly. [Interestingly, this is the type of ko that could cause a difference in score between Area and Territory counting. But the game was not close, so the result would not hinge on whether komi were 6.5 or 7.5.]
    – m_mlvx
    Aug 2, 2020 at 9:15

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